Analysis of social combinations of COVID-19 vaccination: Evidence from a conjoint analysis.

Using a conjoint analysis based on Japanese cases, this study attempts to identify a preferable social strategic combination of who are vaccinated, who are not, and who are waiting. Using two surveys that relied on quota sampling reflecting the Japanese demographic composition (n = 1024 & n = 2975), the results of the descriptive analysis show that the most preferred strategy at the individual level was wait-and-see, allowing for a risk assessment of side effects. Via conjoint analysis, I also found that participants who recalled blood relatives as their familiar entities tended to prefer a wait-and-see strategy for themselves and their blood relatives. The results of these analyses suggest that wait-and-see strategies for vaccination are preferred in Japan, making it difficult to achieve early herd immunity through vaccination.

Introduction

With the spread of the coronavirus disease (COVID-19), its consequent deaths, and the after-effects in several countries, the best hope for mass immunization is vaccine development and administration. However, in society, individual intent to receive vaccination remains low. Representative studies show that the percentage of individuals willing to be vaccinated is 53.6% (undecided: 14.4%, unwilling: 32%) in the United States [1] and 73.9% (undecided: 18.9%, unwilling: 7.2%) in European countries [2], which are the main suppliers or initiators of vaccinations. As reported by Yoda and Katsumata [3], among the 1,100 online survey participants in Japan, 65.7% were willing to be vaccinated, 22.0% were not sure, and 12.3% were unwilling. Further, differences in the vaccination intentions between men and women and among people with different education levels have been investigated: vaccination intention increases 1.13 times in men with a higher level of education, whereas it decreases about 0.83 times in women with the same level of education [4].

Several studies have been conducted based on surveys related to intention to be vaccinated. Studies examining vaccination intent in more detail aim to determine the vaccine properties that individuals prefer, immunization protocols in place, and willingness of individuals to be vaccinated according to these situations [5-7]. These studies adopted discrete choice experiments [6] or conjoint analyses [7] to identify the vaccine property preferences of individuals and to detect the requirements for increased vaccine uptake. In addition to research on these vaccines as “one product,” using these methods, it may be necessary to consider the influence of vaccination status in the society on the intention of individuals to be vaccinated. One problem that previous studies do not address is the need to examine the status of strategic interactions, namely, preferences of who should be inoculated and who should not, with exception of the research on healthcare [8].

While most individuals value scientific evidence and find it desirable that they are vaccinated altruistically, as well as the entire community, the most desirable combination of vaccinations among themselves, familiar individuals, and the society overall can occur in various ways [9]. In addition, some individuals may think strategically, wishing to “receive the vaccine after confirming the mid-/long-term adverse reactions,” rather than a binary choice of simply wishing to receive it or not. Considering this nuanced alternative can also capture certain egoistic (individually rational) attitudes, such as “society as a whole should be vaccinated early, and I should be vaccinated after confirming the side effects and the effect on the variant.” A strategic conflict between altruism and egoism is likely to arise in a society that provides vaccines, requiring a framework to explain the consequences of vaccination.

Similar to previous studies, this study uses a choice-based conjoint analysis for vaccination, to identify the types of strategic vaccine combinations most desirable in the Japanese society. The expectation is that the Japanese case could provide meaningful insights. Japan is a latecomer to the vaccine market, only starting to vaccinate healthcare professionals on February 18, 2021. Thus, vaccination has begun with less uncertainty regarding side effects than in other countries. As in the aforementioned [3, 4], the country has a high proportion of individuals adopting a “wait-and-see policy” as the US and European countries started vaccinations earlier. Japan’s case has important implications to ascertain the social impact of the vaccination program, as many Japanese individuals are aware regarding the vaccine’s side effects, with a low level of uncertainty about the vaccine.

This study can shed light on how people’s egoistic or altruistic intentions may be behind the lack of sufficiently high vaccination intention in areas where COVID-19 is not widespread. Nevertheless, the implication is that, to achieve higher vaccination coverage, which is more medically desirable, adequately spreading information about the safety of vaccines to each generation is crucial.

Empirical strategy

Study participants and period

This study aims to identify vaccine combinations by introducing conjoint analysis into an online survey. Two online survey experiments were conducted, guiding survey panels from Yahoo Crowd Sourcing, Inc. (YCS) (March 14–16, 2021) and Lucid Holdings, LLC. (March 26–28, 2021). Lucid included 1,024 participants and 27 questions, while YCS included 2,975 participants and 43 questions. The selection of participants was based on quotas allocated according to demographic composition. As the further information, Supplemental Materials contain descriptive statistics on respondent variables and demographic composition compared to the census.

