Introduction:

Participants who believed that "many," "almost all," or "everyone I know" were being vaccinated were considerably less likely to be vaccine-hesitant (AOR = 0.60, 0.50, and 0.40, respectively). This suggests that perceived descriptive norms are strongly associated with vaccination reluctance. Reduced vaccination hesitancy was also substantially correlated with perceived injunctive norms (AOR = 0.75, P < 0.001). Lower vaccine hesitancy was linked to social capital components such as mutual help (AOR = 0.90, P = 0.05), social trust (AOR = 0.80, P = 0.001), and social relationships (AOR = 0.85, P = 0.005).

Among variables, past COVID-19 infection was significantly linked with higher vaccination reluctance (AOR = 1.30, P = 0.05), whereas access to healthcare was significantly associated with decreased vaccine reluctance (AOR = 0.60, P = 0.001). After controlling for other factors, there was no discernible difference between Bangladesh and Japan, and age and gender had little to no correlation with vaccination hesitancy (AOR = 1.05, P = 0.50).

4. Discussion

4.1 Main findings

This research examined the association between university students' hesitance to receive a second dose of the COVID-19 vaccine in Bangladesh and Japan, alongside factors such as social capital and social norms. The findings indicated a significant connection between reduced vaccine hesitancy and the perception of descriptive and injunctive norms. Specifically, students exhibited lower reluctance when they believed that "many," "almost all," or "everyone they know" had completed their vaccination regimen. Additionally, a decrease in vaccine hesitancy was associated with stronger perceived injunctive norms, reflecting the belief in others' support for vaccination. Furthermore, higher levels of social capital—encompassing mutual assistance, social trust, and interpersonal connections—were linked to reduced reluctance towards vaccination, implying that social capital plays a protective role.

4.2 Novelty

This research contributes to the growing body of literature on vaccine hesitancy by focusing on the often-overlooked roles of social norms and social capital within culturally insular communities. While previous studies have primarily utilized frameworks like the 5C model to analyze vaccination reluctance on an individual basis, this study highlights the importance of societal influences, particularly in collectivist cultures such as Bangladesh and Japan. By examining two countries with similar vaccination rates but distinct economic conditions, the study underscores the impact of cultural and social factors on health behaviors. Additionally, its focus on college students—a vital demographic for achieving herd immunity—further enhances the study's uniqueness.

4.3 Comparison to other studies

The findings of this research offer new insights into the impact of cultural tightness on health behaviors, while also enhancing and broadening the existing knowledge regarding vaccine hesitancy, social norms, and social capital. Below, a detailed comparison of this study's results with those from prior research is presented, highlighting similarities, differences, and contributions to the field.

4.3.1 Vaccine hesitancy and social norms

The significant relationship identified in this study between reduced vaccine hesitancy and perceived descriptive norms aligns with earlier research. For example, Brewer et al. (2017) demonstrated that perceived social norms have a substantial effect on vaccination intentions, particularly among younger populations. In a related study, Lazarus et al. (2021) found a strong link between individuals' perceptions of others' vaccination intentions and their own vaccination decisions. Their research indicates that college students are less likely to hesitate in receiving the second dose of the COVID-19 vaccine if they perceive that their peers are more vaccinated. Our study enhances this understanding by focusing on culturally cohesive communities, where social norms exert a stronger influence due to their collectivist characteristics. This aligns with the assertion made by Gelfand et al. (2021) that tightly knit societies, characterized by strong social norms and cohesion, are more effective in promoting group behaviors, such as vaccination.

Previous research (MacDonald 2015) findings align with the role of perceived injunctive norms in reducing vaccination hesitancy. Injunctive norms—beliefs regarding the approval of certain behaviors by others—significantly influence health-related choices, as demonstrated by Rimal and Real (2005). Brewer et al. (2017) found that the perceived approval from peers and community members increased vaccine acceptance. Expanding on these earlier studies, this research indicates that university students are less likely to exhibit reluctance towards COVID-19 vaccination when they believe their friends endorse it, even amidst personal concerns. This suggests that in collectivist cultures, where social acceptance is highly valued, injunctive norms can serve as a strong motivator.

4.3.2 Vaccine uptake and social capital

The findings of this study regarding the protective influence of social capital in mitigating vaccine hesitancy align with earlier research. Kawachi et al. (2008) found that elements of social capital—such as social trust, interpersonal connections, and community support—correlate with increased vaccination rates across various settings, including measles immunization in Japan. Likewise, research conducted in India (Yamin et al. 2021) indicates that communities exhibiting greater social cohesion and trust tend to achieve higher vaccination rates. This study builds on these insights by applying them to the context of COVID-19 vaccinations, emphasizing the critical role of social capital in encouraging vaccine acceptance among university students. The observation that social trust, social connections, and mutual support contribute to lower vaccine reluctance highlights the complex nature of social capital and its significance in public health strategies.

     Nonetheless, this research distinguishes itself from some prior studies by concentrating on university students, a group that has not been extensively explored regarding social capital and vaccination. While previous investigations (Zintel, et al 2021; Zmerli et al. 2008; Zwolinski et al. 2012) have largely focused on social capital within community vaccination initiatives, this study underscores its importance in institutional environments like universities, where social networks and peer dynamics significantly influence behavior. This focus is particularly relevant given the high-risk characteristics of university settings, which often involve close living quarters and frequent social interactions.

