Effectiveness and duration of a second COVID-19 vaccine booster

Abstract

Using a prospective national cohort of 3.75 million individuals aged 20 or older, we evaluated the effectiveness against COVID-19 related ICU admissions and death of mRNA-based second vaccine boosters for four different three-dose background regimes: BNT162b2 primary series plus a homologous booster, and CoronaVac primary series plus an mRNA booster, a homologous booster, and a ChAdOx-1 booster. We estimated the vaccine effectiveness weekly from February 14 to August 15, 2022, by estimating hazard ratios of immunization over non-vaccination, accounting for relevant confounders. The overall adjusted effectiveness of a second mRNA booster shot was 88.2% (95%CI, 86.2-89.9) and 90.5% (95%CI 89.4-91.4) against ICU admissions and death, respectively. Vaccine effectiveness showed a mild decrease for all regimens and outcomes, probably associated with the introduction of BA.4 and BA.5 Omicron sub-lineages and immunity waning. The duration of effectiveness suggests that no additional boosters are needed six months following a second booster shot.