The barriers, facilitators and association of vaccine certificates on COVID-19 vaccine uptake: a scoping review

Background Globally, COVID-19 vaccines have proven to be instrumental for promoting population health by reducing illness from SARS-CoV-2. Vaccine certificates emerged as a potentially promising solution for encouraging vaccination and facilitating the safe reopening of society, however, they were controversial due to criticisms of infringing upon individual rights. While there is extensive literature describing the ethical, legal, and public health implications of vaccine certificates, there is currently a gap in knowledge about the association of vaccine certificates on vaccine uptake during the COVID-19 pandemic and barriers and facilitators to their use. Objectives The objectives of this scoping review are to (i) describe the existing literature on the association of vaccine certificates on the rates of COVID-19 vaccine uptake across several countries and (ii) describe the intrinsic and extrinsic barriers or facilitators that moderate this relationship. Methods We conducted a scoping review based on PRISMA Extension for Scoping Reviews (PRSIMA-ScR) guidelines. We searched three bibliographic databases (APA PsychInfo, Embase Classic + Embase, OVID-Medline) and preprint severs during the first week of July 2023. Three reviewers independently screened the studies based on pre-specified eligibility criteria and performed quality assessments of the primary literature and data extraction. Results Sixteen studies met the inclusion criteria. 14 or these were surveys and 2 were modelling studies. The majority documented that vaccine certificates were significantly associated with increased rates of COVID-19 vaccine uptake (n = 12), motivated by factors such as travel/employer requirements, influence from the government/peers, and trust in the safety, efficacy, and science behind COVID-19 vaccines. Three studies had non-significant or mixed findings. Only one study found a significant decrease in COVID-19 vaccine uptake, motivated by pervasive distrust in the QR code-based system of digital vaccine certificates in Russia. Quality of survey studies was generally high. Conclusion Our findings provide insights into the existing literature on vaccine certificates association with vaccine uptake in several different jurisdictions and barriers and facilitators to their uptake. This information can be used to guide future examinations of the implementation of vaccine certificates and more effective implementations. Supplementary Information The online version contains supplementary material available at 10.1186/s12992-023-00969-y.


Introduction
Globally, governments implemented public health measures, including quarantine/stay at home orders, social distancing, lockdowns and closures of various social or commercial venues, travel restrictions, vaccination, and more, to help mitigate the impact of the SARS-CoV-2 virus [1,2].Vaccination is critical to protecting the public from the deleterious health consequences of COVID-19 infection and facilitating the reopening of the economy and society at-large [3][4][5][6].For the latter, vaccine certificates have been introduced as a hybrid approach of gating access to certain privileges (e.g., cross-border travel, return to work, access to certain shared public spaces and venues, etc.) under the condition of vaccination against COVID-19 [7][8][9][10].However, a major criticism, from a human rights and ethical perspective, is that vaccine certificates infringe individual rights and freedoms, particularly their right to bodily autonomy [11][12][13][14].Further, several upstream social determinants, including influence from friends, family, sources of information, and more, impact one's willingness to vaccinate and obtain vaccine certificates [15][16][17].This issue is further complicated by the fact that different countries introduced different types of vaccine certificates, using different approaches, and at different timelines [18,19].For example, it was found that in certain regions, such as the European Union, vaccination certificates served as a means to gate international travel.However, in China and the United States of America, they were more commonly used for gating access to activities of daily life within the country.In Canada, India, South Africa, Korea and the United Kingdom, vaccine certificates were used for gating access to activities of daily life and international travel [20].Altogether, it is not yet clear how vaccine certificates may be associated willingness to vaccinate and whether these effects vary across various settings and timelines.To address this gap in the literature, we have conducted a scoping review to investigate the association between vaccine certificates and willingness to vaccinate against COVID-19 and barriers and facilitators to their impact.

Objectives
The objectives of this scoping review are to (i) describe the existing literature on the association of vaccine certificates on the rates of COVID-19 vaccine uptake across several countries and (ii) describe the intrinsic and extrinsic barriers or facilitators that moderate this relationship.

Methodological approach
We conducted a structured scoping review in accordance with PRISMA Extension for Scoping Reviews (PRISMA-ScR) guidelines to identify and describe both peer-reviewed and grey literature within the topic of COVID-19 vaccine certificates and COVID-19 vaccine hesitancy.
A scoping review was conducted given the expected heterogeneity of the primary data.Implementations of vaccine certificates varied significantly across the globe limiting the ability to draw strong conclusions from a formal evidence synthesis [21].However, substantial value would be garnered from capturing the range and breadth of the studies on this important intervention.
We followed the Preferred Reporting Items for Systematic reviews and Meta-Analyses extension for Scoping Reviews (PRISMA-ScR) Checklist (Appendix 2).