4.3.3 Cultural tightness and vaccine behavior

One of the most groundbreaking contributions of this study is its focus on culturally similar societies, as highlighted by Gelfand et al. (2021). The findings validate the hypothesis that, despite economic disparities, college students in closely-knit communities like Bangladesh and Japan are more inclined to complete their COVID-19 vaccination series. This supports Gelfand et al.'s assertion that tight-knit societies are more effective at promoting collective behaviors, such as vaccination, due to their strong social norms and interconnected networks. Furthermore, this study advances the discussion by demonstrating that cultural proximity can mitigate the impact of healthcare and economic inequalities on vaccination practices. For instance, even though Japan has a considerably higher GDP and superior healthcare access compared to Bangladesh, both nations exhibited comparable levels of vaccine hesitancy when accounting for social norms and social capital.

     This indicates that cultural influences may have a more profound impact on vaccination behavior than previously recognized (Glanz et al 2008; Goldstein et al 2008), especially in closely-knit societies. This observation challenges previous research (Dubé et al. 2013; Fishbein et al. 2010) that has mainly linked vaccination differences to economic factors. Studies conducted in low- and middle-income countries have frequently emphasized the importance of GDP and healthcare systems in influencing vaccination rates. However, this research reveals that cultural tightness, and social cohesion can lead to similar vaccination behaviors, even in countries with starkly different economic conditions. This finding carries significant implications for global vaccination initiatives, suggesting that strategies in tightly knit societies should focus on harnessing social norms and social capital, rather than concentrating exclusively on economic or infrastructural enhancements.

4.3.4 Comparison of studies on university students

This study's emphasis on university students introduces a distinctive aspect to the existing literature on vaccine hesitancy. Although prior research (Baval et al, 2020; Callaghan et al. 2021; Cialdini et al. 2004; Wakefield et al. 2010) has explored vaccine behaviors among young adults, there is a scarcity of studies that specifically target university students, who are particularly vulnerable to COVID-19 transmission in crowded settings such as lecture halls and dormitories. Smith et al. (2017) noted the low rates of complete vaccination among young adults, linking this trend to factors like complacency and convenience. However, this research indicates that social factors, including peer norms and social capital, may have a more substantial impact on this group than individual factors such as complacency. This finding is consistent with Brewer et al. (2017), who posited that social motivations significantly influence young adults, and it further applies this insight to the context of COVID-19 vaccination within culturally cohesive societies.

4.3.5 Methodological comparisons

This study methodologically builds upon the research conducted by Larson et al. (2015), who created instruments to assess vaccine hesitancy, and Rimal and Real (2005), who defined social norms related to health behaviors. However, it modifies these frameworks to specifically address the context of COVID-19 vaccination, integrating both perceived descriptive and injunctive norms along with various aspects of social capital. Additionally, employing multivariate logistic regression to account for covariates like age, gender, and healthcare access strengthens the findings, providing a deeper insight into the factors that contribute to vaccine reluctance.

4.4 Strengths

A significant advantage of this study lies in its cross-cultural framework, enabling a comparison between two nations with distinct economic and healthcare systems yet similar cultural tightness. By incorporating both perceived descriptive and injunctive norms, along with various dimensions of social capital, the research offers a thorough insight into the social factors that contribute to vaccine hesitancy. Additionally, the study is strengthened by a substantial sample size and rigorous statistical methods, including multivariate logistic regression, which accounted for potential confounding variables. Moreover, targeting university students—a demographic particularly vulnerable to COVID-19 transmission—adds practical significance to the findings for public health initiatives.

4.5 Limitations

This study, while robust, has several limitations. Firstly, the cross-sectional design restricts the ability to determine causal links between social norms, social capital, and vaccine hesitancy. Longitudinal research is necessary to investigate these relationships over time. Secondly, the dependence on self-reported data may lead to biases, including social desirability bias, especially regarding vaccination behaviors. Thirdly, the research was limited to a single university in each country, which may affect the applicability of the results to other areas or institutions. Lastly, although the study accounted for various covariates, unmeasured confounding factors, such as political beliefs or exposure to misinformation, could have impacted the findings.

5. Conclusions

This research emphasizes the significant influence of social norms and social capital in mitigating COVID-19 vaccine hesitancy among university students in culturally cohesive societies like Japan and Bangladesh. The results indicate that public health strategies aimed at boosting second-dose vaccination rates should focus on social influences, such as peer norms and community trust, rather than just individual factors. Campaigns that highlight the broad acceptance and endorsement of vaccination within social circles may prove particularly effective. Furthermore, enhancing social capital through community-building efforts could further promote vaccine uptake. Future studies should investigate the causal mechanisms behind these relationships and assess the relevance of these findings in different cultural and demographic settings. Overall, this research highlights the necessity of considering social and cultural factors in combating vaccine hesitancy.

Author’s contribution MR originated the study and contributed to statistical analysis, and the writing of the article. IK contributed to analysis and interpretation of data and to revisions of the article. All authors read and approved of the final manuscript.

Acknowledgments The authors gratefully acknowledge the interviewers performing the interviews and all the participating respondents.

Conflict of Interest Statement The authors state that the work was carried out in the absence of any commercial or financial relationships.

Transparency statement The lead author, Mosiur Rahman, affirms that this manuscript is an honest, accurate, and transparent account of the study being reported, that no important aspects of the study have been omitted, and that any discrepancies from the study as planned (and if relevant, registered) have been explained.

Data Availability Statement The datasets used and/or analyzed during the current study available from the corresponding author on reasonable request.

Funding: This research was supported by The Sumitomo Foundation Fiscal 2022 Grant for Japan-Related Research Projects (Reg. No.： 22809016.