Pandemic is a situation of high uncertainty, constant changes, and high personal threat. In this study, we utilized real-time population-based data that covered the entire period of the COVID-19 vaccine rollout, to systematically investigate determinants associated with the primary vaccine doses and booster dose uptake. Using multi-level variables, our study offers insights into how the vaccination programme in a pandemic context should adapt to the changing situation and related psychological responses to optimize vaccination uptake.
Building on existing literature that vaccine confidence plays the key role in people’s COVID-19 vaccination decision6,16,26, our findings provide more nuanced insights into the changing importance of vaccine confidence attitudes for vaccination uptake with the evolution of the vaccination programme. We found that at the initial stage when COVID-19 vaccines became available to the public, vaccine safety was the most important determinant of primary vaccine doses uptake. Concerns about vaccine safety were mainly driven by the novelty of the vaccine technology and its rapid development40, and further intensified by the negative news regarding COVID-19 vaccine and people’s conspiracy beliefs41,42. As uptake of the primary doses became high, people gain more confidence about the safety of the vaccine. However, the resurgence of COVID-19 outbreaks due to the emergence of new virus variants and waning immunity43 could dampen people’s confidence in the vaccine effectiveness. In other words, people’s attention shifted from the potential harms of the vaccine to its expected benefits in the later stage of the vaccination programme.
Our network analysis also consistently identified that persons with chronic disease status were less likely to take the vaccinations across the whole vaccination programme. Relatedly, older age was found to be negatively associated with vaccine uptake in P2, but the association shifted to be positive in P3. Participants’ verbal reasons for vaccine hesitancy revealed that concern about chronic disease status was the main reason for refusing the primary vaccine doses uptake in older adults (Supplementary Table 6). This explains why uptake of COVID-19 vaccine was low among older people before P3 (the Omicron wave), with around 35%, 50%, and 80% of people aged 60–69, 70–79, and 80 years or above, respectively, had not received any dose of COVID-19 vaccines36, resulting in the highest daily COVID-19 mortality to date37. The low vaccine uptake among older adults could be attributed to the special contexts in Hong Kong44,45. First, older people in Hong Kong particularly those with chronic diseases received insufficient information and no explicit advice from healthcare workers and their family members on COVID-19 vaccination39. This may link to the facts of lacking continuity in health care shaped by the existing healthcare system46 and that most older people lived alone or with their older partners in Hong Kong47. Second, Hong Kong Chinese older adults generally have lower educational attainment, which limits their ability to utilize health information for making a medical decision. There were only 11% of Hong Kong older adults obtained the tertiary education in 202148, comparing to 47.1%, 32.1%, and 38.8%, respectively, in Japan, New Zealand and United Kingdom49 where a higher willingness to take a COVID-19 vaccine was reported among older adults9,50,51. However, older age was positively associated with uptake of COVID-19 vaccine during P3, which may be attributable to the joint effect of intensive news media attention on deaths among older people due to COVID-19 and the implementation of door-to-door mobilization of vaccination for older people during this period36.
