The effect of mandatory COVID-19 certificates on vaccine uptake: synthetic-control modelling of six countries

Abstract

Background: Mandatory COVID-19 certification (showing vaccination, recent negative test, or proof of recovery) has been introduced in some countries. We aimed to investigate the effect of certification on vaccine uptake.

Methods: We designed a synthetic control model comparing six countries (Denmark, Israel, Italy, France, Germany, and Switzerland) that introduced certification (April-August, 2021), with 19 control countries. Using daily data on cases, deaths, vaccinations, and country-specific information, we produced a counterfactual trend estimating what might have happened in similar circumstances if certificates were not introduced. The main outcome was daily COVID-19 vaccine doses.

Findings: COVID-19 certification led to increased vaccinations 20 days before implementation in anticipation, with a lasting effect up to 40 days after. Countries with pre-intervention uptake that was below average had a more pronounced increase in daily vaccinations compared with those where uptake was already average or higher. In France, doses exceeded 55 672 (95% CI 49 668-73 707) vaccines per million population or, in absolute terms, 3 761 440 (3 355 761-4 979 952) doses before mandatory certification and 72 151 (37 940-114 140) per million population after certification (4 874 857 [2 563 396-7 711 769] doses). We found no effect in countries that already had average uptake (Germany), or an unclear effect when certificates were introduced during a period of limited vaccine supply (Denmark). Increase in uptake was highest for people younger than 30 years after the introduction of certification. Access restrictions linked to certain settings (nightclubs and events with >1000 people) were associated with increased uptake in those younger than 20 years. When certification was extended to broader settings, uptake remained high in the youngest group, but increases were also observed in those aged 30-49 years.

Interpretation: Mandatory COVID-19 certification could increase vaccine uptake, but interpretation and transferability of findings need to be considered in the context of pre-existing levels of vaccine uptake and hesitancy, eligibility changes, and the pandemic trajectory.

Funding: Leverhulme Trust and European Research Council.

Figure 1

Daily new vaccinations in France, Israel, and Italy before and after the introduction of a mandatory COVID-19 certificate for various settings compared with a reweighted synthetic control group Data are from Our World in Data and Oxford COVID-19 Government Response Tracker. Shaded areas represent 95% CIs based on a non-parametric bootstrap procedure (1000 runs). The solid vertical line indicates the introduction of COVID-19 certification and the dashed vertical line indicates the 20-days' backdated intervention.

Figure 2

COVID-19 cases by selected countries that introduced COVID-19 certification, March 1 to Nov 1, 2021 Vertical lines indicate dates of introduction of COVID-19 certification. Certification was implemented on April 21, 2021, in Denmark (most restrictions lifted August–September); on July 29, 2021, in Israel (re-introduction of the certificate); on Aug 6, 2021, in Italy; on Aug 9, 2021, in France; on Aug 23, 2021, in Germany; and on Sept 13, 2021, in Switzerland.

Figure 3

Daily new vaccinations by age group in France, Israel, Italy, and Switzerland before and after the introduction of a mandatory COVID-19 certificate Data are from an open platform for French public data, Government Database Israel, Extraordinary Commissioner for the COVID-19 Emergency Italy, and the Federal Office of Public Health Switzerland. The vertical lines represent the date of implementing mandatory COVID-19 certificates.

Figure 4

Daily new vaccinations by age group in Switzerland before and after the introduction of a targeted and general mandatory COVID-19 certificate Data are from the Federal Office of Public Health Switzerland. The vertical dashed line indicates targeted access restrictions (events with >1000 participants and nightclubs) and the vertical solid line indicates the introduction of general access restrictions (events with >30 participants, entire hospitality sector, and leisure activities).