In addition, considering the inoculation schedule, I regard this survey period appropriate for verifying Japan’s vaccination status. The vaccination of healthcare workers began on February 18, 2021, and I conducted my studies after about one month. On March 2, it was reported that a woman in her 60s died of a subarachnoid hemorrhage after vaccination. On March 12 (2 days before the YCS survey and 10 days before the Lucid survey), at a meeting of the vaccine study group held by the Ministry of Health, Labour and Welfare, 17 cases of adverse reactions caused by anaphylactic shock were reported. Considering these series of events, I conducted my survey at the approximate time when Japanese citizens had already decided upon vaccination, that is, when more information about vaccination per se and its side effects was available.

Ethics

This study was undertaken and approved by the Kwansei Gakuin University Committee for Regulations for Behavioral Research with Human Participants (27th February, 2021). In accordance with the Committee’s recommendations, the participants were informed at the beginning of the survey that they may refuse to be presented with sensitive information about COVID-19 and may leave the investigation at any time. They were also informed at the debriefing that, if they felt uncomfortable with the information they received, they could opt not to send in their responses. The compensation for the survey was set to 20 Yahoo points for YCS and 3.8 USD for Lucid.

For details of the consent by the participants and their debriefing, all the survey questionnaires are available in the Supplementary Materials. The survey questionnaire was presented in Japanese to the Japanese participants, but an English translation is available in the Supplementary Materials.

Design of the conjoint analysis

The contrivance in this conjoint analysis is as follows. First, to prevent an attribute of the conjoint analysis from becoming quite complicated, the pattern of attributes was constituted into three main bodies of “myself,” “familial presence,” and “society as a whole.” The participants were asked to select one of the following for “familial presence,” as a reference group: a family member who is older than the participant (e.g., parents), a family member who is younger than the participant (e.g., children), friend, colleague, neighbor, spouse, and significant other/partner. For the criterion for distinguishing familiar individuals, I refer to the reference group setup in a study of relative income with the hypothetical choice experiment by [10, 11]. The participants were instructed to face the conjoint, while recalling a selected alternative as a familiar presence.

Second, the level for each attribute should not be a binary choice between vaccination and non-vaccination, and a third option should be incorporated, namely, “Do not vaccinate now, vaccinate later.” This alternative allows us to capture certain egoistic attitudes such as a watcher, “society as a whole should be vaccinated early, and we should be vaccinated after confirming the occurrence of side effects.” If the effect of a watcher is greater than that of the simple desire for “vaccinate,” the acquisition of herd immunity through the vaccine is not necessarily an optimistic scenario.

According to the above conjoint analysis settings, I set my design of conjoint analysis as in Table 1 and Fig 1. Fig 2 shows an example of the conjoint screen.

Table 1

Proportion of willingness to be vaccinated.

Attributes	Values
Myself	Vaccinate	Do not vaccinate now, vaccinate later	Not vaccinate
Familiar presence	Vaccinate	Do not vaccinate now, vaccinate later	Not vaccinate
Society as a whole	Vaccinate	Do not vaccinate now, vaccinate later	Not vaccinate

Fig 1

Conjoint experiment flow.

Fig 2

Display example of conjoint analysis.

Note: Five tasks were randomly displayed to participants.

I performed conjoint analysis using the Conjoint Survey Design Tool and by introducing the conjoint program into Qualtrics, according to the procedure by [12]. By using the method in [12], the effect of the concerned attribute X can be measured under all other attributes as the average marginal component effect (AMCE), even if the effect of the attribute of interest is heterogeneous, with regard to the distribution of other attributes. Considering the examples in this study, [12] is able to measure the effect of the “myself” intention to vaccinate, based on the overall effects across other attributes: society in general and familiar entities.

Results

Table 2 provides the simple descriptive statistics on the intention to be vaccinated. According to the YCS results, the most common response was “Do not vaccinate now, vaccinate later,” followed by “vaccinate” and finally “not vaccinate.” Conversely, Lucid’s result implies that people prefer “vaccinate” compared to the wait-and-see strategy. However, many people in Japan still use the wait-and-see strategy. I may have obtained such results because I conducted the survey immediately after the reporting of specific information on adverse reactions, and it is likely that more citizens in Japan prefer the wait-and-see strategy compared to those in other countries. A major difference from the results of [3] is expected in the reporting of adverse reactions and the stabilization of infections during the investigation period.

Table 2

Proportion of willingness/wait-and-see/unwillingness to be vaccinated.

	Vaccination	Wait-and-see	No vaccination
YCS	0.418	0.449	0.133
Lucid	0.462	0.401	0.137

Note: Each number refers to a percentage value (%).

