Association Between Toothbrushing Habits and COVID-19 Symptoms.

OBJECTIVES: The association between toothbrushing and coronavirus disease 2019 (COVID-19) infections is unknown. The aim of this study was to test the hypothesis that the change in time and frequency of toothbrushing is associated with having COVID-19 symptoms.
METHODS: In this 8-month retrospective cohort study, we used the data from the Japan COVID-19 and Society Internet Survey (JACSIS; N = 22,366), which was conducted between August and September 2020. The logistic regression analyses were used to calculate the odds ratios (ORs) of having the 3 main COVID-19 symptoms (high fever, cough, and taste and smell disorder). Confounders were age, sex, educational attainment, equivalised income level, self-rated health, health literacy, and living area.
RESULTS: The mean age of the participants was 49 years (SD = ±17.3), and 49.2% were male. Overall 2704 (12.1%) participants changed (increased or decreased) the time and frequency of toothbrushing, whilst 19,662 (87.9%) did not change. Only 60 participants (0.3%) had the 3 main COVID-19 symptoms. All logistic regression models showed that those who had a change in time and frequency of toothbrushing had higher odds of having the 3 main COVID-19 symptoms compared to those who had unchanged time and frequency of toothbrushing. The ORs ranged from 6.00 (95% confidence interval [CI], 3.60-9.99) in the crude model to 4.08 (95% CI, 2.38-6.98) in the fully adjusted model.
CONCLUSIONS: The change in time and frequency of toothbrushing from before to after the COVID-19 pandemic was associated with having the 3 main COVID-19 symptoms.

Introduction

Since the declaration of the coronavirus disease 2019 (COVID-19) caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) as a pandemic in March 2020 by the world health organization (WHO), the standard infection preventive measures such as wearing mask and social distancing were advised by the health organizations worldwide . However, some oral health experts suggested that proper oral hygiene measures such as regular tooth brushing might play a role in the prevention of COVID-19 infections , .

Previous research showed that the oral cavity is an important site for SARS-CoV-2 viral replication, and the saliva could be a potential route of SARS-CoV-2 transmission. However, studies related to the association between oral hygiene and COVID-19 infections is scarce. A very small sample-sized study (8 participants) indicated that viral shedding of SARS-CoV-2 was prolonged by around 15 days in patients with mental disorders who did not brush their teeth. In addition, the previous research examining the association between toothbrushing with similar respiratory diseases such as pneumonia, showed that tooth brushing was associated with lower ໿incidence, duration, and mortality of pneumonia in community dwelling individuals and hospitalized patients , . A systematic review with meta-analysis concluded that the risk of ventilator-associated pneumonia (VAP) was 24% lower in patients receiving chlorhexidine mouthwash combined with toothbrushing than in those receiving chlorhexidine mouthwash only. Also, some growing evidence from the industry of oral care products suggested that certain toothpastes containing zinc, stannous fluoride, or amine fluoride may play a role in temporarily reducing the viral load of the SARS-CoV-2 intraorally. These laboratory studies suggested that these chemicals can render the SARS-CoV-2 virus non-infectious, and prevent its multiplication in the host.

This hypothesized microbial pathway for the association between tooth brushing habits and COVID-19 infections is not yet supported by enough evidence. Also, no epidemiologic study investigated this hypothesis. Hence, the aim of this epidemiological study is to investigate the important public health question of whether the change in the time and the frequency of tooth brushing is associated with having the COVID-19 symptoms in a randomly sampled Japanese population.