Information sources and search strategy
Three bibliographic databases (APA PsychInfo, Embase Classic + Embase, OVID-Medline) were searched for published, peer-reviewed literature, and three repositories (Medrxiv, Biorxiv, L•OVE) were searched to identify pre-print records on the first week of July 2023 for articles related to Covid-19 and vaccine certificates (search terms are included in Appendix 1).The search strategy was co-developed and executed by an experienced medical librarian.A detailed description of the search strategy, including combinations of MeSH terms, can be found in supplementary document 1.The inclusion and exclusion criteria in this study can be found in supplementary document 2.

Eligibility criteria
For articles to have been included in this review, they must have met the following criteria: • Examined the general adult population rather than special/vulnerable populations.
• Included discussion of COVID-19 vaccine certificate characteristics (or synonyms such as immunity passports, green passes, proof of vaccination, etc.) • Included discussion of participants' willingness to receive COVID-19 vaccines (acceptance, delay, ambivalence, hesitancy, etc.) • Evaluated the potential role of COVID-19 vaccine certificate on willingness to vaccinate • Available in English • Considered primary research

Selection of sources of evidence
Titles, abstracts, and relevant full texts of retrieved records were screened by three independent reviewers (DZ, JA, MT) based on pre-specified inclusion and exclusion criteria (Tables 1 and 2).Any conflicts that arose during screening were resolved by a neutral third reviewer (SSM, and MS) who was not involved in the initial review of the papers.

Data charting process and items
Included full text articles that remained after screening then underwent data extraction by 3 reviewers (DTZ, JA, MT), with 2 other independent reviewers performing verification (SSM, MS).Preprint studies remaining after screening were updated with the final peer-reviewed publication if available.
Information was collected and inputted into a data extraction form with prespecified categories.Data extraction endpoints included reference details (author, publication year, study design, location, period of data collection), study population characteristics (demographic information, proportion of vaccinated/unvaccinated), methods of recruitment and assessment (surveys, scales, interviews, etc.), details about the intervention/experimental design (if applicable), theoretical frameworks/models used, vaccine passport/certificate characteristics (types of vaccines, digital technologies used, public attitudes/opinions towards vaccine certificates, reason(s) for seeking a vaccine certificate), and vaccination intention (acceptance, delay, ambivalence, hesitancy).The 3 C's Model of Vaccine Hesitancy was used to categorize barriers to vaccination [21].

Analysis, synthesis, and presentation of results
Studies were analyzed according to article characteristics (i.e., article type such as qualitative, survey, quantitative modelling, etc.), assessment of themes and subthemes, and various elements of content, and the observed association of vaccine certificates on willingness to vaccinate.Articles were then grouped based on study design (i.e., observational, modeling, experimental, and qualitative designs), barriers and facilitators of willingness to vaccinate, and overall association of vaccine certificates on the rates of vaccine uptake.
The quality of all included surveys was assessed using 7 different criteria [37]. 1) Was a clear research question posed?2) Was the target population defined, and was the sample representative of the population?3) Was a systematic approach used to develop the questionnaire?4) Was the questionnaire tested?5)Were questionnaires administered in a manner that limited both response and nonresponse bias?6)Was the response rate reported, and were strategies used to optimize the response rate?7) Were the results clearly and transparently reported?[38].

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 fulltext 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).

Observational 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 students 31 .

Table 1 Eligibility criteria
The following articles were included: Examined the general adult population rather than special/vulnerable populations Included discussion of COVID-19 vaccine certificate characteristics (or synonyms such as immunity passports, green passes, proof of vaccination, etc.) Included discussion of participants' willingness to receive COVID-19 vaccines (acceptance, delay, ambivalence, hesitancy, etc.) Evaluated the potential role of COVID-19 vaccine certificate on willingness to vaccinate English language full text available Primary research (e.g., observational, modeling, experimental, and qualitative studies) The following articles were excluded: No discussion of COVID-19 vaccine certificates (e.g., studies that broadly mentioned "vaccine mandates" without specifying vaccine certificates or synonyms) No discussion of COVID-19 vaccine intention or uptake Studies that generally described public opinions and attitudes on COVID-19 certificates and/or hesitancy but did not evaluate their interaction/ association No English language full text available Review-type or non-empirical studies (e.g., commentaries, editorials, opinion letters, etc.)The effect of vaccine mandate announcements on vaccine uptake in Canada: An interrupted time series analysis [36] N/A, no survey was used in this study and instead a time series analysis was conducted 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].

