Use of oral polio vaccine and the incidence of COVID-19 in the world.

BACKGROUND: Several live attenuated vaccines were shown to provide temporary protection against a variety of infectious diseases through stimulation of the host innate immune system.
OBJECTIVE: To test the hypothesis that countries using oral polio vaccine (OPV) have a lower cumulative number of cases diagnosed with COVID-19 per 100,000 population (CP100K) compared with those using only inactivated polio vaccine (IPV).
METHODS: In an ecological study, the CP100K was compared between countries using OPV vs IPV. We used a random-effect meta-analysis technique to estimate the pooled mean for CP100K. We also used negative binomial regression with CP100K as the dependent variable and the human development index (HDI) and the type of vaccine used as independent variables.
RESULTS: The pooled estimated mean CP100K was 4970 (95% CI 4030 to 5900) cases per 100,000 population for countries using IPV, significantly (p<0.001) higher than that for countries using OPV-1580 (1190 to 1960). Countries with higher HDI prefer to use IPV; those with lower HDI commonly use OPV. Both HDI and the type of vaccine were independent predictors of CP100K. Use of OPV compared to IPV could independently decrease the CP100K by an average of 30% at the mean HDI of 0.72.
CONCLUSIONS: Countries using OPV have a lower incidence of COVID-19 compared to those using IPV. This might suggest that OPV may either prevent SARS-CoV-2 infection at individual level or slow down the transmission at the community level.

Introduction

Coronavirus disease 2019 (COVID-19) has become a major global health concern. By February 13, 2022, it has affected more than 410 million people worldwide; the death toll exceeds 5.8 million [1]. Many countries have begun mass vaccination of their people with a handful of specific vaccines developed thus far that have received authorization. However, there are limitations in the production and distribution of the SARS-CoV-2 specific vaccines worldwide, particularly in resource-limited countries. Furthermore, the specific vaccines currently available were made using the original strain of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which may not be effective against the emerging virus variants [2].

Several studies have shown that the host innate immune system has an important role in combating SARS-CoV-2 [3,4]. “Live” attenuated vaccines (LAVs) stimulate the innate immune system which may provide temporary protection against other viruses [5-9]. A systematic review of 10 cohort and two case-control studies revealed that administration of measles vaccine to children residing in seven developing countries decreased the all-cause mortality by 30% to 86%, far more than that was anticipated based on protection produced solely against measles [8]. Attenuated bacterial vaccines such as Bacillus Calmette-Guérin (BCG) can also provide non-specific immunity against unrelated diseases [10-12]. A recent study reveals that BCG vaccination can not only protect newborns against tuberculosis, but it also provides protection against non-tuberculous infectious disease during the neonatal period [13]. Meta-analysis of five clinical trials showed that BCG vaccination was associated with 30% reduction in all-cause mortality in children under 5 years of age [9]. A recent study revealed that stimulation of the innate immune system by human rhinovirus could block SARS-CoV-2 virus replication through triggering interferon production [14]. It was shown that administration of oral polio vaccine (OPV) to more than 60,000 people resulted in an almost 4-fold decrease in the morbidity and mortality associated with influenza; no clinically important side effects were reported [15,16].

OPV is an LAV. We hypothesized that receiving OPV can prevent COVID-19. Conducting a quality randomized clinical trial to test this hypothesis is not easy, partly because OPV supply is mainly used for the high-priority WHO global polio eradication initiative. However, it is possible to indirectly test the hypothesis at a population level. OPV is made from live attenuated virus that can transmit to contacts of vaccine recipients. In a recent cohort study, we have shown that indirect exposure to the attenuated poliovirus excreted by OPV recipients was associated with decreased symptomatic infection with COVID-19 [17]. Herein, we would like to test the hypothesis in an alternative indirect way. We hypothesized that countries using OPV have a lower cumulative number of cases diagnosed with COVID-19 per 100,000 population (CP100K) compared with those using inactivated polio vaccine (IPV) only.

We thus conducted this study to compare the CP100K between countries using OPV and IPV. There are many other variables that might affect the incidence of the disease, including the quality of the health care system and the surveillance infrastructures that would certainly influence the detection rate and reporting of cases with COVID-19. Therefore, an important part of this study was to account for confounding factors that might influence the conclusions.

Materials and methods

This ecological study was based on the data about the cumulative number of COVID-19 cases diagnosed in each country, the country population and population density, the median age and the life expectancy at birth, the gross domestic product (GDP) per capita, human development index (HDI), and the type of polio vaccine used in each country. We also retrieved the stringency index, a composite metric calculated from 9 response indicators (e.g., school closures, workplace closures, travel bans), ranging from 0 (no restriction) to 100 (highest levels of restrictions). The index was retrieved for each country for each day before April 9, 2021, when all the data were retrieved from “Our World in Data” website [18,19]. The mean stringency index for each country was used for data analyses. Data about the type of polio vaccine used by each country were provided by the World Health Organization Global Polio Eradication Initiative (GPEI). The data for generating the world map were retrieved from Natural Earth, a public domain map dataset [20].

