Table 1. Definitions of the Exposed and Reference Groups for Different COVID-19 Vaccine Effectiveness (CVE) Estimates .

CVE estimate	Exposed group	Reference group
Seasonal CVE	Vaccinated with any COVID-19 vaccine in autumn or winter 2022-2023	Not vaccinated in autumn or winter 2022-2023 or in 6 mo preceding that vaccination campaign
Absolute CVE		Never vaccinated against COVID-19
Relative CVE		Vaccinated with at least a full primary series but not in autumn or winter 2022-2023 or in 6 mo preceding that vaccination campaign
Relative CVE (second booster)	Received second COVID-19 vaccination booster in autumn or winter 2022-2023, but no additional dose	Fully vaccinated with primary series plus 1 booster dose (only); not vaccinated in autumn or winter 2022-2023 or in 6 mo preceding that vaccination campaign