Extrinsic 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 shortand 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, 24-26, 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 [22-24, 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 [26,27,32,33], such as reopening access to various shared public spaces and social events, entertainment venues, religious venues, school venues, restaurants, not needing to use face masks, and more.

Intrinsic facilitators to vaccination
Trust was a recurring theme in efforts to increase willingness to vaccinate.We identified three main aspects of trust: (a) trust and confidence in the safety and quality of COVID-19 vaccines were cited in two studies [22,23], with a small number of participants describing how, if currently available vaccines did not meet their safety or quality expectations, then they will wait until a different or foreign-produced vaccine is made available; (b) trust and confidence in the efficacy of COVID-19 vaccines were cited in two studies [22,26], with many vaccine acceptors believing that vaccination is the most effective strategy to end the COVID-19 pandemic; and (c) trust in COVID-19 vaccine research was cited in one study [24] (e.g., although some vaccine acceptors were concerned at the speed at which COVID-19 vaccines had been developed, they rationalized this in terms of science being more advanced nowadays and having more rigorous scientific and financial focus on vaccine development during the COVID-19 pandemic).A desire to protect themselves and others were also recurring themes that predicted increased vaccination uptake.Specifically, (a) the desire to protect themselves was cited in two studies [24,34], driven by the perception that COVID-19 is a serious illness for which vaccines could reduce the onset, severity, and potential sequelae or complications; (b) the desire to protect family and friends from COVID-19 transmission was cited in one study [32]; and (c) the desire to protect society at-large was cited by two studies [24,26], driven by a perceived civil responsibility to contribute to herd immunity and protect others in society.Convenience and accessibility of vaccine clinics increased willingness to vaccinate in two studies [24,25].Finally, perceived moral responsibility was also evaluated, although none of the included studies mentioned this theme.
There three most frequently referenced countries were: France (vaccine uptake increased by 8-13 percentage-points (p.p.) associated with implementation of vaccine certificates), Germany (vaccine uptake increased by 4.7-6.2percentage-points) associated with implementation of vaccine certificates), and Italy (vaccine uptake increased by 9.7-12 percentage-points associated with the implementation of vaccine certificates) [28][29][30].Mills et al. [30] corroborate these findings for France and Italy, consistently demonstrating a statistically significant increase in COVID-19 vaccine doses at two timepoints: 20 days prior to the implementation of vaccine certificates in those countries (in anticipation of their implementation), with effects lasting up to 40 days postimplementation.In Canada [34], reported that the implementation a vaccine passport resulted in a 39% increase in vaccine uptake [34].Maquiling et al. [36] also reported that in Canada six out of ten provinces saw a statistically significant increase in vaccination following the implementation of vaccine passports [36].The average increase within these six provinces was found to be 6.13 p.p [36].It was also found that the implementation of a vaccine passport resulted in increasing the vaccination rate by 13.98% in Poland, and 19.75% in Lithuania [35].These increases were seen in the youngest age group (18-24 years of age) [35].More detail about each age group is found in Table 5.Interestingly, this trend was inconsistent for Israel: Mills et al. [30] found a small but statistically significant decrease in vaccine uptake at the 20 days pre-implementation period, followed by a large statistically significant increase in uptake at the 40 days postimplementation period.These trends may be moderated