Tables 3 and 4 show the results of the cross-tabulations of vaccination intentions by individual attributes. When examined by age, it is clear that the wait-and-see strategy is the most common strategy for those in their 40s; however, it is also apparent that the vaccination intention is far greater than the intention to use the wait-and-see strategy, among those older than 50. This result is in line with the literature, which shows that vaccination intention increases with age [4]. Regarding sex, men clearly have higher vaccination willingness, and women are more likely to be inclined toward the wait-and-see strategy. In terms of educational level, the wait-and-see strategy was more prevalent among those with a four-year university degree or higher. This suggests that an increase in education level may strengthen the cautious attitude toward the wait-and-see strategy, rather than increasing the vaccination intention. Further, as support of a previous analysis [4], the results of the cross-tabulation between sex and education level, showed that the intention to be vaccinated was higher among men with a university degree or higher, than the wait-and-see attitude. Conversely, the wait-and-see attitude was evident among women with a college degree. These results are common to both the YCS and Lucid results, with a more pronounced trend observed in the Lucid findings.

Table 3

Proportion of willingness/wait-and-see/unwillingness to be vaccinated by demographic composition: YCS.

		Vaccination	Wait-and-see	No vaccination
Age
	20–29	0.016	0.033	0.012
	30–39	0.067	0.094	0.033
	40–49	0.147	0.173	0.047
	50–59	0.117	0.108	0.031
	60–69	0.062	0.034	0.007
	70–79	0.010	0.008	0.001
Sex
	Female	0.136	0.180	0.057
	Male	0.283	0.268	0.075
Education
	Graduated 4-year institution	0.254	0.261	0.063
	Did not graduate	0.165	0.189	0.069

Note: Each number refers to a percentage value (%).

Table 4

Proportion of willingness/wait-and-see/unwillingness to be vaccinated by demographic composition: Lucid.

		Vaccination	Wait-and-see	No vaccination
Age
	20–29	0.054	0.080	0.030
	30–39	0.074	0.112	0.019
	40–49	0.086	0.104	0.031
	50–59	0.100	0.083	0.025
	60–69	0.099	0.032	0.011
	70–79	0.047	0.011	0.002
Sex
	Female	0.174	0.238	0.073
	Male	0.282	0.186	0.047
Education
	Graduated 4-year institution	0.279	0.224	0.0655
	Did not graduate	0.176	0.200	0.066

Note: Each number refers to a percentage value (%).

The majority of the respondents who answered “vaccinate later” or “no vaccinate” declared that they wanted to check the side effects of the vaccine. As predicted, the percentage exceeded 50% in all surveys (Fig 3). Another is an altruistic reason: “there are people who should get vaccinated before me.” Finally, the third most common reason was “once herd immunity is established, it is not necessary to inoculate yourself,” which was marked as a more strategic and egoistic intent.

Fig 3

Reasons for not getting vaccinated and watching.

Note: Abbreviations: side effects = I want to see if there are any adverse reactions to the vaccine. Others’ priority = The product of the desired vaccine manufacturer is not available in Japan. Herd immunity = If others are inoculated first and herd immunity is established, there is no need to inoculate myself as soon as possible. No-deterioration confidence = Even if I am infected, it is unlikely that I will become seriously ill. No-infection confidence = I will not be infected. Infected = I have already been infected with COVID-19.

I then examined the results of a pooled conjoint analysis of all participants. Fig 4 supports the descriptive analysis, revealing that “wait-and-see” is the preferred strategy for self and personal existence (baseline: not vaccinate). It is difficult to find significant differences between the overall results of Lucid’s panel in the AMCEs because of the small sample size. However, there are significant differences among AMCEs, namely, vaccination, watching, and non-vaccination, especially in the case of familiar entities. Furthermore, for the society in general, the results show no noticeable difference in the effects of vaccination among watchers. However, for the society in general, the results show no difference between static and active inoculation. Both results for familiar entities and the society suggest that the altruism of “withholding vaccination for the time being” seems to work, not for the society in general, but only for other people who are particularly close to each other. The results of this study show no support for the altruism in a pure sense, which is observed in the intention to contribute to herd immunity by actively inoculating oneself. More importantly, the wait-and-see strategy is clearly preferred for one’s self and those who have close relations, whereas the watcher strategy for the society in general is not preferred.

Fig 4

Average marginal component effect (AMCE) results.

Note: This plot shows the estimates of the effects of the randomly assigned vaccination attribute values on the probability of being preferred by Japanese participants. The estimates are based on the benchmark OLS model with clustered standard errors, and the bars represent the 99% confidence intervals. The points denote the attribute value, which is the reference category for each attribute.

