Protocol to measure COVID-19 vaccination impact among individuals aged 65 years and over

The Vaccine Effectiveness, Burden and Impact studies using electronic health records (VEBIS-EHR) project monitors the direct effect of each COVID-19 vaccination administered during the autumnal campaign every year, using monthly vaccine effectiveness (VE) estimates. This provides evidence on the level of protection conferred by the vaccine during each circulating wave of COVID-19 [1–4]. Additional VE estimates are obtained during specific periods of the year (e.g. periods when Omicron sub-lineages are predominant or during the summer) to answer specific research questions [5–7].

Although highly relevant, these measures of effectiveness do not consider the level of disease occurrence in the population (i.e. the baseline risk) or the proportion of the population which is vaccinated. Therefore they do not quantify the population-level benefit of vaccination in terms of the absolute decrease in the incidence of COVID-19, or the total number of severe events (cases, hospitalisations or deaths) averted by the autumn vaccination campaigns. 

Population-level impact measures can be evaluated through the number of events prevented by the vaccine. This is an important component of vaccination campaign evaluation and key results can be communicated to support vaccination programmes and promote an increase in vaccination coverage. 

Some studies have measured the impact of COVID-19 vaccination programmes in European Union/European Economic Area (EU/EEA) countries or regions [8–12]. These studies mainly focused on evaluating the impact of vaccination between the primary series and the second booster dose (from 2020 to 2022). To our knowledge, only one study [13] measured the impact of the new COVID-19 adapted vaccines (season 2023–24 season). This protocol proposes a common methodology to measure the impact of the COVID-19 autumn vaccination campaigns 2023 and 2024 in countries/regions participating in the ECDC VEBIS-EHR project.