Ethics

The study protocol was approved by the Petroleum Industry Health Organization (PIHO) R&D Institutional Review Board.

Statistical analysis

R software version 4.1.0 (2021-05-18) was used for data analysis. Wilcoxon rank sum test was used to compare the distribution of two continuous variables not normally distributed. Continuous variables were expressed as median (interquartile range [IQR]) or mean (95% confidence interval [CI]). Spearman’s ρ was used to determine the extent of correlation between the continuous variables.

The CP100K was calculated by dividing the number of cases diagnosed in each country by its population at the midpoint of the study period multiplied by 100,000. Pooled estimation of CP100K for countries using IPV and OPV was calculated by a random-effect meta-analysis model, employing the R package metafor. Because CP100K (dependent variable in our analysis) had overdispersion, negative binomial regression analysis was used (with function glm.nb of the R package MASS). The median age, life expectancy at birth, and GDP per capita had a high correlation with HDI; thus, we only considered HDI and the type of polio vaccine used in each country as independent variables in the model. Interaction of the two independent variables was also examined. A p value <0.05 was considered statistically significant.

Results

Data on the type of polio vaccine used were available for 202 countries—56 used IPV only and 146 used OPV in combination with IPV. The CP100K reported from countries using IPV only was higher than that in countries using OPV (Figs 1 and 2). The pooled estimated mean CP100K, derived using a random-effect (I2 = 1.0) meta-analysis model, for countries using IPV was 4970 (95% CI 4030 to 5900) cases per 100,000 population, significantly (p<0.001) higher than that for countries using OPV—1580 (1190 to 1960).

Fig 1

The distribution of COVID-19 incidence rate in each country.

The fill color reflects the cumulative number of cases diagnosed with the disease per 100,000 population. The countries’ border color shows the type of polio vaccine used in each country. Complete data were not available for gray areas. Made with Natural Earth (https://www.naturalearthdata.com/).

Fig 2

Distribution of data points as well as the box and whisker plot indicating the cumulative number of cases diagnosed with COVID-19 per 100,000 population stratified by the type of polio vaccine used in each country.

The horizontal line in the middle of each box indicates the median. The notch represents the 95% confidence interval of the median. The bottom and top borders of the box show the 25th and 75th percentiles, respectively. The lower whisker indicates the smallest data point within 1.5 times the interquartile range (IQR) less than the 25th percentile; the upper whisker indicates the largest point within 1.5 × IQR greater than the 75th percentile. Points greater than the upper whisker and smaller than the lower whisker were considered outliers. All outliers were included in data analyses.

The distributions of the median age, life expectancy, and GDP per capita were significantly different between countries using IPV and OPV (Table 1). The median HDI in countries using IPV was significantly (0.90 vs 0.70, p<0.001) higher than that in countries using OPV. The population density and the mean stringency index were not significantly different between countries using OPV and those using IPV. The median age, life expectancy at birth, and the GDP per capita, all had a significant (p<0.001) high correlation (ρ>0.92) with HDI, the composite index that accounts for these and some other parameters (Fig 3). To avoid multicollinearity, we have only used HDI and the type of vaccine as independent variables in the regression model. CP100K, the dependent varaible in our model, had a mean of 2496 cases per 100,000; the variance exceeded 9×106. For overdispersion, we used a negative binomial regression.

Table 1

Median (IQR) of studied continuous variables stratified by the type of polio vaccine used.

Variable	Type of polio vaccine used		p value
	IPV (n = 56)	OPV (n = 146)
Median age, yrs	42 (38, 44)	26 (20, 32)	<0.001
Life expectancy, yrs	81 (77, 83)	72 (65, 76)	<0.001
GDP* per capita, ×1000 $US	36 (25, 46)	8 (3, 16)	<0.001
Human development index	0.90 (0.85, 0.93)	0.70 (0.55, 0.78)	<0.001
Population density (people/km2)	108 (45, 214)	77 (36, 209)	0.201
Mean stringency index	53 (44, 58)	54 (41, 64)	0.495

*Gross domestic product.

Fig 3

Distribution of studied continuous variables and their correlation with each other.

Values are Spearman’s ρ. The median age (MedianAge), life expectancy at birth (LifeExpect), and the gross domestic product per capita (GDPperCapita), all had a significant (p<0.001) high correlation with the human development index (HDI).