Previous studies have shown that merely reporting daily case numbers has little impact on behavioural change, including vaccination52,53. Our study found that reporting the number of COVID-19 report cases had a negative association with the primary vaccine doses uptake in P1 and P2, and the association with uptake of the booster dose disappeared in P3 and P4. The pattern at the very beginning could be linked to people's avoidance of public places (such as vaccination centres) to protect themselves against infection. However, the number of reported cases became insignificant in the later stage. This may be attributed to the growing resilience and familiarity of the prolonged public health crisis54 and the fact that vaccination no longer aimed at preventing infection55. Contrary to previous observational studies concluding that more adoption of non-pharmaceutical preventive behaviours was associated with higher vaccine uptake41,56, we observed a consistent negative association between the alternative protective behaviour of “avoiding going out” and vaccination uptake. In addition, adoption of an array of non-pharmaceutical preventive behaviours was all negatively associated with vaccination uptake at P1, when the vaccine was initially rolled out. Several reasons may explain this. First, people who avoided going out may perceive a higher risk of visiting the vaccination venues which was perceived to increase their risk of infection. Second, people who avoided/were able to avoid going out may perceive a low risk of exposure to the viruses and hence perceived a low need for vaccination. Third, the negative associations between different non-pharmaceutical preventive behaviours and vaccine uptake at P1 indicates that people tended to adopt alternative behaviours that they may perceive to be safer when evaluating the pandemic risk against the vaccine risk. Fourth, the booster dose was promoted through the implementation of vaccine pass which required people to take a booster dose for proof to access certain premises. This may induce psychological reactance, particularly among people with higher vaccine hesitancy57. Although there may be reverse causality that people who had received the vaccine tended to be more relaxed about taking the non-pharmaceutical measures, we ruled out this possibility by running additional chi-square tests between vaccine uptake status and adoption of non-pharmaceutical preventive behaviours. We found that the negative association was mainly driven by the greater proportion of adopting non-pharmaceutical preventive behaviours in the vaccination non-uptake group (see Supplementary Fig. 2). Overall, this finding suggests that the non-pharmaceutical preventive measures remained important for people who would like to avoid vaccine risk39. Studies consistently found that people’s positive traditional Chinese medicine (TCM) value can induce negative attitudes toward western biomedicine including vaccination39,45,58. In Hong Kong, some individuals, particularly older adults39, are more familiar with traditional Chinese medicine (TCM) and perceive that TCM is less invasive compared with western biomedicine59. Adoption of non-pharmaceutical measures is important for pandemic control at the initial stage when vaccines are not available60. However, it also induces complacency psychology and illusory optimism that vaccines are no longer needed61, which was found to be the main barrier for booster dose uptake in our qualitative analysis of participants’ verbal reasons for vaccine hesitancy. In the later stage of a pandemic, strategies should focus on mitigation rather than containment, during which stringent social distancing measures may induce tremendous societal costs and thereby vaccination is of paramount importance particularly for individuals at higher risk of severe disease37. In the later stage, an illusory belief about the effectiveness of non-pharmaceutical preventive behaviours may be detrimental to promoting vaccination uptake62.
We also identified potential innovation adopters that could be targeted at the early stage of a vaccination campaign. Our results suggest that people with higher educational attainment and reported married status were more likely to take the primary vaccine doses in P1 and a booster dose in P3 when the new recommendation or policy for vaccination was initiated. People with higher education have a greater ability to comprehend new interventions and policies and are thereby usually the early innovation adopters63,64. While married people’s early adoption of novel intervention is likely to be driven by prosocial motivation, to take the risk and endure the uncertainty of vaccine safety to protect their loved ones44. Prior studies found no clear associations between educational attainment and marital status with COVID-19 vaccination acceptance in their survey time16,65. Our study found that these demographics only predict vaccination uptake at the initially established phase of the vaccination campaign when taking the vaccination was yet to be normative. Future programmes can leverage these early adopters to spread the pro-vaccination norm in the whole population66.
Previous literature consistently reported that trust in government was directly correlated with one’s vaccine uptake9,10,41. Our study found that trust in government is mainly connected with one’s vaccination decision through vaccine confidence attitudes. Specifically, the verbal reasons of vaccine hesitancy suggested that low trust in government was a frequently mentioned reason for refusing the primary vaccine doses uptake among the youngest people (aged 18–24).
This study has both theoretical contributions and practical implications for informing more efficient vaccination programme in the future. One recent systematic review including 47 studies concluded that there are multiple determinants underlying COVID-19 vaccine hesitancy involving individual vaccine confidence beliefs, trust in authorities, self-efficacy, information influence, emotional state (i.e., fear and anxiety), and social influence67. A strength of the current study is that it considered the complexity of vaccination decisions through a network lens, which allowed us to depict the complex interactions among multiple determinants associated with vaccination uptake. The included determinants can be mapped onto multilevel: individual-level (i.e., COVID-19 vaccine confidence, trust in government, demographics), community-level (i.e., residential community vulnerability level), and wider contextual level (pandemic-related situation evolution such as COVID-19 report cases and death numbers). The various data sources enable us to construct relatively comprehensive models to understand vaccination uptake. Another strength of this study is the dynamic perspective in investigations of COVID-19 vaccination. This is especially relevant as pandemic circumstances involved constant changes of multiple contexts including disease incidence in the community, media focus, policies, control measures and associated public risk perceptions. The dynamic view and investigations warrant a more accurate picture of real-time public concerns and agenda on COVID-19 and its vaccines, thereby providing timely insights and instructions for effective risk communication and vaccination promotion.
