The impact of mandatory COVID-19 certificates on vaccine uptake: Synthetic Control Modelling of Six Countries

BackgroundCOVID certification has been introduced, yet there are no empirical evaluations of its impact on vaccine uptake. MethodsMirroring an RCT, we designed a synthetic control model comparing six countries (Denmark, Israel, Italy, France, Germany, Switzerland) that introduced certification (May-August 2021), with 20 control countries. Our estimates provide a counterfactual trend estimating what would have happened in virtually identical circumstances if certificates were not introduced. The primary outcome was daily COVID-19 vaccine doses, with supplementary analyses of COVID-19 infections. FindingsCOVID-19 certification led to increased vaccinations 20 days prior to implementation, with a lasting effect up to 40 days after. Countries with lower than average pre-intervention uptake had a more pronounced increase. In France, doses exceeded 25,895 vaccines per million capita (pmc) or in absolute terms, 1,749,589 doses prior to certification and 11,434 pmc after (772,563 doses). There was no effect in countries with higher uptake (Germany) or when introduced during limited supply (Denmark). There was higher uptake for 1,000) were associated with higher uptake <20 years. When extended to broader settings, uptake remained high in the youngest group, but also observed in older age groups. The relationship of the intervention with reported infections was difficult to assess based on available data. InterpretationWe provide the first empirical assessment of the relationship between COVID-19 certification and vaccine uptake. Interpretation should recognise additional factors, including age eligibility changes and pandemic trajectories. We provide evidence that certification could increase vaccine uptake. Funding and Competing Interest StatementMCM receives funding from the Leverhulme Trust (Large Centre Grant), European Research Council (835079) and participates in UKs SAGE SPI-B (behavioural insights) committee. The funders had no role in study design, data collection, analysis, interpretation, or writing of the report. Research in ContextO_ST_ABSEvidence before this studyC_ST_ABSThe introduction of COVID-19 certification or vaccine passports has been linked to lower self-reported vaccine intentions, yet national media and health offices report increases in vaccinations. No empirical studies could be located that had examined the impact of the implementation of mandatory COVID-19 certification on vaccine uptake. Added value of this studyTo our knowledge, this is the first empirical analysis of the relationship of the introduction of COVID-19 certification on vaccine uptake. Implications of all the available evidenceOur study provides the first evidence that mandatory COVID-19 certification restricting access to certain settings can influence vaccine uptake for those groups affected by the intervention. Given higher vaccine complacency in certain groups, such as youth who perceive lower risks of infection, this intervention could be an additional policy lever to increase vaccine uptake and population level immunity. Future studies examining more countries and variation by eligibility criteria and factors beyond age are warranted.