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

Mills, M. C.; Ruettenauer, T.

Background. COVID certification has been introduced, yet there are no empirical evaluations of its impact on vaccine uptake. Methods. Mirroring 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. Findings. COVID-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 <20 years and 20-29 years. Access restrictions linked to certain settings (nightclubs, events >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. Interpretation. We 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 Statement. MCM receives funding from the Leverhulme Trust (Large Centre Grant), European Research Council (835079) and participates in UK SAGE SPI-B (behavioural insights) committee. The funders had no role in study design, data collection, analysis, interpretation, or writing of the report.