Intrinsic facilitators to vaccination
Trust/confidence in the quality/safety of COVID-19 vaccines 2 (12.5%) [23,25] Trust/confidence in the efficacy of COVID-19 vaccines 2 (12.5%) [23,24] Trust/confidence in the science behind COVID-19 vaccines 1 (6.3%) [33] Desire to protect themselves 2 (12.5%) [26,35] Desire to protect family and friends 1 (6.3%) [26] Desire to protect society 2 (12.5%) [24,26] Perceived moral responsibility 0 (0.0%) N/A Convenient/accessible vaccine clinics 2 (12.5%) [22,26] by the population characteristics and implementation strategy of the vaccine certificates: Okamoto et al. [32] documented that vaccine certificates implemented for facilitating "travel across prefectures" was associated with the greatest increase in vaccine uptake (10 p.p. (percentage points)), followed by vaccine certificates implemented for "going out without masks" (7 p.p.), "dining out after 8 pm" (6 p.p.), and "joining social gatherings or events" (4 p.p.).In their subgroup analysis, these effects tended to be amplified among vaccine-ambivalent survey respondents (15 p.p., 10 p.p., 9 p.p., and 6 p.p., for vaccine certificate-mediated travel, going out without masks, dining, and social events, respectively), although they tended to be diminished among vaccine-hesitant survey respondents (3 p.p., 4 p.p., 3 p.p., and 1 p.p., for vaccine certificate-mediated travel, going out without masks, dining, and social event privileges, respectively).Only one study (6.3%; n = 1/16) found that COVID-19 vaccine certificates significantly associated with a decrease in vaccination [23], and three studies (18.8%; n = 3/16) reported mixed or non-significant findings (Table 5) [25,27,39].Notably, Boguslavsky et al. [23] documented that, among Russian survey respondents, 26.59% may avoid COVID-19 vaccines if QR code-based vaccine certificates were to be introduced.The primary concern was not the idea of vaccine certificates itself, but rather, the low receptivity of the Russian population to the proposed digital QR code-based system of public health surveillance: Boguslavsky et al. [23] found that approximately 94% of individuals who refused to be vaccinated and approximately 87% of their whole sample was opposed to the introduction of a QR codebased approach to digital vaccine certificates in Russia.Boguslavsky et al. [23] proposes two main reasons for this.First, there are prevalent concerns among the Russian public that QR code-based vaccine certificates will lead to potential segregation of the Russian public (those that do not have them will be "castaways" in society); denied access to shops, markets, work, various social venues, transportation, and other public and private sectors; perceived endangerment of digital privacy, lack of financial protection (especially with respect to online banking systems), and potential fraud related to QR codes; etc [23].Second, the Russian government and media may have also fostered anti-vaccination sentiments and creating negative views of COVID-19 vaccines and vaccine certificates over two critical periods (August 23-October 20, 2021; and November 25-January 15, 2022) during which the government strongly pushed for the introduction of a QR code-based system of digital vaccine certificates and widely promoted this across Russian news media platforms [23].However, it is important to note that the general Russian population has very low trust in their government and in turn, the high number of individuals who refused to be vaccinated point to the importance of trust and social capital as facilitators in the implementation of vaccine certificates.

Discussion
To the best of our knowledge, this is the first scoping review that overviews the association between vaccine certificate implementation and willingness to vaccinate on a global scale and barriers and facilitators to their use.Multiple novel interventions were implemented during the pandemic and researchers have attempted to study their impact.There has been substantial variability in the quality of this research and the subsequent evidence produced [21,41].
Given the impact of vaccine certificates and their potential for future use a scoping review provides a broad overview of the emerging literature on this topic.
In our scoping review, the majority of studies (75%; n = 12/16) found that vaccine certificates had a positive association on the rate of vaccine uptake across multiple countries (Table 5).This positive relationship was most commonly observed for three European countries: France, Germany, and Italy (Table 3) [24,[28][29][30].Interestingly, only one study [23] in this review linked the implementation of vaccine certificates to a reduced COVID-19 vaccine uptake (Tables 3 and 5).Boguslavsky et al. [23] propose that this was primarily due to the QR code-based platform that the Russian government was planning to use for their digital COVID-19 vaccine certificates.
Our findings need to be taken into context given the heterogeneity of settings and implementation strategies for vaccine certificates.We attempted to characterize some of this heterogeneity by describing internal and external barriers and facilitators to the impact of vaccine certificates.
The intrinsic facilitators that we identified in (Table 4) reflect "carrot"-type strategies to promote vaccine uptake by disseminating information about the safety and efficacy of COVID-19 vaccines, as well as appealing to the public's social responsibility to protect themselves, their family and friends, and society at-large.These intrinsic facilitators to vaccination were cited less frequently in our review compared to the extrinsic facilitators, although the literature emphasizes their important role in the implementation of vaccine certificates and promoting vaccine uptake.Notably, in our review, a study [39] documented that effective messaging about the safety, efficacy, and medical or hedonistic benefits of COVID-19 vaccines has the potential to mitigate vaccine-hesitant attitudes and promote vaccine uptake; although, these findings were non-significant.Steinert et al. [39] suggest that widespread conspiracy beliefs and low health literacy undermines and reduces the effect  Mixed findings or no effect (n = 3) [25] Predicted impact of COVID-19 vaccine certificates on vaccine uptake (based on logistic regression analysis) (in Israel) OR: 1.12 (95% CI: 0.82-1.55)Vaccine certificates had a positive impact on vaccine uptake, but nonsignificant of this messaging, which could serve as future targets for public health interventions and should be considered when implementing vaccine certificate and vaccine campaigns.These findings were corroborated by other studies, which suggested that messaging and framing designed to garner increased trust in the safety/efficacy of the COVID-19 vaccines and better understanding of the potential benefits of vaccines for population health and the economy/society atlarge -in other words, framing COVID-19 messaging to better appeal to the intrinsic facilitators that we identified in (Table 4)-appeared to be instrumental for the effective implementation of vaccines and vaccine certificates [42][43][44][45].
Ultimately, our findings support the existing literature about the importance of incorporating framing and messaging about these intrinsic facilitators during COVID-19 vaccine and vaccine certificate campaigns.Second, the external facilitators that we identified in (Table 4) reflect "stick"-type strategies to promote vaccine uptake by leveraging vaccine certificates as a "gatekeeping" system to restrict access to various social, work, and travel privileges for individuals lacking proof of vaccination.Our review found that travel (both domestically and globally) and work privileges contingent upon having vaccine certificates were among the most frequently cited facilitators to COVID-19 vaccination, which is consistent with the surrounding literature [20,46,47].A global survey of 23 countries reported in July 2021 that there was generally strong support for travel and work mandates contingent on proof of vaccination, with an average of 74.4% and 62.3% of respondents agreeing with requiring vaccine certificates for international travel and employment, respectively [48].Support for these mandates was lowest in Russia (52.5% and 30.9% of Russian respondents supported travel and work mandates, respectively) [48] which is consistent with the findings in our review.Interestingly, this survey found that the three European countries for which we observed a strong positive impact of vaccine certificates on vaccine uptake -France, Germany, and Italy -had below-average support for travel and work mandates contingent on proof of vaccination: only 66.6%, 66.3%, and 73.0% of French, Germany, and Italian respondents supported travel mandates contingent upon proof of vaccination, respectively, and only 49.3%, 40.3%, and 57.6% of French, Germany, and Italian respondents supported work mandates contingent upon proof of vaccination, respectively [48].
We did not systematically examine the impact of vaccine certificate introduction on other end points.However, several of our included studies did examine the positive impact of vaccine certificate introduction on the economy and on reducing health care burden.Future studies should systematically examine the potential association of vaccine certificates on health and economic outcomes.Future studies should also explore how mechanisms of implementation affected the impact of vaccine certificates.

