Table 1.

Summary of key findings organized by predictor and by population.

Predictors of Vaccine Hesitancy	Population	Key Findings
	Older and younger adults in China	Reasons for vaccinating varied by age with susceptibility to disease predicting influenza vaccine uptake in older adults and perceived effectiveness predicting vaccination in younger adults [17].
Age	Adults and young adults in the U.K.	Vaccine attitudes differed across age groups, with adults 50 to 59 reporting more confidence in vaccines than adults 20 to 29 [18]
	Parents in the U.S.; Parents in Malaysia	Younger parental age was associated with vaccine hesitancy as measured by the Parent Attitudes about Childhood Vaccines Questionnaire [19,20]
	Adults in the U.S.; Adults in Germany	Increased social media use contributed to negative vaccine attitudes, and centennials and millennials were the primary users of social media platforms [21–23]
Implications: Observed age group differences in vaccine hesitancy have connotations for developing effective vaccination campaigns. Findings of increased hesitancy among young adults indicate a need for communications targeted at those cohorts (i.e., sharing evidence-based content over social media).
Socioeconomic Status	Families in the U.S. with children under 6	Families who refused vaccines for their children were more likely to reside in higher-income communities than families who vaccinated [24]
	Parents of dependent children in the U.S.	Parents of under-immunized children were largely middle-class and college-educated [25]
	Families with infants aged 12–23 months in West Africa	Standard of living was associated with the vaccination status of children, with well-off families being more likely to have children who were vaccinated than families living in poverty [26]
	Postpartum mothers in the U.S.	Low income was associated with less trustful attitudes toward vaccination [27]
Implications: Vaccine campaigns should not be reduced to targeting communities based on SES, as vaccine hesitancy is an issue across social classes. Future research is required to explain why income relates differently to vaccination behaviours and attitudes in different samples.
Education	Canadian parents	Higher educational attainment related to fewer concerns over vaccine safety, according to results of the National Childhood Immunization Coverage Survey [30]
	Greek parents of 6-year-old children	Paternal education of high school or higher predicted age-appropriate immunizations of children [31]
	Families in the U.S. with children under 6	Families who refused vaccines for their children tended to reside in communities with higher educational attainment, based on census data [24]
	Mothers from several low- to middle-income countries	Education was unrelated to vaccine hesitancy in a multi-ethnic sample, using the WHO Vaccine Hesitancy Scale [34]
Implications: Educational campaigns may be effective means for addressing knowledge gaps and correcting misinformation. Public education should explain the mechanisms of action of vaccines using everyday terminology and plain language. Education on the history of transmissible disease and the role of vaccination programs in controlling outbreaks will create pertinent conversations in combatting the COVID-19 pandemic. Vaccine uptake strategies geared toward individuals with adequate education and health literacy should depart from educational agendas and, instead, focus on perceived risks and fears.
Parental Status	British adults responding to an adapted WHO Vaccine Hesitancy Scale	Participants with young children experienced the most aversion to potential side effects and risks of vaccines [18]
	Parents in Malaysia	The most common reason for vaccine hesitancy among parents with young children was concern over side effects [20]
	Parents in West Africa; Parents in the U.S.; Parents in Greece	Family size was a consistent predictor of vaccine uptake, and families with 3+ children were more likely to refuse immunizations [26,31,41]
	Postpartum mothers in the U.S.; Parents in the U.S.	Parental vaccine decisions were strongly linked to health care provider recommendations. Parents who were vaccine-hesitant were more likely to have care providers who questioned the overuse of immunizations [25,27,42]
Implications: Findings highlight the importance of knowledge sharing between health care providers and parents. Parent-centred information on vaccines should be distributed by primary care providers in ways that are efficient and effective. Brochures, pamphlets, and web-based aids for parents are all evidence-based outlets shown to positively affect parents’ intent to vaccinate. Much of this research has taken place with parents of young children, representing a notable knowledge gap given that age guidelines are currently 12+ for the Pfizer/BioNTech vaccine and 18+ for the Moderna vaccine. a
Rurality	Parents in Greece	Longer travel time to vaccine administration sites has been cited as a barrier to being vaccinated [31]