The model could explain more than 70% (Nagelkerke’s R2 = 0.734) of the variance observed in CP100K (Table 2). It showed an interaction between HDI and the type of vaccine used. Use of OPV compared to IPV could independently decrease the CP100K by an average of 30% at the mean HDI of 0.72 (Table 2); the protection provided was higher for countries with lower HDI. For example, the value corresponding to an HDI of 0.55, the 25th percentile HDI in countries using OPV (Table 1), was 75%. HDI was also an independent predictor of CP100K; each 0.1 unit increase in HDI increased CP100K by an average of 2.47-fold; it was 1.62-fold for countries using IPV and 2.90-fold for countries using OPV (Table 2).

Table 2

Results of negative binomial regression.

Variable	Coefficient (95% CI)	Adj IRR* (95% CI)	p value
HDI	4.82 (0.18 to 9.46)	123.37 (1.19 to 12765.75)	0.042
OPV vaccine†	-4.58 (-8.80 to -0.36)	0.01 (0.00 to 0.70)	0.033
Interaction‡	5.84 (0.98 to 10.69)	342.15 (2.66 to 44008.53)	0.019
Intercept	4.21 (0.10 to 8.32)	67.17 (1.10 to 4086.18)	0.045

*Adjusted incidence rate ratio.

†Using OPV vaccine compared to IPV (reference) (OPV = 1, IPV = 0).

‡HDI × Type of vaccine (OPV = 1, IPV = 0).

Nagelkerke’s R2 = 0.734.

Discussion

Use of OPV by a country was an independent predictor for a lower CP100K diagnosed and reported in that country. The pooled mean CP100K was significantly lower in countries using OPV compared with those using IPV. However, this was the result of a univariate analysis and could be affected by other variables. For example, the observed difference might be attributed to the limited availability of diagnostic tests (e.g., RT-PCR) in the low- and middle-income countries, which mostly use OPV, compared to high-income nations, which mostly use IPV. Difference in the climate of countries using IPV vs those using OPV could be another cause of the observed difference [21].

To avoid rare cases of vaccine-associated paralytic polio (VAPP), most countries with higher HDI switched from OPV to IPV after they became free from wild poliovirus circulation. Most countries that are still using OPV are also free from the wild virus, but cannot switch to using IPV only because of the economic and logistic considerations. For this reason, IPV-only countries have a higher GDP per capita and expectedly, should have a better health care infrastructure (as evident by higher life expectancy at birth and median age of their population, Table 1). A better infrastructure expectedly leads to a better and early diagnosis of the disease (more tests, having active surveillance, etc.) that results in a higher number of cases diagnosed, CP100K. At first glance, the under-diagnosis of cases with COVID-19 in low- and middle-income countries might explain the significantly lower observed CP100K in these countries where mostly use OPV (Figs 1 and 2). However, after taking into account the effect of HDI, a variable highly correlated with the level of investment in health care and the necessary infrastructure in a given country, use of OPV still remained an independent predictor of lower CP100K (Table 2).

The effects of lockdowns and international travel bans were hard to account for in our study since policies changed with time. However, the mean stringency index, an index reflecting the level of restrictions imposed in a given country, was not significantly different between countries using OPV vs those using IPV (Table 1). The population density was also not significantly different between the two groups of countries. None of these variables was thus taken into account in the model. HDI exhibits a strong correlation with GDP per capita, life expectancy at birth, and the median age of population in a country (Fig 3). This high correlation was not surprising because HDI, a measure reflecting the average wellbeing of people in a country, is in fact calculated based on the life expectancy at birth (and thus, the median age of the population), gross national income (and thus, the GDP) per capita, and other factors. The type of polio vaccine used in a given country strongly depends on the HDI—countries with higher HDI prefer and can afford to use IPV, which is significantly more expensive than OPV. This explains the significant interaction between HDI and the type of vaccine used observed in the regression model (Table 2). This might also explain the higher impact of each 0.1 unit increase in HDI on CP100K in countries using OPV (2.90-fold increase) compared to that in countries using IPV (1.62-fold increase); an 0.1 unit increase in HDI in low- and middle-income countries (mostly using OPV), where the HDI is relatively low (median 0.7), would expectedly result in a more tangible improvement in the health infrastructure and reporting systems comapred to the same increase in HDI in high-income countries (mostly using IPV), where the HDI is relatively high (median 0.9) and a good health infrastructure has already been established.

The type of vaccine used is another independent variable found significantly associated with CP100K; countries using OPV had a lower CP100K compared to those using IPV (Table 2). We hypothesize that this correlation might reflect the protective effect of the live attenuated poliovirus in the OPV against SARS-CoV-2. Recently, we have shown that mothers whose children recently received OPV do not develop symptomatic COVID-19 for at least 6 months, probably for the indirect exposure they had to the attenuated poliovirus in the vaccine [17].