Strengths and limitations
This study has numerous strengths.First, our evidence base included a wide-ranging set of study designs (observational, modeling, experimental, and qualitative studies) and was not limited to only peer-reviewed articles (our search strategy included preprint servers such as Medrxiv and Biorxiv).Second, our search strategy was not constrained to specific settings or populations, therefore, enabling us to evaluate vaccine certificate and vaccination campaigns across multiple countries at a global scale and make cross-national comparisons.Third, we evaluated the quality of the studies that employed a survey using a standard instrument.
There were also several limitations in this study.First, the pandemic created multiple natural experiments that provided an opportunity for evaluation.The variability in the quality of these evaluations limits their potential generalizability of their findings.We found, in our analysis, that the quality of the studies was generally good.However, there is substantial variability to how vaccine certificates were implemented and the local culture that contributes to their impact that limits the generalizability of our findings.We attempted to capture some aspects of this through our analysis of barriers and facilitators but there are many other confounding variables that impact the relationship between vaccine certificate implementation and vaccination rates.A general consistency of effect across multiple jurisdictions does suggest potential for improvement in vaccination rates.However, substantial heterogeneity and potential for co-interventions limits the ability to make any causal assessments.As such, the results of this review should be viewed as exploratory and hypothesis generating.Second, this review was limited to articles published in the English language.Future studies should aim to include articles published in other languages, to ensure a comprehensive evaluation of the impact of vaccine certificates on willingness to vaccinate, which is especially important given the global scale of this issue.Future studies should also systematically examine the impact of these interventions on mortality and the economy.Third, there is no standard tool for reporting on the survey studies included in our analysis [38].Fourth, there is a potential for publication bias, where studies that showed no effect of vaccine certificates were not submitted for publication.

Conclusion
Achieving high vaccine coverage during the COVID-19 pandemic was crucial to reducing the transmission of SARS-CoV-2 and mitigating the impact of the pandemic on the healthcare system and society at-large.Within this rapidly evolving and transitional period, the ability to track (e.g., using vaccine certificates) those who have been vaccinated, versus those who refused or delayed vaccination, was potentially valuable for governments and public health officials to make evidence-based policy decisions about how to safely return society to normalcy.However, this approach has not been without controversy and had potential negative effects.Our scoping review provides insights about the various facilitators and barriers to COVID-19 vaccination related to vaccine certificates, as well as an overview of the observed impacts of vaccine certificates on COVID-19 vaccine uptake across multiple countries.These findings reflect important considerations for future implementation of vaccine certificates for later stages of the current pandemic as well as other emergent public and global health threats.

Table 3
General characteristics of included studies

Table 5
Impact of COVID-19 vaccine certificates on willingness to vaccinate