Publication date: Dec 30, 2024

Background The Winter Coronavirus (COVID-19) Infection Study (WCIS) was a sample-based household study in England and Scotland aiming to inform COVID-19-related epidemiology and health pressures over the 2023-2024 winter period. We aim to estimate vaccine effectiveness (VE) against both SARS-CoV-2 infection in general and specifically symptomatic infection (COVID-19) within the WCIS cohort in England. Methods Data from self-reported lateral flow device (LFD) tests for SARS-CoV-2 were collected from participants alongside self-completed questionnaires from November 2023 to March 2024. A test-negative case-control design was used to estimate VE of the Autumn 2023 COVID-19 boosters against SARS-CoV-2 infection and symptomatic infection compared with being unboosted, regardless of previous vaccination history. Symptomatic infection as an outcome was restricted to participants who reported an ARI symptom associated with their test. Logistic regression was used to calculate VE, with LFD test result as the outcome, vaccination status as primary exposure variable, and adjustment for covariates. Analyses were also stratified by bivalent BA4/5 and monovalent XBB vaccines to assess VE of individual vaccine types. Results The analysis included 109,929 English residents primarily in the 60-80 age range (54%), of White ethnic background (95%), and in the least deprived quintile (31%). Participants contributed 114,066 eligible tests in the analysis, of which 42,340 were associated with a symptomatic outcome. VE against infection peaked at 49% (95% Confidence Interval (CI): 35-60%) at 2-4 weeks post-vaccination, with waning to a null effect occurring after 10 weeks (VE: 5% (95% CI: -5-14%)). Similarly, VE against symptomatic infection peaked at 49% (95% CI: 32-63%) after 2-4 weeks, waning after 10 weeks (VE: 5% (95% CI: -7-16%)). The bivalent vaccine showed low and mainly non-significant evidence of protection against either outcome, whereas the monovalent vaccine showed a peak VE of 49% (95% CI: 34-60%) at 2-4 weeks against infection and 49% (95% CI: 31-63%) at 2-4 weeks against symptomatic infection. Conclusions The Autumn 2023 COVID-19 vaccine campaign provided moderate protection against infection and symptomatic infection during the 2023 winter period, with a differential effect between vaccine type. Household studies such as WCIS are useful to understand impacts of vaccination campaigns within the community, especially in the post-pandemic period where testing capacity is restricted to hospital settings.

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