Early real world evidence on the relative SARS-COV-2 vaccine effectiveness of bivalent COVID-19 booster doses: a rapid review.

The objective of this review is to give an overall view of COVID-19 bivalent vaccines knowledge and to explore their early available real world effectiveness evidence in the Omicron era. Presently, bivalent vaccines are generally offered to all groups eligible for their next booster, as defined by the national vaccination campaign, with varying policies between countries. The use of bivalent vaccines is supported by immunogenity studies, which, nevetheless, have led to contradictory conclusions, and are not generally designed to measure clinical impact. In order to critically appraise the available research on real world effectiveness, a systematic literature search was performed: out of 876 references examined, 14 studies were finally included and extracted. The findings of this review demonstrate modest to moderate additional protection of vaccination with bivalent BA.4-5 or BA.1 mRNA-booster vaccines against COVID-19 associated illness and hospitalization, -if compared with having received a monovalent dose as booster-, during a period when BA.5 and other Omicron sublineage viruses predominated globally, Considering the complexity of the current immunity situation at global level, and the high level of heterogeneity both at study and at review level, these findings must be taken with caution. Further research on SARS-CoV-2 vaccine effectiveness against emerging SARS-CoV-2 variants is encouraged.