Next, I examined the conjoint results relating to familiar entities. As shown in Fig 5, the most common choices, in descending order, were as follows: spouse and then family members living together, who were older and younger than the participants. This order was the same in both studies. Clearly, close family was the most frequent choice. In this context, Fig 6 shows the results of the conjoint, sorted according to who is selected as a familiar entity, and its baseline is set as ‘not to be inoculated’. Strikingly, those who select younger family members are less likely to prefer vaccinations for familiar entities and are more likely to hope that younger members are watchers. Furthermore, even the group that selected elderly people who are living family members, as the highest risk group at the time of infection, did not intend to receive vaccination, among those who were close to them (especially, in Lucid’s case), and the wait-and-see strategy played a central role.

Fig 5

Selection of familiar entities.

Fig 6

Average marginal component effect (AMCE) results by group.

Note: This plot shows the estimates of the effects of the randomly assigned vaccination attribute values on the probability of being preferred by Japanese participants. Coefficient plots are sorted by groups, as per familial presence recalled by participants. The estimates are based on the benchmark OLS model with clustered standard errors, and the bars represent 99% confidence intervals. The points denote the attribute value, which is the reference category for each attribute.

Conclusions

With fewer COVID-19 deaths in Japan than in other countries, the intention to receive vaccination is low, with individuals adopting the “wait-and-see” strategy. Results of a conjoint analysis that delved further into intentions, showed a strong preference for the wait-and-see strategy, not only for the participants themselves but also for those with whom they were familiar. This suggests that altruism does not lead to herd immunity through active voluntary vaccination; instead, a reduction in the side effects of vaccination leads to increased willingness to inoculate, the objective being to protect oneself or one’s friends and families. In addition, this study reveals that the preference of a familiar person to adopt the wait-and-see strategy was stronger in those who recall blood relations as familiar entities.

Clearly, in the case of Japan, the altruism surrounding vaccines did not manifest as a psychological mechanism for wanting the vaccination to be given as soon as possible to close relatives, but rather as a mechanism to wait until one is fully aware of the side effects. Beyond simple altruism, egoism for the safety of close relatives has been also embossed from this research. In countries where vaccination is not widely spread, the longer the observation period for side effects, the more likely it is that the strategy of observing the consequences and waiting until the safety from side effects is sufficiently assured, will be adopted. Hence, my analysis indicates that the most preferred social combinations of vaccination are as follows: society in general, may or may not be vaccinated early, but my loved ones and I can wait long enough to be vaccinated.

Whatever the reason for the wait-and-see behavior and non-vaccination, these results suggest that it would be difficult to establish herd immunity by increasing the number of vaccinated individuals at an early stage. To suppress opportunistic strategies and cultivate intentions for mass vaccination, the public should be fully informed that the risk of infection is higher than the risk of side effects. Furthermore, considering the result that people are more cautious about allowing their relatives who are from a different generation to receive vaccination, proving the safety of vaccines for each generation and publicizing its safety, is an efficient way to enhance the intention to be vaccinated. However, even with such countermeasures in place, empirical results pessimistically predict that vaccination rates would remain at a low level among Japanese citizens.

The first limitation of this study, however, is that the results of this analysis are limited to Japanese participants, and there is a possibility that other factors may function as the main motivation behind the adoption of the wait-and-see and non-inoculation strategies in other countries (e.g., some campaign discourse on vaccine toxicity). For the second limitation, as this study did not handle panel data obtained from several waves of surveys, the design may not be sufficiently controlled for covariates (unit-specific effects). To address such challenges, this paper relies on the results of analyses using multiple surveys at roughly the same time point. However, the limitation is that it still fails to capture the changes that occur over time within each unit.