Because of its nature, ecological studies cannot prove causal relationships; they are mostly considered “hypothesis generating” studies. In this case, the hypothesis about protective effect of OPV against COVID-19 is also supported by other independent evidence [17]. The non-specific protective effects provided by LAVs against unrelated infections have been demonstrated previously [8-11]. The mechanism involved is believed to be mediated by stimulation of the host innate immune system, including interferon production [5,7]. OPV is given mostly to infants who shed significant amounts of vaccine poliovirus in stool, and thus can transmit it to their caregivers and other close contacts. This can initiate chains of transmission resulting in exposure of a significant number of people to attenuated poliovirus, explaining why the virus can be readily found in the environment in countries using OPV [22]. This immunization through secondary exposure to vaccine virus creates strong herd immunity against poliovirus, and may also contribute to higher resistance to other infections by stimulating innate immunity. The community transmission of the virus is expected to be lower in countries with higher sanitation and better health care infrastructures, which are strongly correlated with higher HDI. This could support our hypothesis that OPV might provide protection against SARS-CoV-2 and be an additional reason why the protective effect of OPV was lower in countries with higher HDI (with presumably better sanitation) compared with communities where the virus can transmit more effectively.

An important but yet unknown aspect of the secondary effect of LAVs on susceptibility to unrelated infections is the duration of the protection. Interferon production is a transient phenomenon, but activation of other innate immunity pathways can be more durable. After immunization with OPV, the attenuated poliovirus can trigger epigenetic changes through methylation and acetylation of nuclear histones, marking the genes necessary for the host defense and making them more readily available to have stronger expression upon subsequent stimuli by viral particles, say SARS-CoV-2 [7,23]. This phenomenon, the so-called “trained immunity,” could produce long-lasting effects reducing susceptibility to infections at the individual level; it can also reduce circulation of pathogens in the population.

A recently published study could not find any evidence for the protective effect of polio vaccination against SARS-CoV-2 infection [24]. However, authors of this study failed to distinguish between OPV (a live vaccine) and IPV (inactivated vaccine). Since non-LAVs were shown to lack the non-specific protective effects demonstrated for LAVs, no conclusion can be made from this study regarding use of an LAV like OPV.

An important consideration is whether the use of OPV against COVID-19 is a viable option, because many countries have stopped its use due to its ability to cause rare but serious VAPP cases and trigger the generation of circulating vaccine-derived polioviruses (cVDPV). Therefore, reintroduction of OPV into such countries should be considered very carefully. Furthermore, before this is done, direct clinical evidence of the effectiveness of OPV (and for this matter, other live vaccines) must be generated. This could be done in countries that still use OPV in their immunization programs. If successful, this could open several possibilities. Besides the classical OPV developed based on inherently genetically unstable Sabin strains, rationally designed novel OPV was recently created by targeted genetic manipulation to prevent its reversion to virulence [25]. In addition, a new class of broadly-specific vaccines employing innate immunity stimulation could be created for rapid response to emerging infections, before traditional pathogen-specific vaccines can be developed and introduced. They could become an important instrument in the pandemic preparedness toolbox.

Limitations

The most important limitation of this study is that ecological studies demonstrating correlations cannot provide the final proof of causality, but rather should be used as hypothesis-generating tools. In this particular case, the correlation presented in this study agrees well with other observations of broadly protective effects of OPV against influenza [15,16] and other infections [17], making our conclusions more plausible. Another limitation of our study is that we did not consider the number of diagnostic tests daily performed in each country. That might be a better index compared to HDI for assessment of the health care quality in a given country. However, this information is missing for many countries and we thus decided to use HDI, which was available for most countries in the database we used. Furthermore, the model seems to be good; it could already explain more than 70% of the variance observed in CP100K. We also did not consider the coverage of vaccination against COVID-19 in our study. However, at the time of our data collection, most low- and middle-income countries either did not introduce COVID-19 vaccines or immunized a very small part of their populations. In contrast, high-income courtiers had a much more successful rollout of these vaccines, that are expected to reduce COVID-19 morbidity. Taking this factor into account would further strengthen our hypothesis that at a population level, use of OPV might reduce the SARS-CoV-2 incidence. Nor did we consider the coverage of other LAVs in the studied countries. It has been shown that many of LAVs such as MMR and BCG, can provide non-specific protection against other infections [5,10,26]. However, the coverage of these vaccines was not much different in the studied countries. BCG is routinely administered to all people in almost 90% of countries in the world; in some countries only at-risk groups are vaccinated [27]. On the other hand, while the attenuated poliovirus in the OPV can contaminate the close contacts of the vaccinated child (particularly where the hygienic standards are low) and thereby protect them from other infections [17], MMR and BCG can only affect the vaccinated person. Furthermore, recent experimental evidence suggests that antibodies generated in response to other viral pathogens may cross react with SARS-COV-2 and provide heterologous protection [28]. Controlling these covariates is complicated mainly because of the nature of our study—the inherent inability to make individual level causal inferences based on the aggregate data used in an ecological study.

