Reinfection with SARS-CoV-2: outcome, risk factors and vaccine efficacy in a Scottish cohort

BackgroundThe objective of this study was to investigate how protection against COVID-19 conferred by previous infection is modified by vaccination. MethodsIn 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. FindingsRates 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. InterpretationThe combination of natural infection and vaccination provides maximal protection against new infection with SARS-CoV-2: prior vaccination does not impair this protection. FundingNo specific funding was received for this work. Research in contextO_ST_ABSEvidence before this studyC_ST_ABSIn 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 addsIn 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%. Implications of all the available evidenceThe combination of natural infection and vaccination provides maximal protection against COVID-19: prior vaccination does not seriously impair this protection.