As of October 20, 2021, fortunately, against the implication of this study, 7 months after the survey was conducted in March 2021, the percentage of those who have received at least one shot of vaccination and the percentage of those who have received two shots in Japan were 76% and 67.8%, respectively (Source: Nikkei Shimbun HP about Vaccination Status URL: https://vdata.nikkei.com/newsgraphics/coronavirus-japan-vaccine-status/), far exceeding the levels predicted by the results of the analysis in this study. Research suggesting that vaccination reduces the number of effective reproductions (R(t)) has, is ongoing [13]. Compared to the results of the survey on vaccination intention in the U.S., as of October 2020 (i.e., 53.6% [1]), and the vaccination rate as of October 2021 (i.e., 57.5%), the prior vaccination intention and actual vaccination rates are close (Source: New York Times URL: https://www.nytimes.com/interactive/2020/us/covid-19-vaccine-doses.html). In the EU, the percentage of the completion of partial vaccination, as of September 2021, was 60s in Western Europe and around 20% in Eastern Europe, compared to prior vaccination intention of over 70% (Source: European Centre for Disease Prevention and Control URL:https://www.ecdc.europa.eu/en/publications-data/data-covid-19-vaccination-eu-eea)Ċompared to countries where these prior vaccination intentions and actual vaccination rates are consistent, and where actual vaccination rates remain low when compared to prior vaccination intentions, the question of why the Japanese society was able to overcome its initial low vaccination intentions and reach a high vaccination completion rate remains; together with the question of why this enables Japan to act to lower R(t), these are issues to be unraveled by future research and comprise the third limitation of this study. The hypotheses for these questions include (1) a vigorous public outreach program with medical experts, (2) a vaccination system that was quickly promoted by both local public administrative bodies and corporate companies, and (3) the contribution of social peer pressure as an opportunity to transform individuals’ vaccination intentions. Thus, additional surveys are needed to test these hypotheses in future research.

Supplemental R-code for descriptive statistics and demographic composition.

(R)

Click here for additional data file.

Example of a display screen of conjoint analysis in the original Japanese language.

(PDF)

Click here for additional data file.

Supplemental data for Fig 3.

(CSV)

Click here for additional data file.

(CSV)

Click here for additional data file.

(PDF)

Click here for additional data file.

Supplemental R-code for Fig 3.

(R)

Click here for additional data file.

Supplemental data for Fig 4.

(R)

Click here for additional data file.

(R)

Click here for additional data file.

Supplementary materials for Fig 4.

(PDF)

Click here for additional data file.

Supplemental R-code for Fig 5.

(R)

Click here for additional data file.

Supplemental data for Fig 6.

(R)

Click here for additional data file.

(R)

Click here for additional data file.

The survey questionnaire can be accessed here.

(PDF)

Click here for additional data file.

19 Oct 2021

Analysis of Social Combinations of Coronavirus Vaccination: Evidence from a Conjoint Analysis

PLOS ONE

Dear Dr. Ohmura,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

Please submit your revised manuscript by Dec 02 2021 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

Please include the following items when submitting your revised manuscript:

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'.

A marked-up copy of your manuscript that highlights changes made to the original version. You should upload this as a separate file labeled 'Revised Manuscript with Track Changes'.

An unmarked version of your revised paper without tracked changes. You should upload this as a separate file labeled 'Manuscript'.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter. Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

If applicable, we recommend that you deposit your laboratory protocols in protocols.io to enhance the reproducibility of your results. Protocols.io assigns your protocol its own identifier (DOI) so that it can be cited independently in the future. For instructions see: https://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols. Additionally, PLOS ONE offers an option for publishing peer-reviewed Lab Protocol articles, which describe protocols hosted on protocols.io. Read more information on sharing protocols at https://plos.org/protocols?utm_medium=editorial-email&utm_source=authorletters&utm_campaign=protocols.

We look forward to receiving your revised manuscript.

Kind regards,

Prof. Anat Gesser-Edelsburg, Ph.D.

Academic Editor

PLOS ONE

Journal requirements:

When submitting your revision, we need you to address these additional requirements.

1. Please ensure that your manuscript meets PLOS ONE's style requirements, including those for file naming. The PLOS ONE style templates can be found at

https://journals.plos.org/plosone/s/file?id=wjVg/PLOSOne_formatting_sample_main_body.pdf and

https://journals.plos.org/plosone/s/file?id=ba62/PLOSOne_formatting_sample_title_authors_affiliations.pdf

2. Please update your submission to use the PLOS LaTeX template. The template and more information on our requirements for LaTeX submissions can be found at http://journals.plos.org/plosone/s/latex.

3. Thank you for stating the following in the Acknowledgments Section of your manuscript:

“Corresponding author: Professor, School of Policy Studies, Kwansei Gakuin University. 2-1, Gakuen, Sanda-shi,

Hyougo-ken, Japan, 6691337. hanakohmura@kwansei.ac.jp. This work was supported by Grant-in-Aid for Young

Scientists, Japan Society for Promotion of Science 19K13615”

We note that you have provided funding information that is not currently declared in your Funding Statement. However, funding information should not appear in the Acknowledgments section or other areas of your manuscript. We will only publish funding information present in the Funding Statement section of the online submission form.

Please remove any funding-related text from the manuscript and let us know how you would like to update your Funding Statement. Currently, your Funding Statement reads as follows:

“This work was supported by Grant-in-Aid for Young Scientists 19K13615.”