Conclusions

Use of OPV for routine immunization was associated with a lower incidence of COVID-19 in countries using OPV than in those using only IPV. We hypothesize that the live attenuated poliovirus in the OPV might protect people against SARS-CoV-2. Although this hypothesis has recently been supported by a cohort study [17], it should be tested in well-controlled clinical trials. Besides the possibility of using traditional or novel OPV to mitigate COVID-19, the results of such studies could open the possibility of creation of a new class of broadly specific vaccines, alongside the traditional antigen-specific vaccines.

(CSV)

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1 Dec 2021

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Additional Editor Comments:

This observational study provides a possible relationship between oral Polio vaccine and low incidence of COVID-19 infection. As there is no solid molecular evidence to support the relationship, more discussion and a list of limitations might be necessary for this revision.

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Reviewer #1: The authors performed a population study utilizing publicly available data on the number of COVID cases per country and their usage of oral poliovirus vaccine vs inactivated poliovirus vaccine, as well as their human development index. The authors' data support their conclusion, but there are many variables that would go into the spread of the virus during the pandemic beyond HDI, GDP, and overall health care.

The authors should add in population density as another variable to assess.

The authors need to fix figure 3 which is cutoff.

The authors should include additional discussion on the potential roles that lockdowns and international travel could have played in the observed differences.

The authors should specifically state whether the outliers shown in figure 2 were included in the analyses or excluded. If they were excluded, they should provide the statistical criteria for the exclusion, and provide a secondary graph with them included.

Reviewer #2: The overall observation of this study might be true but the role of OPV is more hypothetical. What the authors failed to conclude with sufficient evidence is whether the observed protection for SARS-CoV-2 in countries with OPV is really due to OPV, BCG (widely used in LMICs) or many other rampant viral and bacterial infection with high frequency. Its well documented that non specific innate immunity provides protection to some degree from several other pathogens. Not only innate immunity, recent experimental evidences suggest that antibodies generated (humorl response) in response to other viral pathogens such as HIV and Flu cross reacts with SARS-COV-2 and might provide protection indirectly through antibody dependent virus clearance rather than direct neutralization. To conclude the specific role of OPV, a more direct comparison need to be done within the same environmental setting by comparing the number of COVID-19 infection among the population with OPV and without OPV vaccination. The circulating OPV strain among the population through improper hygiene in LMICs is not expected to provide significant dosing for indirect protection against SARS-CoV-2.

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8 Dec 2021

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Additional Editor Comments:

This observational study provides a possible relationship between oral Polio vaccine and low incidence of COVID-19 infection. As there is no solid molecular evidence to support the relationship, more discussion and a list of limitations might be necessary for this revision.

Thank you very much for your comment. Please note that the real molecular data are rarely available for causal relationships in such diseases, including todays SARS-CoV-2 and CoVID-19. Some (like HIV) are an exception. However, if by this you mean a reasonable and known molecular mechanism of a likely explanation, it would certainly be by activating TLR and related receptors reacting to PAMP signals and thereby activating IFN-Type 1 genes, which in turn affect various immediate antiviral pathways. NK cells are also activated. However, we certainly agree we have not shown that here. We have already mentioned limitations of the study. In the revised version, we clearly show the Limitations section. We also added the probable protective effect of other live attenuated vaccines such as BCG and the probable role of cross-reacting antibodies on the results, as other limitations of the study.

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Partly

________________________________________

2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

________________________________________

3. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

________________________________________

4. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

________________________________________

5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: The authors performed a population study utilizing publicly available data on the number of COVID cases per country and their usage of oral poliovirus vaccine vs inactivated poliovirus vaccine, as well as their human development index. The authors' data support their conclusion, but there are many variables that would go into the spread of the virus during the pandemic beyond HDI, GDP, and overall health care.

The authors should add in population density as another variable to assess.

Thank you very much for your comment. We added the population density to our study. It was not significantly different (p=0.201) in countries using OPV vs IPV (Table 1). Nevertheless, we’ve also added it to the model but it did not change anything significantly; it was not a significant predictor in the model. Therefore, we decided not to change the model.

The authors need to fix figure 3 which is cutoff.

We appreciate this suggestion and have complied by making this change to Figure 3.