	Mothers in rural and urban areas of China	Mothers from rural regions report significantly lower vaccination rates for their children compared to mothers in urban areas [43]
	Adults in the U.K.	Vaccine confidence was stronger among urban residents than individuals residing in rural areas in the U.K [18]
	Adults in the U.S.	Surveys completed in December 2020 revealed greater vaccine hesitancy among rural respondents than the general population, with 35% of rural participants indicating that they would probably not or definitely not get a COVID-19 vaccine [44]
Implications: Discrepancies in rural versus urban settings need to be addressed by efforts to boost vaccine confidence in rural regions and by attempts to decentralize vaccination clinics. Otherwise, geographical differences in vaccine hesitancy could lead to disparities in vaccination coverage and localized COVID-19 outbreaks for the foreseeable future.
Mistrust in Authority	Parents in Canada, the U.S., the U.K.	Mistrust in the medical profession predicted vaccine hesitancy in parents in Canada, the U.S., and the U.K [47]
	Parents in the U.S.	Individuals who lacked trust in their family physician were more likely to consult the Internet for advice on vaccinations, which, in turn, negatively affected vaccination attitudes and behaviours [48]
	Adults in the U.S.	Mistrust was a common reason for not planning to get a COVID-19 vaccine. Fifty-five percent of survey respondents stated they lacked trust in the government to ensure vaccine safety and effectiveness [44]
	Adults in the U.K.; Adults in Ireland	Adults who self-identified as vaccine-hesitant on a COVID-19 vaccine survey reported mistrust in authorities and a reluctance to obtain information from traditional sources [45]
Implications: One strategy for addressing mistrust is to share knowledge through relatable sources (i.e., peers). Other areas of public health, such as substance use prevention and intervention, have emphasized peer-led initiatives. Individuals experiencing mistrust in government or the health system may be more responsive to members of the public sharing their intentions to immunize or providing information they receive from credible sources. Incorporating a community voice and highlighting collaboration between experts, leaders, and peers could help build vaccine confidence among this population.
Disgust Sensitivity	Parents in the U.S., U.K., and Canada	Respondents who scored higher on global measures of disgust sensitivity were more vaccine-hesitant, as per scores on the Parent Attitudes about Childhood Vaccines Scale [47]
	Undergraduate university students in the U.S.	Positive associations have been found between pathogen disgust sensitivity and vaccine hesitancy among university students [52]
	Parents in the U.S.	Purity values (another measure of disgust sensitivity) were associated with high levels of vaccine hesitancy among parents. High-hesitancy respondents were over twice as likely to endorse strong purity values [53]
Implications: Associations between disgust sensitivity and vaccine hesitancy highlight a need to integrate broader themes into vaccine discussions. Disgust-sensitive individuals may respond better to content that uses technical and logical terms and avoids potential triggers for disgust, such as images of needles puncturing skin. Emphasizing the vaccine’s mechanism for building immunity as a natural bodily response may resonate with individuals with strong purity values.
Risk Aversion	Israeli Parents	Trait anxiety is closely linked to risk aversion, and individuals high in anxiety have been found to experience greater levels of vaccine hesitancy [55,57]
	Mothers with young infants	Qualitative research has provided insight into a preference for passive risk (i.e., not vaccinating) over taking a risk through active behaviour, in a phenomenon known as the omission bias [56]
	Adults in low- and middle-income countries	Studies using the WHO Vaccine Hesitancy Scale have found that a majority of people believe new vaccines carry more risk than older vaccines [34]
	Parents and caregivers in Taiwan	Fear and risk aversion can promote vaccination when individuals perceive the vaccine-preventable disease as being prevalent and/or dangerous [58]
Implications: Building a sense of safety among the public will be essential to addressing the outlined fears. Work is needed to ensure that transportation to vaccine clinics and the clinics themselves are organized in a way that limits the opportunity for transmission, which will help address some concerns. Accurate information around the outcomes and risks of the disease versus outcomes and risks of vaccination should be provided to encourage individuals to accurately calculate risk and make informed decisions. Public health communications must acknowledge that decision making is not purely cognitive but driven heavily by emotions, especially fear.

^a In Canada.