Selection of studies

Our search strategy initially identified 675 articles. After duplicates (n = 12) were automatically removed by Covidence, title and abstract screening resulted in the exclusion of 592 articles from 663 articles, and full-text screening resulted in the exclusion of an additional 55 articles. The remaining 16 articles are included in the manuscript. A comprehensive overview of the screening process is presented in a PRISMA flowchart (Fig. 1).

Fig. 1Article types and general characteristics

Of the 16 total studies included, there were four main types of study designs: observational (n = 8), modeling (n = 4), experimental (n = 3), and qualitative (n = 1) (Table 3).

Table 3 General characteristics of included studiesObservational studies

The eight observational studies were all based on a cross-sectional design and spanned eight different countries (i.e., Netherlands, Russia, France, Israel, China, Canada, Lithuania and Poland). Six studies focused on the general adult population within their respective countries [23, 25, 26, 34,35,36], whereas the others focused on specified subpopulations, including ethnic/racial minorities 30 and university students31. These studies involved similarly low rates of COVID-19 vaccination at baseline — namely, 2–14% [22], 17.28% [23] 10.1–13.5% [26], and 3–4% [35] — except for a study conducted immediately after the COVID-19 vaccines were made available, therefore, had no vaccinated participants at baseline [25], and a study that recruited only participants that had received their vaccination dose(s) at a specified vaccine clinic, therefore, all participants were vaccinated at baseline [24]. Another study had an 87% vaccination rate (defined as 2 doses) at baseline [34]. Another study had an average vaccination rate among all provinces at 82.01% at baseline [36]. Two studies used the Health Belief Model as their theoretical framework [24, 25], one study used the 5 C’s Model of Vaccine Hesitancy [34] and one used the 3 C’s Model of Vaccine Hesitancy [22].

Modeling studies

The four modeling studies primarily examined countries in Europe (e.g., France, Italy, Germany, Denmark, Switzerland, and the United Kingdom) as well as a few non-European countries (e.g., Canada, Israel, and the United States). Only one of these studies used a theoretical framework — innovation diffusion theory — which describes how innovations (particularly, vaccines) are spread and taken up; specifically, in this study, innovation diffusion theory was used to establish the counterfactual estimates [29]. Data on COVID-19 cases, deaths, hospital admissions, vaccination rates, and more, were collected from multinational databases (e.g., Our World in Data, European Centre for Disease Prevention and Control, Oxford COVID-19 Government Response Tracker, etc.) to generate the predictive models [27,28,29,30]. All four modeling studies examined trends in the general adult population in their respective countries and were not narrowed to specific subpopulations. The time period these studies took place ranged from one month — April 2021 [27]or December 2021 [29]— to several months in length, July 2021–October 2021 [28] or April 2021–November 2021 [30].

Experimental studies

The three experimental studies primarily examined Japan [32], the United States [31], and several European countries [39], and were all focused on the general adult population rather than any specific subpopulations. The theoretical frameworks used were the Health Belief Model [32] and Health Preferences Research (HPR) [31]. These experimental studies evaluated the effects of varying characteristics (varying levels efficacy, side effects, settings, presence of vaccine certificates, etc [32]), knowledge translation (messaging about COVID-19 risk reduction, vaccine certificates, and hedonistic or altruistic benefits [39]), and incentives (incentives such as access to travel, restaurants, social gatherings, and going out without masks) associated with vaccine certificates, and to what extent they influenced willingness to vaccinate. These experimental studies used various experimental designs such as a conjoint experimental design [32], a randomized control design [39] and a discrete choice design [31].

Qualitative studies

Only one qualitative study was captured [33], which focused on examining public attitudes towards COVID-19 vaccines in the United Kingdom. The main questions in the focus groups covered themes such as vaccination intention, perceptions on vaccine certificates, and other vaccine-related experiences and behaviors. The authors employed the Continuum of Vaccine Hesitancy Model as their theoretical framework, which treats willingness to vaccinate as a continuum between complete acceptance and complete refusal [33]. This study was conducted between March 2021–April 2021.

Quality assessment

Fourteen out of 16 studies in this review had used a survey. All 14 studies posed a clear research question, indicating a focus on specific objectives. 13 studies met the criterion of defining the target population and ensuring sample representativeness. 8 studies used a systematic approach to develop the questionnaire and 8 studies were found to have followed a systematic process to construct their survey instruments. In terms of administering questionnaires, 12 studies employed methods that aimed to limit both response and nonresponse bias, indicating an effort to collect accurate and unbiased data. Additionally, 13 studies reported their response rates and discussed strategies used to optimize response rates. All 14 studies presented their results clearly and transparently.