The authors should include additional discussion on the potential roles that lockdowns and international travel could have played in the observed differences.

Thanks for raising this important point. The effects of lockdowns and international travel bans were hard to account for in our study since policies changed with time. However, we added the mean stringency index, an index reflecting the level of restrictions imposed in a given country, to our study. The mean was not significantly different between countries using OPV vs those using IPV. We have now added this in detail in the methodology section, Table 1, and Discussion.

The authors should specifically state whether the outliers shown in figure 2 were included in the analyses or excluded. If they were excluded, they should provide the statistical criteria for the exclusion, and provide a secondary graph with them included.

Thank you very much for making this suggestion. All the outliers were included in the analyses.

Reviewer #2: The overall observation of this study might be true but the role of OPV is more hypothetical. What the authors failed to conclude with sufficient evidence is whether the observed protection for SARS-CoV-2 in countries with OPV is really due to OPV, BCG (widely used in LMICs) or many other rampant viral and bacterial infection with high frequency. Its well documented that non specific innate immunity provides protection to some degree from several other pathogens. Not only innate immunity, recent experimental evidences suggest that antibodies generated (humorl response) in response to other viral pathogens such as HIV and Flu cross reacts with SARS-COV-2 and might provide protection indirectly through antibody dependent virus clearance rather than direct neutralization. To conclude the specific role of OPV, a more direct comparison need to be done within the same environmental setting by comparing the number of COVID-19 infection among the population with OPV and without OPV vaccination. The circulating OPV strain among the population through improper hygiene in LMICs is not expected to provide significant dosing for indirect protection against SARS-CoV-2.

Thank you very much. The reviewer has raised an important point. We agree this is in fact a limitation of this study, mainly due to the nature of the ecological studies. We addressed this point in the Limitations section of the Discussion.

Submitted filename: Answers to Reviewers Comments.docx

Click here for additional data file.

11 Feb 2022

A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). You should upload this letter as a separate file labeled 'Response to Reviewers'.

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We look forward to receiving your revised manuscript.

Kind regards,

Wen-Wei Sung, M.D., Ph.D.

Academic Editor

PLOS ONE

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #2: (No Response)

Reviewer #3: (No Response)

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #2: Partly

Reviewer #3: Partly

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #2: Yes

Reviewer #3: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #2: Yes

Reviewer #3: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #2: Yes

Reviewer #3: Yes

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #2: (No Response)

Reviewer #3: This is a manuscript examining the correlation between the use of oral polio vaccine (OPV) and a reduced population case rate for COVID-19. As was noted in prior review, this study is correlative and hypothesis generating but does not provide a firm mechanism for the phenomenon other than “stimulation of the host innate immune system”. The analysis is simple and clear, demonstrating the correlation between OPV use and a lower incidence of diagnosed COVID-19 cases. As noted in the discussion, there are many potential explanations for these findings that are not directly associated with the action of OPV, and while not exhaustive, the points are made.

I have several concerns.

1. This isn’t a meta-analysis in the strict sense of being a comparison of point estimates in randomized controlled trials. This paper is an analysis of aggregated data that were scraped from publicly available databases. If this use of “meta-analysis” is acceptable to PLoS One, I defer to the editorial decision. That being said, there is no shame in analyzing data scraped from public sources—this is how weather forecasting is done—and it would be more appropriate to call this what it is: an analysis of publicly available data.

2. Figure 2, while simple, obscures the distribution of the data. It would be helpful to show the underlying datapoints (whether in the main figure or in a supplemental figure) to emphasize the clustering of datapoints.

More importantly, I am concerned about features of the analysis overall. Specifically, I am concerned that the conclusions drawn may be examples of Simpson’s paradox, specifically because A) there is a significant geographic split between the countries in the two groups and B) populations of radically different sizes are being compared as single datapoints. That being said, the analysis as presented is reasonable, but certainly subject to significant caveats.

3. In the second paragraph of the discussion, it is stated, “To avoid rare cases of vaccine-associate paralytic polio (VAPP), most countries with higher HDI prefer to use IPV instead of OPV. Countries using IPV are thus free from poliovirus circulation.” This is true but confused. It is true that the exclusive use of IPV prevents the circulation of vaccine type poliovirus (and rare revertant cases) in those countries, but the decision to make that change was based on the disappearance of wild-type poliovirus. For example, in the US the change was made after wild-type poliovirus circulation was eliminated in the Western hemisphere, thus tipping the risk-benefit ratio away from OPV use (see MMWR v46 RR-3 (1997)). The elimination of wild-type poliovirus circulation is certainly correlated with health care infrastructure, and it is highly likely that the continued use of OPV has more to do with vaccine distribution networks and the ease of OPV use in those settings.