Methods

Study design, setting and participants

In this 8-month retrospective cohort study, we used the data gathered between August 25th 2020 to September 30th 2020 from the Japan COVID-19 and Society Internet Survey (JACSIS study) which targeted participants with a wide age range (from 15 to 79 years old). Figure 1 illustrates the timeline of data gathering in relation to the development of the COVID-19 pandemic. The JACSIS is a self-reported internet survey conducted to assess the effects and the changes in the socioeconomic status, the lifestyle and the health behaviors of the participants due to the COVID-19 pandemic. This internet survey was administered by a big internet research agency called Rakuten insight, Inc., a Japanese online market research firm with approximately 2.3 million qualified Japanese respondents registered in their database as of march 2016.. These individuals are users of the Rakuten Group services such as telecommunication services and online shopping. To compensate for the shortcomings of the online surveys, simple random sampling using a computer algorithm developed to select the target participants from the Rakuten insight databases was conducted. This sampling was adjusted to match the population distribution in Japan by age, sex, and living area and covered all the 47 prefectures of Japan using the respondents postal codes originally collected by Rakuten Inc. The data from the National Survey on Living Standards, which is nationally representative of Japan, was used as a reference for the adjustment, weighting and other processing schemes during the sampling , . The survey, with informed consent at its top, was distributed to 224,389 individuals. When the predetermined response rate of 12.5% (28,000/224,389) was reached, the online survey was stopped. The survey design ensured that all survey questions must be answered. However, the participants could choose not to respond or to discontinue at any point during answering the survey. Those who discontinued answering the survey were counted as non-respondents. There were no missing values in the raw dataset due to the described survey design. However, there was a possibility of inconsistent responses. Thus, we excluded 2,386 participants because they showed inconsistent responses and were assumed to have answered the questionnaire haphazardly without reading it (1,955 participants failed to correctly answer a dummy question asking them to choose the 2nd response from the bottom of a list of five available answers, 331 participants selected all items in a list of seven substance in a question about drug abuse and 100 participants selected all sixteen diseases available in the list about existing comorbidities). The raw data of 25,614 respondents (91.4%) was used in this study. Of the 25,614 eligible participants, 3,248 were excluded because they had only one or two of the three main symptoms of COVID-19 as described in the following paragraph. The final sample size in this study was 22,366 respondents, 79.8% of the total survey respondents.

Figure 1

The timeline of the data collection showing the 8-month retrospective design of this study.

Outcomes

According to the publicly available information from the National health services (NHS) in the United Kingdom (UK), the three main symptoms of COVID-19 are high fever, cough and taste and smell disorder. In the questionnaire, the participants were asked “Have you had any of the following symptoms in the past month? 1. Fever, 2. Cough, 3. taste and smell disorder. The answers were binary “yes/no”. Having the three main COVID-19 symptoms altogether was used as a proxy for being infected with COVID-19.

Exposures

The change in the time and the frequency of tooth brushing before and after the COVID-19 pandemic was used as an independent variable. It was a self-reported retrospective question as follows; “Does the time and frequency of tooth brushing changed in the last month compared with the period before January 2020?”, with three available choices, “increased, same as before and decreased”. At first, the trichotomized variable was used to investigate the increase and the decrease in the time and the frequency of tooth brushing. Then, we dichotomized the responses into a binary variable (changed and unchanged) to examine the total effect of the change in the time and frequency of tooth brushing.

Confounders

In this study, we adjusted for demographics, socioeconomic status (SES), general health status, health literacy and living area. The age and sex of the participants were used as demographic confounders. The education attainment and the equivalised income level were used as socioeconomic determinants of health. The educational attainment was combined into four categories (High school or lower, vocational school or college, university degree, and graduate school and others). The equivalised income level was calculated as the annual pre-tax household income divided by the squared root of the number of people in the household, and was categorized into (<25000 $/year, 25000-45000 $/year, >45000 $/year and do not want to answer or do not know). The self-rated health was used as an indicator of the general health status. It was categorized into (good, fairly good, normal, not very good and not good). For health literacy, we used the question “In the past month, did you refrain from going out unnecessarily or on a business trip?”. The available answers were (always, sometimes, almost never and not at all). To account for the effects of regional variations in the rate of COVID-19 infections, the living area was adjusted in the fully adjusted model after being categorized into the following seven living areas of Japan arranged from north to south (Hokkaido-Tohoku, Kanto, Hokuriku-Ko-shin-etsu, Tokai, Kansai, Chugoku-Shikoku and Kyushu-Okinawa).