Please include your amended statements within your cover letter; we will change the online submission form on your behalf.

4. Please include your full ethics statement in the ‘Methods’ section of your manuscript file. In your statement, please include the full name of the IRB or ethics committee who approved or waived your study, as well as whether or not you obtained informed written or verbal consent. If consent was waived for your study, please include this information in your statement as well.

Additional Editor Comments (if provided):

[Note: HTML markup is below. Please do not edit.]

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 article presents a very current and necessary topic. It is well written and has a great methodological quality. The only suggestion I have is:

- in the introduction, provide an international overview of the subject.

- improve the limitations of the study.

- the practical and/or clinical implications of the study need to be further emphasized. It should make clearer the importance of the study for society and the next steps for the future.

Reviewer #2: Manuscript Number: PONE-D-21-21028

Manuscript Title: Analysis of Social Combinations of Coronavirus Vaccination: Evidence from a Conjoint Analysis

October 17, 2021

Reviewer

Comments to the authors:

This study provided some important evidence on vaccination dynamics in Japan using a conjoint analysis to breakdown the dynamics into groups. This is very important to determine the attributes of the groups in this pandemic. The aim of this study is laudable and evidentially justified. The statistical analyses, results, and discussions were aligned, which helped in understanding and reviewing this paper.

Revise grammatical errors in the manuscript throughout.

I would recommend the following:

Abstract:

-Provide the sample size and indicate whether this is a nationally representative sample or not.

-Are these results statistically significant or based on descriptive statistics? Please include this information.

-What conclusion(s) could you draw from these findings? I would suggest including this information.

-Check your quotations. Some of the quotations have apostrophes in addition. The quotation marks are also different.

Introduction

-Paragraph one: What are the vaccination statistics in Japan? Provide the vaccination statistics in Japan to compare these information to that of other countries you mentioned, instead of continents.

-Paragraph two: Rewrite this “(for example McPhedran and Toombs, 2021; Motta, 2021; Kreps et al., 2020)” as “(McPhedran and Toombs, 2021; Motta, 2021; Kreps et al., 2020)”.

-Paragraph four: Add this paragraph to paragraph five as its starting paragraph.

-Paragraph six: State your study aim and objectives here. No need stating “section” etc., as original research papers are organized in this manner.

Methods

Study subjects and period: Did you collect sociodemographic data on the subjects? If yes, I would recommend that you provide these information for us to better understand the dynamics of vaccination by subgroups.

Ethics: Was ethical approval obtained? This was not clearly stated.

Design of conjoint analysis: Please revise the grammar at this section. You were using both past tense and present tense. For instance, you used a present tense in “Subjects are asked to select one of the following, . . .” but used a past sentence in “The subjects were instructed. . .”

-Revise this sentence “According to the above conjoint analysis settings, we set our design of conjoint as in Table 1 and Figure 1, and Figure 2shows an example of the conjoint screen” as “According to the above conjoint analysis settings, we set our design of conjoint as in Table 1 and Figure 1, and Figure 2 shows an example of the conjoint screen.”

Results

-Paragraph two: Tables 1 and 2 should be formatted to remove a lot of the borderlines.

-What could be some possible limitations to your study? It is important to acknowledge this in a research.

-Provide some potential empirical studies that could have explained vaccination dynamics or perceptions about vaccination. You could also identify studies that agree or disagree with your findings.

Conclusion

No comments

Thank you very much for your important work.

**********

6. 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 #1: Yes: Mateus Dias Antunes

Reviewer #2: 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.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email PLOS at figures@plos.org. Please note that Supporting Information files do not need this step.

16 Nov 2021

Reviewer #1: I would like to express my sincere gratitude for the favorable review of my manuscript and would appreciate it if you could review my responses to the three points as follows.

- in the introduction, provide an international overview of the subject.

In addition to the descriptions of the U.S. and EU for international comparisons in the Introduction section, I have added statistics on vaccination intentions for Japan, corresponding to the point raised by Reviewer 2. In the Conclusions section, I have included a description about a cross-comparison of vaccination dynamics in Western countries and Japan. These newly added descriptions are founded upon international comparisons in previous studies rather than simply presenting the actual situation based on statistics.

- improve the limitations of the study

As a limitation of this study, I first note in the Conclusion section that the findings of this study are limited to subjects in Japan. Also, as a limitation of this study and a suggestion for new research, I have also reported that the current vaccination dynamics in Japan are so advanced that they have reached a situation that overturns the results of this analysis. Here, I also mentioned that a puzzle for new research has arisen.