4. The major problem with the conclusions of the paper is the fallacy that correlation equals causation. This study is a correlative, hypothesis generating study. It should not be sold as anything more than that. Generating hypotheses is important, but stating a correlation is not proof of mechanism.

5. The final conclusion, that vaccination with OPV could protect against SARS-CoV-2, is a vast overreach. That conclusion ignores the potential negative effects of unleashing poliovirus shedding in populations where vulnerable persons are at risk. There is an ever-growing population of people on immune modulating therapies, infants at risk from revertant poliovirus, and others who could have a negative outcome. The use of IPV in countries that can provide it was based on sound public health principles—suggesting a drastic change needs to have a similar level of care.

And perhaps more importantly, do the authors really think this is a viable solution? For example, in the US, poor vaccination rates have nothing to do with availability. I highly doubt that people who refuse a COVID vaccine will line up to take OPV, and they will be even more incensed at the idea of unleashing another virus over which they have no control.

Minor comments.

1. Page 6, this sentence is confusing / poorly worded / has an oddly placed comma.

The distribution of all studied continuous variables but the population density and the stringency index, was significantly different in countries using IPV compared with OPV (Table 1).

2. Page 6, “overdispersion” is misspelled.

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

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Reviewer #2: No

Reviewer #3: No

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.]

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14 Feb 2022

Comments to the Author

We would like to take this opportunity to thank the respected reviewers for their fruitful comments. We revised the manuscript according to their comments. Now, we believe that the revised version has substantially been improved and is much better than before.

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #2: (No Response)

Reviewer #3: This is a manuscript examining the correlation between the use of oral polio vaccine (OPV) and a reduced population case rate for COVID-19. As was noted in prior review, this study is correlative and hypothesis generating but does not provide a firm mechanism for the phenomenon other than “stimulation of the host innate immune system”. The analysis is simple and clear, demonstrating the correlation between OPV use and a lower incidence of diagnosed COVID-19 cases. As noted in the discussion, there are many potential explanations for these findings that are not directly associated with the action of OPV, and while not exhaustive, the points are made.

I have several concerns.

1. This isn’t a meta-analysis in the strict sense of being a comparison of point estimates in randomized controlled trials. This paper is an analysis of aggregated data that were scraped from publicly available databases. If this use of “meta-analysis” is acceptable to PLoS One, I defer to the editorial decision. That being said, there is no shame in analyzing data scraped from public sources—this is how weather forecasting is done—and it would be more appropriate to call this what it is: an analysis of publicly available data.

Thank you very much for pointing this out. We do completely agree with you that the term “meta-analysis” in the sense that you mentioned above should not be used in this context. We prefer to use the term “systematic review with meta-analysis” for the purpose the respected referee mentioned. The term meta-analysis was used in this manuscript to describe the set of statistical methods used to aggregate data obtained from various sources. We changed the statements to reflect this point and hope that, if necessary, journal editors could perform further editing according to the journal’s style.

2. Figure 2, while simple, obscures the distribution of the data. It would be helpful to show the underlying datapoints (whether in the main figure or in a supplemental figure) to emphasize the clustering of datapoints.

Thank you very much for raising this important point. Data points were added to the main figure, as suggested.

More importantly, I am concerned about features of the analysis overall. Specifically, I am concerned that the conclusions drawn may be examples of Simpson’s paradox, specifically because A) there is a significant geographic split between the countries in the two groups and B) populations of radically different sizes are being compared as single datapoints. That being said, the analysis as presented is reasonable, but certainly subject to significant caveats.

Thank you very much for the important point you raised. Based on the distributions of the data points in countries using IPV and OPV (see revised Fig 2) and taking into account that we tried to identify and control, as much as possible, the confounding factors and covariates in the final model, it is unlikely that the observed findings resulted from Simpson’s paradox. You are completely correct that the population sizes were significantly different, but we tried to address this disparity by comparing the cumulative incidence of the disease (corrected for the population size) rather than the number of infections. We also tried to adjust the model for the marked differences observed between the countries in the two groups by considering a number of covariates the most important of which was the Human Development Index (HDI). Anyway, Figure 2 presents the results of a univariate analysis and we did not rely on it. We mainly used the results obtained from the regression model. Nevertheless, we have strengthened in the Discussion the point about possible caveats.

