We completed a national, cross-sectional survey to evaluate Canadians’ intentions to receive booster (third and annual) COVID-19 vaccine doses, at a time when recommendations on additional COVID-19 vaccine doses were evolving. We found that 70% of all respondents, and 78% of those who had completed a two-dose series, indicated they would accept a third dose of COVID-19 vaccine. The proportion of two dose recipients willing to receive a third dose of vaccine was somewhat lower than findings from surveys conducted in other countries during the same time period (79–95.5%) [8,9,10, 21,22,23,24]. However, more than 15% of respondents in our study reported they remained undecided about receiving a third dose, including over 10% of those who had not yet received any COVID-19 vaccines. Therefore, even in a population with high primary series coverage (almost 90% in our study), there is potential to improve uptake of both primary and additional doses.
Acceptance of an annual dose (65%) was lower than third dose acceptance (70%), indicating that, if COVID-19 vaccines are required on an ongoing basis, uptake may decline over time. Factors associated with accepting an annual COVID-19 vaccine were similar to third dose intentions; however, those of male gender, lower income, and visible minority identity appeared less likely to accept annual vaccination. Given that almost 20% of respondents remain undecided about annual COVID-19 vaccination, and the important influence of government and healthcare provider recommendations on vaccination decisions, acceptance is likely to be improved by clear guidelines around annual dose vaccinations, if or when they are required.
We found that many respondents with established risk factors for COVID-19 morbidity and mortality (e.g., higher age, pre-existing chronic conditions) were significantly more likely to report acceptance of additional COVID-19 vaccine doses. The relationship between increasing age and additional dose acceptance is similar to findings on third dose acceptance from other countries [9,10,11, 22, 24]. Literature indicates mixed results on the impact of a pre-existing chronic condition on uptake of additional COVID-19 vaccines [8,9,10,11, 23], though we defined chronic conditions more narrowly, as those at highest risk for COVID-19 morbidity/mortality. People who live with disability may also be at increased risk for both infection and negative disease outcomes, though risk varies with both type and severity of impairment [25, 26]. However, self-reported disability was not associated with higher odds of additional dose acceptance in our study; instead, these respondents were more likely to be undecided about additional dose acceptance. For some respondents, this could be related to concerns around vaccine service accessibility; approximately 8% of survey respondents indicated that improved vaccine service accessibility for all abilities would be important for making vaccination easier. To ensure those who plan to receive additional vaccine doses are able to act on their intentions, and facilitate uptake in those who are undecided, vaccine services must consider the needs of all abilities , and involve those who live with disabilities in service planning . Improved understanding of potential barriers to vaccine uptake in this population is also required .
Our results indicated both concerning and encouraging relationships between sociodemographic factors and additional vaccine dose acceptance. Concerningly, lower socioeconomic status (i.e., lower household income and educational attainment) and visible minority status were related to less positive intentions toward receiving additional COVID-19 vaccines, though results were not always statistically significant. These findings are consistent with third dose vaccination intentions in the United Kingdom , though other research suggests socioeconomic status has little impact [11, 30]. Encouragingly, other populations thought to be at risk for undervaccination (e.g. newcomers, first language not English or French, Indigenous identity) in Canada , reported third dose vaccination intentions that were not significantly different from the general population or more positive. Results may reflect the success of specialized public health measures within these communities, minimizing previously existing health disparities . However, some inequities remain, requiring further investigations.
It is also concerning that neither parental status nor employment as a healthcare worker was associated with more positive intentions around additional COVID-19 vaccine doses, as both populations have significant potential to influence the health and vaccination status of others. Parental intention for self-vaccination is a significant predictor for COVID-19 vaccination intent for their children . Healthcare workers are consistently identified as important influences on vaccine uptake decisions, both here and in other literature , and those who are personally vaccinated are more likely to recommend vaccination to patients . However, it is important to acknowledge that there is significant heterogeneity around vaccine acceptance within both parents  and healthcare workers , though stressing altruistic reasons for receiving additional doses (e.g. protecting children, protecting patients) may be effective for both [20, 35].
Unsurprisingly, both previous COVID-19 vaccination and seasonal influenza vaccination intention were significant predictors of favorable intentions towards additional doses. However, previous COVID-19 vaccine receipt did not uniformly predict acceptance of additional doses; more than 20% of those who had received two doses, and 60% of those who had received only one dose, did not accept additional doses. Our results indicate that COVID-19 disease history, and motivation for receiving initial doses, may explain some of that difference. In our study, those who had experienced COVID-19 disease were more likely to refuse additional vaccine doses. This relationship between previous infection and future dose refusal is not consistently found in the literature [8, 10, 11, 36], though timing of infection relative to vaccination may be important [10, 11]. Given that the evidence around vaccination after SARS-CoV-2 infection is still emerging , confusion over the need for, and timing of, vaccination post-infection is likely. As the number of people who have experienced SARS-CoV-2 infection continues to grow, clear messaging around the effectiveness of additional vaccine doses will be required. We also found that vaccination mandates or restrictions were the main motivation for previous COVID-19 vaccine receipt for significant portions of both the refusal (almost 50% of 419 respondents) and undecided (almost 20% of 837 respondents) groups, compared to less than 3% of the acceptance group (4179 respondents). Thus, while coercive measures may positively influence initial vaccine receipt, vaccination experience is not enough to overcome hesitancy toward additional vaccine doses. This supports observations that initial hesitancy about COVID-19 vaccination appears to persist, even after two-dose completion . Other factors, including belief that sufficient protection is acquired through a two-dose series [8,9,10], experiencing side effects from previous doses [8, 37], and concerns over receiving additional vaccine doses while other countries are struggling to secure first doses may also play a role .
While our results indicate that pharmacy-based delivery and drop-in appointments may increase COVID-19 vaccine uptake, offering co-administration of the COVID-19 vaccine with influenza or routine vaccines may not have the positive impact on uptake that is expected. In general, respondent attitudes regarding vaccine co-administration mirrored their intentions for additional COVID-19 vaccine doses; however, while almost 95% of the refusal group (N = 419) did not agree (disagree or neutral) with co-administration, only around three quarters of the acceptance group (N = 4179), and less than 17% of the undecided group (N = 837), agreed with co-administration. Thus, while more than 60% of all respondents agreed with COVID-19 vaccine co-administration, hesitancy towards co-administration is greater than hesitancy towards the COVID-19 vaccine alone. While there is some evidence to support this finding , one study found that a combination influenza/COVID-19 vaccine had higher acceptance than a COVID-19 vaccine alone . More work is required to understand whether and how acceptance of co-administration differs from acceptance of combination vaccines, and whether these options differentially impact uptake across populations. Providing public choice around vaccine co-administration will be important to avoid negatively impacting future uptake.
Strengths and limitations
Our study benefits from a large sample size that was representative of the Canadian population in age, sex, and region of residence. Another strength of our study was the targeted effort to include respondents of populations typically underrepresented in research and of particular focus for COVID-19 vaccination programs. However, there are a number of limitations associated with our sample that limit generalizability of our results. First, our sample was drawn from a pre-existing panel with internet access who could communicate in English or French. Second, while we sought to recruit a heterogenous sample, and weighted responses to selected population proportions, we were unable to account for all population variables. For example, the education level of our participants was higher than in the general Canadian population (51.6% of our sample reported university education, compared to 31.6% of the Canadian population ). Finally, the cross-sectional design of our study prevents identification of any trends in additional vaccine dose acceptance. A number of changes to additional vaccine dose recommendations have been made in Canada since the time of our data collection, which may impact COVID-19 vaccination acceptance.