- the practical and/or clinical implications of the study need to be further emphasized. It should make clearer the importance of the study for society and the next steps for the future.

As practical and clinical implications of this study, at the end of the Introduction and in the Conclusions sections, I have added the need to prove and publicize the safety of vaccines for each generation. In particular, since this survey was conducted in early March 2021, some studies have begun to show that the vaccination rate in Japan has increased significantly since then, and that this has pushed down the effective reproduction number (R(t)). It is also hypothesized that what contributed to this increase in vaccination rates and subsequent decrease in R(t) was the government’s publicity on TV and public commercials by experts about the limited side effects of vaccines. Since a detailed examination of these hypotheses is an issue for the future, a paragraph on the dynamics of vaccination in Japan and its implications has been added at the end of the Conclusions section.

Reviewer #2

I am very grateful for the positive review and the many important suggestions for improvement. The following responses address each of these points, and I hope you will find them helpful.

-Revise grammatical errors in the manuscript throughout.

Thank you for the suggestion. I have hired an English-language editing company to proofread the entire paper.

Abstract:

-Provide the sample size and indicate whether this is a nationally representative sample or not.

I have included a description of the sample size for the two surveys and how they match up to national demography.

-Are these results statistically significant or based on descriptive statistics? Please include this information.

Since the empirical results consist of descriptive statistics and a conjoint analysis with tests of statistical significance, I have rewritten the description of the results to distinguish between these analyses.

-What conclusion(s) could you draw from these findings? I would suggest including this information.

-Check your quotations. Some of the quotations have apostrophes in addition. The quotation marks are also different.

I have added the conclusion (and implications) of this study in the Abstract section and also corrected the quotation marks.

Introduction:

-Paragraph one: What are the vaccination statistics in Japan? Provide the vaccination statistics in Japan to compare these information to that of other countries you mentioned, instead of continents.

In response to this comment, I have added statistics on prior vaccination intentions for Japan. Rather than simply adding statistics, I have included statistics from studies such as Yoda et al. (2021) and Ishimaru et al. (2021) with the intention of introducing related studies.

-Paragraph two: Rewrite this “(for example McPhedran and Toombs, 2021; Motta, 2021; Kreps et al., 2020)” as “(McPhedran and Toombs, 2021; Motta, 2021; Kreps et al., 2020)”.

All literature citations have been changed to a number formation. All references have been renumbered, and vague descriptions such as “for example” have been removed.

-Paragraph four: Add this paragraph to paragraph five as its starting paragraph.

I would like to thank you for your useful suggestions. I have re-written the paragraphs as you suggested.

-Paragraph six: State your study aim and objectives here. No need stating “section” etc., as original research papers are organized in this manner.

I also would like to thank you for your useful suggestions. As you pointed out, I have presented the purpose of this study and a brief conclusion in the last paragraph of the Introduction.

Methods:

Study subjects and period: Did you collect sociodemographic data on the subjects? If yes, I would recommend that you provide these information for us to better understand the dynamics of vaccination by subgroups.

In accordance with this helpful comment, I have presented the results on vaccination intention according to sociodemographic characteristics in Tables 3 and 4. By discriminating between vaccination intentions of subgroups, I have added the following explanations: (1) women are more likely to use the wait-and-see strategy than are mem, (2) this tendency is more conspicuous for women with a higher education, and (3) the results are consistent with the findings of previous studies such as Ishimaru (2021).

Ethics: Was ethical approval obtained? This was not clearly stated.

Design of conjoint analysis: Please revise the grammar at this section. You were using both past tense and present tense. For instance, you used a present tense in “Subjects are asked to select one of the following, . . .” but used a past sentence in “The subjects were instructed. . .”

-Revise this sentence “According to the above conjoint analysis settings, we set our design of conjoint as in Table 1 and Figure 1, and Figure 2shows an example of the conjoint screen” as “According to the above conjoint analysis settings, we set our design of conjoint as in Table 1 and Figure 1, and Figure 2 shows an example of the conjoint screen.”

Thank you for pointing out the grammatical errors. I have corrected these errors. In addition, ethical approval had been obtained. I have added those details to the paper.

Results

-Paragraph two: Tables 1 and 2 should be formatted to remove a lot of the borderlines.

As you suggested, I tried to see if I could remove the lines, but I think this is the limit of what I can do to maintain readability. I would appreciate it if you would consider it.

-What could be some possible limitations to your study? It is important to acknowledge this in a research.