Barriers and facilitators to COVID-19 vaccination

(Table 4) describes major themes in motivation to refuse (“barriers”) or accept (“facilitators”) COVID-19 vaccination. These motivations were further categorized into external influences on vaccination, such as family, friends, community, and other structural influences (“extrinsic barriers and facilitators”) or intrinsic influences on vaccination, such as personal goals, values, concerns, and belief systems (“intrinsic barriers and facilitators”). The 3 C’s model of vaccine hesitancy was also incorporated into our analysis [40].

Table 4 Facilitators and barriers to vaccination uptakeExtrinsic barriers to vaccination

Privacy concerns were brought up in two studies [23, 33] with regards to themes such as fears of total digitalization, accumulation of digital information in government databases, possible fraud, lack of financial protection (e.g., some Russian banks have integrated digital vaccine certificates into online banking systems), protection especially for children who are issued digital vaccine certificates, and the perceived “Orwellian” nature of vaccine certificates. Technological concerns were mentioned in one study [23] and were closely related to privacy concerns, such as pervasive public distrust of the digital infrastructure underlying vaccine certificates (e.g., distrust of QR code system). Ethical concerns were discussed in two studies [23, 33] and centered around the idea that vaccine certificates, from a human rights perspective, restrict personal autonomy and freedoms such as gatekeeping access to many shared public spaces or social events, travel across borders, employment, and many other privileges. Lack of reliable sources of COVID-19 information, or exposure to COVID-19 misinformation and conspiracy theories, were mentioned in five studies [24, 26, 31, 33, 34] and discussed themes such lack of information about the safety of efficacy of COVID-19 vaccines, lack of information about the short- and long-term side effects of COVID-19 vaccines, and exposure to COVID-19 vaccine-related conspiracy theories or “echo chambers”. Finally, the lack of convenience and accessibility were cited in six studies [22, 24,25,26, 31, 33], such as barriers to accessing COVID-19 vaccine centers or the unavailability of specific brands (e.g., some are willing to accept particular vaccine brands, but not others).

Intrinsic barriers to vaccination

Distrust and lack of confidence in certain aspects of COVID-19 vaccines, or towards specific social institutions, were frequently mentioned themes. This distrust and lack of confidence were (a) directed towards government leaders in four studies [22, 23, 26, 33] with regards to themes such as vaccines and vaccine certificates serving as agents of social control; (b) directed towards public health or pharmaceutical agencies in two studies [22, 26] with regards to themes such as a lack of trust in the sources, manufacturers, and countries of origins of COVID-19 vaccines; (c) directed towards the quality and safety of COVID-19 vaccines in seven studies [22,23,24, 26, 31, 33, 39], focusing on themes such as concerns about potential adverse events from COVID-19 vaccines (particularly long-term side effects), belief that COVID-19 vaccines were developed too quickly and did not undergo sufficient safety testing, belief that COVID-19 vaccines contain harmful substances, a lack of trust in vaccine research and the refusal to be used as a “guinea pig” in vaccine research. Further, complacency was frequently cited as a barrier to vaccination. Specifically, complacency (a) with respect to the perception that COVID-19 is not a serious illness (e.g., “just like the flu”) and does not pose a threat to health and wellbeing was cited in three studies [22, 32, 39]; and (b) with respect to the perception that COVID-19 vaccines are unnecessary since alternative forms of precautions and protection are sufficient to prevent COVID-19 infection and sequelae (e.g., personal protective equipment, masks, natural immunity, and herd immunity) were cited in five studies [22, 24, 32, 33, 39].

Extrinsic facilitators to vaccination

Discourse about vaccine certificate-mediated privileges centered around travel and employment. Seeking COVID-19 vaccines and vaccine certificates to facilitate both regional and international travel were cited in four studies [22, 26, 32, 33]. Seeking COVID-19 vaccines and vaccine certificates to satisfy employer recommendations or mandates were cited in four studies [22, 24, 26, 27]. We also identified six external sources of influence regarding vaccination: (a) high levels of trust in the government and mandates facilitated vaccination in three studies [22, 26, 39]; (b) influence from the government via monetary incentives facilitated vaccination in two studies [34, 35]. (c) recommendations from friends or family to get vaccinated also predicted increased willingness to vaccinate in three studies [22, 24, 26]; (d) recommendations from physicians and other healthcare providers to get vaccinated led to increased vaccination in one study [26]; (e) influence from the media was not identified to be a facilitator to vaccination in any of the included studies; and (f) influence from other sources was mentioned in one study [26], which discussed the provision of medical absences to allow time for vaccination and relaxing mandatory post-vaccination isolation measures predicted increased uptake of COVID-19 vaccines. Accepting the COVID-19 vaccine to help reopen the economy and society was cited by four studies [26, 27,