Coronavirus disease 2019 (COVID-19) booster vaccination has been proposed and is undergoing experiments or pilots in response to the new challenges of COVID-19 brought by highly contagious variants [1
]. A striking example of such a COVID-19 variant is the B.1.617.2 (delta) variant, which has led to a surge in infection cases across the globe [2
]. Moreover, the effectiveness of approved COVID-19 vaccines against variants remains unclear. One recent study reported that the effectiveness of two-dose BNT162b2 (mRNA) and ChAdOx1 (adenovirus vector) vaccines remained as high as 88% and 67%, respectively, among patients with the Delta variant [3
]. Another laboratory study found that the antibodies produced by mRNA vaccines still offered protection against the B.1.1.7 (alpha) and B.1.351 (beta) variants, but the protection was much less strong [2
]. Until May 2021, three inactivated vaccines, one adenovirus vector vaccine, and one recombinant subunit vaccine against COVID-19 have been used in China, and all of them were domestically produced [4
]. Among them, inactivated vaccines possessed the largest market share and were the most frequently administered [5
]. An Indian case-control study published in November 2021 showed that the adjusted effectiveness of the two-dose BBV152 (inactivated vaccine) at least 14 days before testing was 47% and suggested that the relatively low effectiveness might be due to the high prevalence of the circulating delta variant in India [6
]. Infectious disease experts have carefully weighed the need for booster shots for certain vulnerable groups or the entire population to protect against the circulating new variants and improve immunity level, as the duration of protection remains unknown [7
]. As concerns about variants and protection duration continue to alarm the public of the importance of booster shots, it is imperative to make some preparations in advance to understand the demands for booster vaccination.
The World Health Organization (WHO) has also considered annual boosters for high-risk individuals and boosters every two years for the general population [8
]. An increasing number of countries are now delivering a third booster shot for the public, and Israel took the initiative to supply boosters for the elderly aged over 60 years on 30 July 2021 [9
]. In China, the mass immunization program has been progressing smoothly since its launch on 31 December 2020 [10
], and the country has delivered 2.47 billion vaccine doses as of 27 November 2021 [11
]. Clinical evidence supports the widely acknowledged two-dose vaccination schedule [12
], but limited clinical evidence is available to support the necessity of boosters in China [13
]. However, it was reported in late September 2021 that China would launch COVID-19 booster vaccinations for priority populations aged 18–59 years old who had completed two-dose immunization six months before. Later, local areas such as Zhejiang and Henan provinces officially announced the implementation of booster vaccination programs, which could soon be launched throughout the country.
Although there have been many studies examining public acceptance of current COVID-19 vaccination showing that acceptance varied substantially globally [14
], little is known about booster acceptance in China. A declining trend of COVID-19 vaccination acceptance rates has been found in China [17
] and the United States [18
], and it has been noted that many factors could be considered in the interpretation of varied vaccine acceptance in different countries [14
]. Another issue with the blooming controversy is whether the COVID-19 vaccine needs annual (or regular) boosters to maintain high levels of immunity against both the original virus and variants, similar to annual seasonal influenza shots. Therefore, there is an urgent need to understand the public acceptance of COVID-19 boosters to prepare for effective promotion strategies [14
To fill the gap in booster vaccination acceptance in China, this study aims to examine the public acceptance rate of COVID-19 boosters and its influencing factors using a self-administered online survey conducted in June 2021. This study also attempts to explore the willingness to pay (WTP) for annual boosters in case of yearly surges of the COVID-19 pandemic. The study of booster acceptance will help provide empirical evidence to improve COVID-19 booster delivery.
To our knowledge, this is the first study to investigate public acceptance of COVID-19 booster vaccination in China, and it is prudent to prepare for such a need as a precaution. We found that 84.80% of respondents aged 18–59 years old reported their willingness to accept COVID-19 booster vaccination, and perceived benefits and barriers were two important HBM dimensions associated with booster acceptance. The primary reasons for accepting or not accepting booster vaccination were “protection against current strains” and “concern about vaccine safety”, respectively. Additionally, over 90% reported WTP for an annual booster vaccination between 0 and 300 CNY (0–46.29 USD). Our findings have important implications for effective and proper interventions for future booster vaccination campaigns against COVID-19 and its variants.
This study was conducted approximately six months after the implementation of national COVID-19 vaccination programs in China, when new variants, especially the delta variant, appeared to threaten global public health [11
]. Our findings indicated that a large proportion of respondents (84.80%) expressed their intention to receive COVID-19 booster vaccinations. At present, few studies have been published on booster acceptance, and it is difficult to make direct domestic or inter-country comparisons. We compared our findings of booster acceptance rates with the public acceptance of non-booster COVID-19 vaccine acceptance rates, and found that generally, the booster acceptance rate was close to some non-booster vaccine acceptance rates reported in China, such as the 88.5% shown in a longitudinal study in the well-contained phase (Nov–Dec 2020) [17
]. However, the booster acceptance rate in this study was much higher than the non-booster vaccine acceptance rate (55.3%) reported by Zhao et al. in Jan–Apr 2021 [39
]. The higher acceptance rate of boosters reported in this study may be partly explained by higher public expectations of boosters facing the new challenges of variants [11
], as well as the observed increasing trend in COVID-19 vaccination acceptance in the first half of 2021 [39
] since this study was conducted in June 2021, several months after Zhao et al.’s survey.