Statistical Analysis

A descriptive analysis was performed to examine the characteristics of participants. Then, we used the logistic regression analyses to calculate the odds ratios (ORs) for having the three main COVID-19 symptoms altogether. For sensitivity analyses, we examined the association between the change in the time and frequency of tooth brushing with each one of the three main COVID-19 symptoms as a separate outcome. As supplementary analyses, we examined the reverse association based on the hypothesis that COVID-19 infection led to changes in the time and frequency of tooth brushing after adjusting for the relevant confounders (the outcome, the change in the time and the frequency of tooth brushing, was trichotomized; 0=decreased, 1=unchanged and 2=increased). We used the directed acyclic graphs (DAGitty) version 3.0 to structure the hypothesized framework of this study (Figure 2 ). Stata 14 software from StataCorp LP (College Station, Texas, USA) was used for the analyses, and the STROBE guidelines for cohort studies were followed.

Figure 2

The hypothesized framework using the directed acyclic graph (DAG) for the association between the change in the time and the frequency of tooth brushing with the three main COVID-19 symptoms.

Ethical approval

This study protocol was approved by the ethics committee at the Osaka International Cancer Institute, Japan (approval number: 20084).

Results

Table 1 shows the characteristics of participants. From the 22,366 participants included in the analyses, the age range of the participants was from 15 to 79 years (Mean=49.0 years, SD=±17.3 years). In total, 11,014 participants were males (49.2%) and 11,352 were females (50.7%). Also, 2,076 participants (9.3%) had increased time and frequency of tooth brushing, while, 628 participants (2.8%) had decreased time and frequency of tooth brushing and 19,662 participants (87.9%) had unchanged time and frequency of tooth brushing. Only 60 participants (0.3%) had the three main COVID-19 symptoms. The proportions of those who had the three main COVID-19 symptoms was the highest among those with decreased time and frequency of tooth brushing, the youngest age group (15-29 years old), males, the group with the highest educational attainment (graduate school and other), the lowest income group (<25000 $/year), the group with the lowest self-rated health, those who almost never refrained from going outside unnecessarily in the past month and those living in Kansai area.

Table 1

The descriptive statistics of the participants, and its stratification by having the three main COVID-19 symptoms (n=22,366)

	Total		Having the three main COVID-19 symptoms
			No		Yes
	No.	%	No.	%	No.	%
The change in Time and Frequency of Tooth brushing
Increased	2076	9.3	2058	99.1	18	0.9
Unchanged	19662	87.9	19629	99.8	33	0.2
Decreased	628	2.8	619	98.6	9	1.4
Age
15-29 years	3817	17.1	3789	99.3	28	0.7
30-49 years	7499	33.5	7477	99.7	22	0.3
50-59 years	3758	16.8	3755	99.9	3	0.1
60-79 years	7292	32.6	7285	99.9	7	0.1
Sex
Male	11014	49.2	10974	99.6	40	0.4
Female	11352	50.8	11332	99.8	20	0.2
Education attainment
High school or lower	6656	29.8	6642	99.8	14	0.2
Vocational school or College	4957	22.2	4947	99.8	10	0.2
University degree	9614	43	9584	99.7	30	0.3
Graduate school and others	1139	5.1	1133	99.5	6	0.5
Equivalised income level
<25000 $/year	4628	20.7	4606	99.5	22	0.5
25000<45000 $/year	7301	32.6	7284	99.8	17	0.2
>45000 $/year	5738	25.7	5722	99.7	16	0.3
Do not want to answer or Do not know	4699	21	4694	99.9	5	0.1
Self-rated health
Good	4618	20.6	4604	99.7	14	0.3
Fairly good	7188	32.1	7180	99.9	8	0.1
Normal	7858	35.1	7846	99.8	12	0.2
Not very good	2209	9.9	2191	99.2	18	0.8
Not good	493	2.2	485	98.4	8	1.6
Health literacy (refrain from going out unnecessarily)
Always	13612	60.9	13585	99.8	27	0.2
Sometimes	6453	28.9	6429	99.6	24	0.4
Almost never	1312	5.9	1306	99.5	6	0.5
Not at all	989	4.4	986	99.7	3	0.3
Living areas
Hokkaido and Tohoku	2424	10.8	2421	99.9	3	0.1
Kanto	7930	35.5	7912	99.8	18	0.2
Hokuriku Ko-shin-etsu	2017	9.0	2012	99.8	5	0.2
Tokai	2025	9.1	2018	99.7	7	0.3
Kansai	3702	16.6	3682	99.5	20	0.5
Chugoku and Shikoku	1866	8.3	1865	99.9	1	0.1
Kyushu and Okinawa	2402	10.7	2396	99.8	6	0.2
Total	22366	100	22306	99.7	60	0.3

Table 2 shows the findings of the logistic regression analyses for the trichotomized outcome.

