Duration of vaccine protection against breakthrough infections during five COVID-19 waves among healthcare workers primarily vaccinated with CoronaVac.

We aimed to characterise vaccine-induced protection against COVID-19 during five waves caused by Variants of Concern (VOCs). This is a nested case-control study of 3,972 HCW primarily vaccinated with CoronaVac (98%) that evaluated symptomatic SARS-CoV-2 breakthrough infections (BI) in almost two-years follow-up until the 3rd Omicron wave. Predictors of protection against SARS-CoV-2 BI were analysed using conditional logistic regression models. We included 1,491 SARS-CoV-2 breakthrough cases, mostly mild, and 2,962 controls. Most participants (90%) had received at least one booster before the onset of the Omicron waves, mainly BNT162b2. A multivariate logistic regression showed that vaccine-induced protection against BI wanes after six months regardless of the number of monovalent booster doses. Additionally, booster dose with BNT162b2 showed a trend for higher protection compared to CoronaVac during the Omicron waves. In conclusion, immunity of monovalent booster doses against SARS-CoV-2 is short-lasting. Individuals previously vaccinated with an inactivated vaccine should receive a BNT162B2 booster dose.