Previous SARS-CoV-2 infections and their impact on the protection from reinfection during the Omicron BA.5 wave – a nested case-control study among vaccinated adults in Sweden

We evaluated the protection afforded by SARS-CoV-2 infection-induced immunity against reinfection among working age vaccinated during a calendar period from June to December 2022 when Omicron BA.5 was the dominating subvariant in Scania county, Sweden. The study cohort (n = 71 592) mainly consisted of health care workers. We analysed 4 144 infected cases during Omicron BA.5 dominance and 41 440 sex and age matched controls with conditional logistic regression. The average protection against reinfection was marginal (16%, 95% confidence interval CI 7 to 23%) during the study period, but substantially higher for recent infections. Recent infection (3–6 months) with Omicron BA.2 and BA.5 both offered strong protection (86%, 95% CI 68 to 94% and 78%, 95% CI 69 to 84%), whereas more distant infection (6–12 months) with Omicron BA.1, BA.2 and the variants before Omicron offered marginal or no protection. These findings suggest that infection-induced immunity contributes to short-term population protection against infection with the subvariant BA.5 among working age vaccinated, but wanes considerably with time independent of virus variant.