Relative vaccine effectiveness of mRNA COVID-19 boosters in people aged at least 75 years during the spring-summer (monovalent vaccine) and autumn- winter (bivalent vaccine) booster campaigns: a prospective test negative case-control study, United Kingdom, 2022

Background: Understanding the relative vaccine effectiveness (rVE) of new COVID-19 vaccine formulations against SARS-CoV-2 infection is a public health priority. A precise analysis of the rVE of monovalent and bivalent boosters given during the 2022 spring -summer and autumn-winter campaigns, respectively, in a defined population remains of interest. Aim: We assessed rVE against hospitalisation for the spring -summer (fourth vs third monovalent mRNA vaccine doses) and autumn-winter (fifth BA.1/ancestral bivalent vs fourth monovalent mRNA vaccine dose) boosters. Methods: We performed a prospective singlecentre test-negative design case-control study in >= 75-year-old people hospitalised with COVID-19 or other acute respiratory disease. We conducted regression analyses controlling for age, sex, socioeconomic status, patient comorbidities, community SARS-CoV-2 prevalence, vaccine brand and time between base-line dose and hospitalisation. Results: We included 682 controls and 182 cases in the spring-summer booster analysis and 572 controls and 152 cases in the autumn-winter booster analysis. A monovalent mRNA COVID-19 vaccine as fourth dose showed 46.6% rVE (95% confidence interval (CI): 13.9-67.1) vs those not fully boosted. A bivalent mRNA COVID-19 vaccine as fifth dose had 46.7% rVE (95% CI: 18.0-65.1), compared with a fourth monovalent mRNA COVID-19 vaccine dose. Conclusions: Both fourth monovalent and fifth BA.1/ancestral mRNA bivalent COVID-19 vaccine doses demonstrated benefit as a booster in older adults. Bivalent mRNA boosters offered similar protection against hospitalisation with Omicron infection to monovalent mRNA boosters given earlier in the year. These findings support immunisation programmes in several European countries that advised the use of BA.1/ancestral bivalent booster doses.