Our study has several limitations. One limitation of this study is that in the later stage of the pandemic when the society gradually returned to new normalcy, we did not measure the full set of variables at bi-weekly basis in P4, which resulted in only two weeks’ data included in this period. However, the sample size in each survey week remained sufficient to estimate population characteristics (see Methods). Second, vaccination uptake was self-reported. Despite this, we found a high correlation between the self-reported vaccine uptake rates in our surveys and the actual vaccine uptake rates reported by the government (r(15) = 0.995, p < 0.01; see Supplementary Table 4), indicating that the self-reported vaccination uptake was a reliable indicator of actual vaccination uptake. Third, the repeated cross-sectional study design does not allow us to establish causal relationships of determinants with vaccine uptake. However, our study design answers the relative importance of determinants and is suitable for surveillance on vaccine uptake among the public throughout the vaccination campaign period. It supplemented the shortcomings of high attrition rates and costly efforts in maintaining the cohort across consecutive time points. Fourth, our networks did not exhaustively include all variables that were associated with vaccination uptake which contributed to the relatively low predictability of the networks for vaccination uptake. Similar studies have reported explained variances ranging from 10–78%68–71. However, those studies used vaccine intention instead of vaccine uptake as the main outcome. Comparatively, our study showed a similar explained variance of ~ 13% as another study that reported 15% of explained variance in vaccine uptake72.
Hong Kong’s experience in meeting the challenges of the COVID-19 pandemic has implications for future vaccination campaign against a pandemic in other regions of the world except when contexts are highly divergent. First, communications should highlight and address the salient attributes of the vaccine concerns dynamically. While communicating about vaccine safety is important at the early stage of the vaccination campaign, reinforcing the public's confidence in vaccine effectiveness should be prioritized for promotion of booster dose uptake. This can be done by giving timely feedback on how vaccine uptake helps to reduce people’s risk of infection or disease complications. Second, there could be complacency psychology and illusory optimism due to overconfidence in the effectiveness of non-pharmaceutical preventive behaviours, which could dampen motivation for vaccination when the vaccines become available. This is particularly the case when the disease incidence in the community is kept at a low level with the implementation of stringent social distancing measures. Vaccination campaigns should highlight the unique contribution of vaccination uptake, the potential societal costs of prolonged social distancing measures, and the importance of mitigation rather than containment at the later stage of the pandemic. Third, it is possible to leverage the early innovation adopters including the better-educated and married adults to make their vaccination decision more visible and positive to other wait-and-see groups. Fourth, it is important to establish trust in the public to promote vaccine uptake by enhancing people’s confidence in the COVID-19 vaccines. A possible approach to establish trust is through partnerships with influential figures such as political figures of various political ideologies to reduce hesitancy and mitigate the polarization of vaccines73. Fifth, older people were identified as the most hesitant group to take a novel vaccine, possibly attributing to the contexts in Hong Kong. Interventions should specifically focus on older adults and persons with chronic conditions to reduce their vaccination concerns. A potentially effective approach is to leverage the doctors’ longstanding relationship with older patients to clarify the safety of COVID-19 vaccines and address their concerns about their weak physical function to endure the vaccine side effects39,44. Overall, future vaccination campaigns should timely identify and respond to the various determinants of vaccine uptake by periodically reviewing the evolution of the pandemic to mitigate potential loss brought by people’s unnecessary delayed vaccination decisions.