3. In the second paragraph of the discussion, it is stated, “To avoid rare cases of vaccine-associate paralytic polio (VAPP), most countries with higher HDI prefer to use IPV instead of OPV. Countries using IPV are thus free from poliovirus circulation.” This is true but confused. It is true that the exclusive use of IPV prevents the circulation of vaccine type poliovirus (and rare revertant cases) in those countries, but the decision to make that change was based on the disappearance of wild-type poliovirus. For example, in the US the change was made after wild-type poliovirus circulation was eliminated in the Western hemisphere, thus tipping the risk-benefit ratio away from OPV use (see MMWR v46 RR-3 (1997)). The elimination of wild-type poliovirus circulation is certainly correlated with health care infrastructure, and it is highly likely that the continued use of OPV has more to do with vaccine distribution networks and the ease of OPV use in those settings.

We appreciate this comment and agree that the sentence is confusing. While the main reason for the switch from OPV to IPV was triggered by the need to stop VAPP, it was predicated on the absence of wild poliovirus circulation. The situation in less affluent countries is slightly different. While in most countries where OPV is still being used wild-type poliovirus circulation was stopped many years ago, and VAPP and cVDPV present a problem, the continued use of OPV in these countries is related to economic and logistical considerations. We have edited this sentence to make it less confusing.

4. The major problem with the conclusions of the paper is the fallacy that correlation equals causation. This study is a correlative, hypothesis generating study. It should not be sold as anything more than that. Generating hypotheses is important, but stating a correlation is not proof of mechanism.

Thank you very much for raising this very important point. We do agree with you that an ecological study cannot provide any cause-and-effect relationship. It can just provide association. We have revised different parts of the manuscript, played down any causal inference mentioned and exclusively stated that that’s all a hypothesis that might be correct and although a recent cohort study has supported this hypothesis, it should be tested in a clinical trial.

5. The final conclusion, that vaccination with OPV could protect against SARS-CoV-2, is a vast overreach. That conclusion ignores the potential negative effects of unleashing poliovirus shedding in populations where vulnerable persons are at risk. There is an ever-growing population of people on immune modulating therapies, infants at risk from revertant poliovirus, and others who could have a negative outcome. The use of IPV in countries that can provide it was based on sound public health principles—suggesting a drastic change needs to have a similar level of care.

Thank you very much for your comment, this is indeed an important point. The risk of OPV-associated complications must be considered before the decisions about the mass use of OPV is made. In this paper we do not propose this but rather call for prospective clinical trials to prove the beneficial effect of OPV use against COVID-19 and potentially other emerging infections. If it is confirmed, other possibilities are available, including the use if safer novel OPV that was recently developed. We have modified this Discussion to reflect this point.

And perhaps more importantly, do the authors really think this is a viable solution? For example, in the US, poor vaccination rates have nothing to do with availability. I highly doubt that people who refuse a COVID vaccine will line up to take OPV, and they will be even more incensed at the idea of unleashing another virus over which they have no control.

We would like to thank the reviewer for this comment, and regret that we came across in a simplistic way as if we propose immediate use of OPV as a vaccine against COVID-19. The main point that we want to make is that broadly-specific protective effects of live vaccines are totally underappreciated and deserve to be closely studied in well-controlled prospective clinical trials. While the wisdom of reintroduction of OPV in the US and most European countries may be questioned, there are countries where such trials will not pose a great risk, if conducted properly. The information generated in such studies could be extremely valuable and be used for creation of a new kind of broadly specific vaccines, alongside the traditional pathogen-specific ones. We have added the discussion of this point in the Discussion, to better reflect our position.

Minor comments.

1. Page 6, this sentence is confusing / poorly worded / has an oddly placed comma.

The distribution of all studied continuous variables but the population density and the stringency index, was significantly different in countries using IPV compared with OPV (Table 1).

Thank you very much for pointing out this ambiguous statement. It was reworded as:

The distributions of the median age, life expectancy, and GDP per capita were significantly different between countries using IPV and OPV (Table 1). The median HDI in countries using IPV was significantly (0.90 vs 0.70, p<0.001) higher than that in countries using OPV. The population density and the mean stringency index were not significantly different between countries using OPV and those using IPV.

2. Page 6, “overdispersion” is misspelled.

Thank you very much for pointing out that mistake. It was corrected.

________________________________________

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #2: No

Reviewer #3: No

Submitted filename: Answers to Reviewers Comments 2.docx

Click here for additional data file.

4 Mar 2022

Use of oral polio vaccine and the incidence of COVID-19 in the world

PONE-D-21-33498R2

Dear Dr. Robert C. Gallo,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

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Kind regards,

Wen-Wei Sung, M.D., Ph.D.

Academic Editor

PLOS ONE

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #3: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #3: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #3: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #3: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #3: Yes

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #3: I applaud the authors for addressing all of my concerns, including my strident ones. I appreciate their work.

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #3: Yes: M. Anthony Moody

9 Mar 2022

PONE-D-21-33498R2

Use of oral polio vaccine and the incidence of COVID-19 in the world

Dear Dr. Gallo:

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