Based on HBM, our results suggested that perceived benefits and perceived barriers to vaccination were important dimensions associated with the acceptance of COVID-19 boosters, which is consistent with the HBM-related findings of the existing H1N1 and COVID-19 vaccination studies [29
]. Vaccine efficacy has been reported as an important predictor of vaccine acceptance and uptake [15
], and a discrete choice experiment (DCE) study conducted in China found that high effectiveness of COVID-19 vaccines was the most favored attribute [41
]. The importance of perceived barriers has likewise been reported in previous studies of COVID-19 vaccines [15
], and it was emphasized that strict surveillance during booster development should be organized. It is important to improve health promotion and reduce barriers to booster vaccination. Hence, public health intervention programs should focus on increasing beliefs about vaccine effectiveness and reducing perceived adverse effects and safety barriers if boosters are widely approved for mass vaccination against COVID-19 variants in the future.
Our findings also indicated that COVID-19 vaccination history was strongly associated with increased odds of accepting COVID-19 booster vaccination. Prior studies also found that vaccination history had a positive effect on individual vaccination intention [27
]. For example, one study in Australia found that the acceptance of previous influenza vaccination had the largest effect on the willingness to be vaccinated against H1N1 during the 2009 pandemic (OR = 5.03) [27
]. Previous vaccination history should be carefully considered when designing vaccination schedules and targeted measures. The multiple logistic regression of sociodemographic characteristics indicated that respondents aged 41–50 and with a higher education level expressed significantly lower booster acceptance, while employment or belonging to priority groups for vaccination remained indicators of higher booster acceptance. These results were inconsistent with previous studies in China concerning COVID-19 vaccine acceptance that highlighted the impacts of gender, marital status, and regional or rural/urban differences [17
]. Further studies are needed to investigate the differences in sociodemographic characteristics between vaccine acceptance and booster acceptance. The associations between sociodemographic characteristics and booster acceptance in this study could provide preliminary results to help the design of booster-targeted vaccination strategies to increase coverage.
The reasons for accepting or not accepting booster vaccination were further explored, with protection against early circulating strains being the primary reason for booster acceptance, and safety concerns being the priority issue for refusers. Public concern about vaccine safety continues to be an obvious obstacle for booster uptake, and the top priority should be to strengthen public trust in both vaccination and booster safety by adhering to post-marketing surveillance and improving the compensation policy after adverse events [45
]. For booster refusers, there were also differences between previously vaccinated and unvaccinated groups. The previously vaccinated group was more concerned about vaccine recommendations or the need to be vaccinated again, while the unvaccinated group posed more concerns about contradictions and vaccine efficacy. Therefore, for previously vaccinated respondents, more efforts could be made concerning recommendations from reliable information sources and emphasis on the need for and importance of boosters; for previously unvaccinated respondents, more attention could be paid to the correct understanding of vaccine contradictions and the effectiveness of booster shots.
Vaccine price may be an important obstacle for the acceptance of self-paid vaccines [48
]. The Chinese government is providing COVID-19 vaccination free of charge to the general public, but it is unknown whether this policy will be maintained. Our findings revealed that the mean and median WTP for annual booster vaccination were 118.62 CNY (18.30 USD) and 60 CNY (9.26 USD), respectively, which was lower than those reported for full-course COVID-19 vaccination when vaccines were not yet available (mean: 254 CNY (39.19 USD); median: 100 CNY (15.43 USD)) [24
]. This decline may be explained by the annual requirement and currently free vaccination. The purpose of exploring WTP is not to encourage the transition from free vaccination to out-of-pocket or partially out-of-pocket payment, but to provide a reference for policy makers to make decisions on future vaccination policies.
There are several limitations in this study. First, due to the intrinsic disadvantages of cross-sectional online surveys, sampling bias may exist to limit the representativeness of the results [50
]. This study tried to reduce bias by recruiting adults using a stratified sampling method. Second, self-reported responses may be subjective to recalling bias and a tendency to report socially desirable responses. We designed an anonymous survey with most questions asking respondents’ thoughts and feelings at the moment, which may have helped to minimize the effect of self-reporting bias. Third, booster acceptance and WTP were derived based on hypothetical COVID-19 booster vaccination before the approval of final products [52
]. Future research to gather more accurate acceptance of COVID-19 boosters is encouraged to prepare for the booster campaign.