I apologize for the inadequate description of the limitations of the study and thank you for your remarks. As the other reviewer also pointed out, we did not fully address the limitations of the study: one is that the findings are limited to Japan; so even if there is a large number of quiescent or non-vaccinated people in other countries, this may not be due solely to avoidance of side effects in blood relatives. In other countries, even if there are a large number of people who are watchers or do not want to be vaccinated, it may not be due only to avoidance of side effects on blood relatives. This is the first limitation of this study.

Surveys for this study were conducted in March 2021, and it has been almost 8 months since then. This has led to the discrepancy between the prior vaccination intention rates and actual ones. A relatively high vaccination rate is being achieved in Japan, even beyond the expectations of this study. This discrepancy partially constitutes a limitation of this study, but I recognize that it can be a critical future issue to be addressed. Thus, I have emphasized on answering why lower prior vaccination intentions are being overcome and higher vaccination rates are being achieved in Japanese society has been set as a future topic of research. To answer this question, three hypotheses are presented as well in the end of the Conclusion section.

-Provide some potential empirical studies that could have explained vaccination dynamics or perceptions about vaccination. You could also identify studies that agree or disagree with your findings.

Thanks to your comment, I followed up on recent studies on vaccination dynamics and, in relation to the above point, to mention the discrepancy between vaccination dynamics and the predictions of this study. In the Conclusions section, I have mentioned the need to examine why prior vaccination intentions and actual vaccination diffusion tend to coincide in the U.S. and European countries, whereas they tend to diverge in Japan.

Submitted filename: response_letter_plosone202111.pdf

Click here for additional data file.

24 Nov 2021

30 Nov 2021

Reviewer #2

I would like to thank you for your second detailed review of this paper and for your valuable suggestions. I have responded to each comment as follows and would appreciate your confirmation.

You were using "I" but at some points in the method section, you were using "our" and "us". Please, be consistent.

I have ensured that all instances of “us” and “our” were deleted in this newly revised version.

Methods

Design of conjoint analysis: This sentence may not be complete, “According to the above conjoint analysis settings, I set my design of conjoint as in Table 1 and Figure 1, and Figure shows an example of the conjoint screen.” I think ". . . Figure shows an example of the conjoint screen." should have a number as ". . . Figure 2 shows an example of the conjoint screen."

Thank you for pointing this out. The figure number was indeed missing, so I have added it accordingly.

Results

Lines 137-138: ". . . as predicted, the percentage exceeded 50% in all surveys (Figure )." The figure is missing a number.

I would like to thank you for your careful review of this as well. I have added the figure number accordingly. I have also embedded each percentage value in the bar graph in the figures. I hope that the readability of the figures has improved.

Conclusion

The information you provided as a limitation does not reflect a study limitation. Your limitation should focus on the measures, study design and sampling, analytical procedures, etc. For instance, this is a cross-sectional study, so you could not establish causality and temporality.

I would like to thank you for your kind remarks. One of the limitations of this study is that the data were not panel data consisting of several waves, the unit specific effects were not controlled, and there was a difficulty in handling covariates. I would appreciate it if you could confirm this (l.204–l.212).

Submitted filename: response_letter_plosone2021201.docx

Click here for additional data file.

2 Dec 2021

Analysis of Social Combinations of COVID-19 Vaccination: Evidence from a Conjoint Analysis

PONE-D-21-21028R2

Dear Dr. Ohmura,

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.

An invoice for payment will follow shortly after the formal acceptance. To ensure an efficient process, please log into Editorial Manager at http://www.editorialmanager.com/pone/, click the 'Update My Information' link at the top of the page, and double check that your user information is up-to-date. If you have any billing related questions, please contact our Author Billing department directly at authorbilling@plos.org.

If your institution or institutions have a press office, please notify them about your upcoming paper to help maximize its impact. If they’ll be preparing press materials, please inform our press team as soon as possible -- no later than 48 hours after receiving the formal acceptance. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information, please contact onepress@plos.org.

Kind regards,

Prof. Anat Gesser-Edelsburg, Ph.D.

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Reviewers' comments:

9 Dec 2021

PONE-D-21-21028R2

Analysis of Social Combinations of COVID-19 Vaccination: Evidence from a Conjoint Analysis

Dear Dr. Ohmura:

I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

If your institution or institutions have a press office, please let them know about your upcoming paper now to help maximize its impact. If they'll be preparing press materials, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information please contact onepress@plos.org.

If we can help with anything else, please email us at plosone@plos.org.

Thank you for submitting your work to PLOS ONE and supporting open access.

Kind regards,

PLOS ONE Editorial Office Staff

on behalf of

Prof. Anat Gesser-Edelsburg

Academic Editor

PLOS ONE