Table 2. Definition of vaccine effectiveness indicators estimated in this study, VEBIS SARI VE network, Europe, 15 February–31 August 2023.

Vaccine effectiveness indicator	Vaccinated with bivalent vaccine (‘vaccinated’)a	Not vaccinated with bivalent vaccine (‘unvaccinated’)b
Relative vaccine effectiveness (rVE)	Vaccinated with any bivalenta booster dose	Vaccinated with at least primary series of vaccinationb, received > 6 months before the bivalent campaign began
Incremental vaccine effectiveness (iVE)	Vaccinated with primary series vaccination plus two booster doses, with the second booster being a bivalenta vaccine	Vaccinated with primary series of vaccination plus one monovalent booster doseb, received > 6 months before the bivalent campaign start

SARI: severe acute respiratory infection; VE: vaccine effectiveness; VEBIS: Vaccine Effectiveness, Burden and Impact Studies.

^a Any adapted bivalent mRNA COVID-19 vaccine (Comirnaty (BNT162b2; Pfizer-BioNTech) and Spikevax (mRNA-1273; Moderna) Original/Omicron BA.1 and Comirnaty and Spikevax Original/Omicron BA.4–5). Any booster dose received after the introduction of the bivalent vaccines in each country was considered to be a bivalent COVID-19 vaccine.

^b Any monovalent COVID-19 vaccine. Any dose received before the introduction of the adapted bivalent mRNA COVID-19 vaccines in each country were considered to be a monovalent COVID-19 vaccine.