Effect of the 2022 COVID-19 booster vaccination campaign in people aged 50 years in England: Regression discontinuity analysis in OpenSAFELY-TPP

SARS-CoV-2 vaccines are highly effective in preventing severe COVID-19 but require boosting to maintain protection. Changes to circulating variants and prevalent natural immunity may impact on real-world effectiveness of boosters. With NHS England approval, we used linked routine clinical data from >24 million patients to evaluate the effectiveness of the 2022 combined COVID-19 autumn booster and influenza vaccine campaign in non-clinically vulnerable 50-year-olds in England using a regression discontinuity design. Our primary outcome was a composite of 6-week COVID-19 emergency attendance, COVID-19 unplanned hospitalisation, or death. By 26 November 2022, booster vaccine coverage was 11.1 % at age 49.75 years increasing to 39.7 % at age 50.25 years. The estimated effect of the campaign on the risk of the primary outcome in 50-year-olds during weeks 7-12 after the start of the campaign was -0.4 per 100,000 (95 % CI -7.8, 7.1). The results were similar when using different follow-up start dates or when estimating the effect of vaccination (rather than the campaign). This study found little evidence that the autumn 2022 vaccination campaign in England was associated with a reduction in severe COVID-19-related outcomes among non-clinically vulnerable 50-year-olds. Possible explanations include the low risk of severe outcomes and substantial pre-existing vaccine- and infection-induced immunity. The booster campaign may have had effects beyond those we estimated, including reducing virus transmission and incidence of mild or moderate COVID-19.