Background The objective of this study was to investigate how protection against COVID-19 conferred by previous infection is modified by vaccination.
Methods In a cohort of all 152655 individuals in Scotland alive at 90 days after a positive test for SARS-CoV-2 (confirmed by cycle threshold < 30, or two tests) followed till 22 September 2021, rate ratios for reinfection were estimated with calendar time or tests as timescale.
Findings Rates of detected and hospitalised reinfection with COVID-19 while unvaccinated were respectively 6.8 (95% CI 6.4 to 7.2) and 0.18 (95% CI 0.12 to 0.25) per 1000 person-months. These rates were respectively 68% and 74% lower than in a matched cohort of individuals who had not previously tested positive. Efficacy of two doses of vaccine in those with previous infection was estimated as as 84% (95 percent CI 81% to 86%) against detected reinfection and 71% (95 percent CI 29% to 88%) against hospitalised or fatal reinfection. The rate of detected reinfection after two doses of vaccine was 1.35 (95% CI 1.02 to 1.78) times higher in those vaccinated before first infection than in those unvaccinated at first infection.
Interpretation The combination of natural infection and vaccination provides maximal protection against new infection with SARS-CoV-2: prior vaccination does not impair this protection.
Funding No specific funding was received for this work.
Evidence before this study In a recent systematic review of cohort studies reported up to July 2021, the average reduction in COVID-19 infection rates in those with previous infection compared with those without evidence of previous infection was 90%. There is little information about the protective effect of previous infection against severe COVID-19, or about how the protective effects of previous infection against reinfection and severe disease are modified by vaccination.
What this paper adds In unvaccinated individuals the protection against hospitalised COVID-19 conferred by previous infection is similar to that induced by vaccination. In those with previous infection, vaccination reduces the rates of reinfection and hospitalised COVID-19 by about 70%.
McKeigue, P., McAllister, D., Robertson, C., Stockton, D. & Colhoun, H. 2021, 'Reinfection with SARS-CoV-2: outcome, risk factors and vaccine efficacy in a Scottish cohort'. To be published in MedRxiv [Preprint]. Available at: https://doi.org/10.1101/2021.11.23.21266574