Table 2

The findings of the logistic regression analyses for the association between the trichotomized change in the time and the frequency of tooth brushing with the three main COVID-19 symptoms (n=22,366)

	Crude model (1)			Model (2) adjusted for Age & Sex			Model (3) adjusted for Age, Sex & SES			Fully adjusted model (4)
	Odds ratio	95%CI		Odds ratio	95%CI		Odds ratio	95%CI		Odds ratio	95%CI
Time and Frequency of Tooth brushing
Unchanged	Reference			Reference			Reference			Reference
Increased	5.20	2.92	9.26	4.56	2.54	8.21	4.52	2.51	8.15	4.19	2.31	7.61
Decreased	8.65	4.12	18.15	5.48	2.57	11.73	5.13	2.39	11.04	3.83	1.71	8.59
Age
15-29 years				Reference			Reference			Reference
30-49 years				0.50	0.28	0.89	0.50	0.28	0.90	0.48	0.27	0.88
50-59 years				0.14	0.04	0.48	0.15	0.05	0.52	0.14	0.04	0.49
60-79 years				0.18	0.08	0.42	0.18	0.08	0.42	0.20	0.09	0.48
Sex
Male				Reference			Reference			Reference
Female				0.44	0.26	0.76	0.47	0.27	0.82	0.49	0.28	0.87
Education attainment
High school or lower							Reference			Reference
Vocational school or College							1.08†	0.47	2.47	1.12†	0.49	2.58
University degree							1.13†	0.59	2.16	1.23†	0.64	2.38
Graduate school and others							1.88†	0.70	5.05	2.09†	0.77	5.66
Equivalised income level
<25000 $/year							Reference			Reference
25000 - 45000 $/year							0.53†	0.28	1.02	0.59†	0.31	1.14
>45000 $/year							0.54†	0.28	1.06	0.61†	0.31	1.20
Do not want to answer or Do not know							0.25	0.09	0.65	0.26	0.10	0.70
Self-rated health
Good										Reference
Fairly good										0.39	0.16	0.93
Normal										0.64†	0.29	1.40
Not very good										3.22	1.56	6.64
Not good										3.87	1.51	9.93
Health literacy (refrain from going out unnecessarily)
Always										Reference
Sometimes										1.59†	0.90	2.80
Almost never										1.40†	0.56	3.50
Not at all										0.69†	0.20	2.46
Living area
Hokkaido and Tohoku										Reference
Kanto										1.60†	0.46	5.49
Hokuriku Ko-shin-etsu										1.87†	0.44	7.95
Tokai										2.67†	0.68	10.45
Kansai										4.22†	1.24	14.40
Chugoku and Shikoku										0.41†	0.04	3.96
Kyushu and Okinawa										1.81†	0.45	7.30

Abbreviations; SES=socio-economic status.

Model 4 is adjusted for all confounders; age, sex, educational attainment, equivalized income level, self-rated health, health literacy and living area.

All p-values were <0.05, except those with the sign † p-values>0.05

All models showed that those who had increased or decreased time and frequency of tooth brushing had high odds of having the three main COVID-19 symptoms compared to those who had unchanged time and frequency of tooth brushing. However, the values of the odds ratio (OR) for those who had decreased time and frequency of tooth brushing were consistently higher than those who had increased time and frequency of tooth brushing in all models except the fully adjusted model.

Table 3 shows the findings of the logistic regression analyses for the dichotomized outcome (unchanged vs changed time and frequency of tooth brushing). All models showed that those who had a change in time and frequency of tooth brushing had higher odds ratios of having the three main COVID-19 symptoms compared to those who had unchanged tooth brushing habits. The ORs ranged from 6.00 (95% CI: 3.60–9.99 p-value <0.001) in the crude model to 4.08 (95% CI: 2.38– 6.98 p-value <0.001) in the fully adjusted model.

Table 3

The findings of the logistic regression analyses for the association between the dichotomized change in the time and the frequency of tooth brushing with the three main COVID-19 symptoms (n=22,366)

	Crude model (1)			Model (2) adjusted for Age & Sex			Model (3) adjusted for Age, Sex & SES		Fully adjusted model (4)
	Odds ratio	95%CI		Odds ratio	95%CI		Odds ratio	95%CI		Odds ratio	95%CI
Time and Frequency of Tooth brushing
Unchanged	Reference			Reference			Reference			Reference
Changed (increased & decreased)	6.00	3.60	9.99	4.83	2.86	8.16	4.70	2.78	7.96	4.08	2.38	6.98
Age
15-29 years				Reference			Reference			Reference
30-49 years				0.50	0.28	0.88	0.50	0.28	0.89	0.49	0.27	0.88
50-59 years				0.14	0.04	0.47	0.15	0.05	0.51	0.15	0.04	0.49
60-79 years				0.18	0.08	0.41	0.18	0.08	0.41	0.20	0.09	0.49
Sex
Male				Reference			Reference			Reference
Female				0.44	0.26	0.75	0.47	0.27	0.82	0.49	0.28	0.87
Education attainment
High school or lower							Reference			Reference
Vocational school or College							1.08†	0.47	2.47	1.12†	0.49	2.58
University degree							1.13†	0.59	2.16	1.23†	0.64	2.38
Graduate school and others							1.87†	0.70	5.02	2.10†	0.77	5.68
Equivalised income level
<25000 $/year							Reference			Reference
25000 - 45000 $/year							0.53†	0.28	1.00	0.59†	0.31	1.15
>45000 $/year							0.53†	0.27	1.05	0.61†	0.31	1.21
Do not want to answer or Do not know							0.24	0.09	0.65	0.26	0.10	0.70
Self-rated health
Good										Reference
Fairly good										0.39	0.16	0.93
Normal										0.64†	0.29	1.39
Not very good										3.20	1.56	6.58
Not good										3.82	1.50	9.73
Health literacy (refrain from going out unnecessarily)
Always										Reference
Sometimes										1.58†	0.90	2.78
Almost never										1.38†	0.55	3.45
Not at all										0.68†	0.19	2.38
Living area
Hokkaido and Tohoku										Reference
Kanto										1.59†	0.46	5.48
Hokuriku Ko-shin-etsu										1.87†	0.44	7.95
Tokai										2.67†	0.68	10.47
Kansai										4.24	1.24	14.44
Chugoku and Shikoku										0.41†	0.04	3.94
Kyushu and Okinawa										1.81†	0.45	7.30

Abbreviations; SES=socio-economic status.

Model 4 is adjusted for all confounders; age, sex, educational attainment, equivalized income level, self-rated health, health literacy and living area.

All p-values were <0.05, except those with the sign † p-values>0.05

The sensitivity analyses (Supplementary tables 2-4) showed similar patterns compared to the main analyses for having high fever and taste and smell disorder as separate outcomes in all models. Having cough as an outcome was associated with the change in tooth brushing habits in all models except for those with decreased time and frequency of tooth brushing in the fully adjusted model.

Supplementary table 1. shows the findings of the reverse association. Those who had the COVID-19 symptoms had consistently higher odds of increasing the time and frequency of their tooth brushing in all the four ordered logistic regression models when compared to those who did not have the COVID-19 symptoms (the OR ranged from 2.25 (95%CI: 1.14 - 4.46) in the crude model to 2.52 (95%CI: 1.27 – 5.03) in the fully adjusted model).

Discussion

To the best of our knowledge, this is the first epidemiological study to examine the association between the change in the time and the frequency of tooth brushing and having the main COVID-19 symptoms. Compared to before the COVID-19 pandemic, both the increase and the decrease in the time and frequency of tooth brushing after the pandemic were associated with having the three main COVID-19.

Our findings are consistent with the scarce evidence from previous studies which reported that decreased tooth brushing was associated with prolonged viral shedding of SARS-CoV-26. The disruption of the equilibrium of the oral microbiome could be a possible way to explain the association between the decrease in the time and the frequency of tooth brushing and having the three main COVID-19 symptoms. While the reverse causation could explain the association between the increase in the time and the frequency of tooth brushing and having the three main COVID-19 symptoms (supplementary table 1). To explain these pathways in details, it is possible that those who decreased the time and the frequency of tooth brushing could not have benefited from the mechanical cleansing effect of tooth brushing and the protective antiviral effect of the chemicals in the tooth paste, and had higher SARS-CoV-2 viral load intraorally and eventually had higher chances of having the three COVID-19 symptoms , , , , . (Figure 2) illustrates this hypothesized microbial pathway. For the reverse causation, it is possible that those who had the three COVID-19 symptoms became more health cautious out of fear and panic after being infected with a new disease as COVID-19 and started to brush their teeth more as a protective mechanism. Supplementary figure 1 illustrates this pathway.

Our findings should be interpreted with caution considering the limitations of our study. First, the inherent self-reported nature of our outcome as a proxy for COVID-19 infections was not supported by a clinical or a laboratory diagnosis. However, we followed the most conservative approach during the selection of the three main COVID-19 symptoms as listed by the NHS as the outcome, which increases the like-hood that those who had the three selected symptoms at the same time were infected with COVID-19. Second, we could not account for the asymptomatic COVID-19 cases, the cases with only two symptoms, or the cases with symptoms other than the three main symptoms used in this study. However, the sensitivity analyses using high fever, cough and taste and smell disorder as separate outcomes showed nearly similar patterns for the association with the change in time and frequency of tooth brushing. In addition, we used the self-reported measurements, which cause non-differential misclassification. In this context, there is a possibility that the prevalence of the COVID-19 symptoms in this study did not reflect the actual situation. In fact, this study's prevalence for having the three main COVID-19 symptoms (0.3%) was excessively lower than the national average cumulative confirmed cases at the time of JACSIS data gathering (1.6%) which was calculated from the national statistics , . However, this misclassification was expected to occur equally in all tooth brushing categories, which in turn lead to the underestimation of the association between the exposure and the outcome. Third, although our study sample is a relatively large and the respondents were recruited to represent the Japanese population in terms of age, sex, and residential area, the generalizability of these findings is limited. This is because the respondents to a web-based survey might not fully represent the Japanese population. Fourth, an exact measurable change in the time and the frequency of tooth brushing could not be measured in this study due to data limitation. However, the proportions of those who brushed their teeth twice a day in a population-based study in japan was 79.1% in 2017 . This prevalence is consistent with the proportions of those who reported unchanged time and frequency of tooth brushing in this study (87.9%). This implies that those who reported an increased or decreased time and frequency of tooth brushing either brush their teeth >2 times a day or <2 times a day respectively.

On the other hand, and as strength points, laboratory studies and randomized controlled trials (RCTs) take time and might be not-feasible to investigate the research question of this study. This is where the importance of this epidemiologic study comes; providing an evidence regarding this association. Also, we used a relatively large dataset which improves the statistical power and the precision of the estimates. In addition, we adjusted for a wide range of possible confounders which improves the robustness of the findings.

Public health implications

This epidemiological study raises the argument that there might be a microbial pathway for the association between tooth brushing and COVID-19 infections. Also, it suggests that maintaining tooth brushing habits at an appropriate time and frequency, for example twice a day based on the consensus recommendation of the American dental association , might have a possible protective mechanism against COVID-19 infections. In relation to this, tooth brushing patterns are generally known to be a lifestyle habit that does not change much, however, 12.1% of the study participants changed their tooth brushing habits, and this is not considered to be a small percentage in the public health field. Besides, the findings of this study need to be confirmed by further research in the future including randomized controlled trials and cohort studies using clinical and/or laboratory data. In addition, similar cross-national comparative epidemiological studies would provide better insights about the association between oral hygiene measures and COVID-19 infections in different countries.

Conclusion

The change (decrease and increase) in time and frequency of tooth brushing from before to after the COVID-19 pandemic was associated with having the three main COVID-19 symptoms. To briefly explain these findings, it is possible that those who decreased their tooth brushing habits might not have benefited from the cleansing and antiviral effects of regular tooth brushing, and thus had higher SARS-CoV-2 viral load intraorally, and eventually had higher chances of having the three main COVID-19 symptoms. While those who had the three main COVID-19 symptoms might have increased their tooth brushing habits out of fear and panic after being infected with COVID-19.

Declaration of Competing Interest

We declare no conflict of interest.