Protection against infection with the Omicron BA.5 subvariant among people with previous SARS-CoV-2 infection - surveillance results from southern Sweden, June to August 2022

We evaluated the protection afforded by SARS-CoV-2 natural infection against reinfection among vaccinated during a calendar period from June to August 2022 when Omicron BA.5 was the dominating subvariant in Scania county, Sweden. We formed a study cohort (n = 71 592) mainly consisting of health care workers by restricting to people 18-64 years old who received their first vaccine dose relatively early (24 April 2021 or sooner). We used continuous density case-control 1:10 sampling matched for sex and age within the study cohort, and thereby obtained 1 114 cases during Omicron BA.5 dominance and 11 140 controls who were analysed with conditional logistic regression. Limited protection against reinfection was suggested from prior infection of virus variants before Omicron (11%, 95% confidence interval [CI] -10 to 28%]. By contrast, prior Omicron infection offered clear protection (65%, 95% CI 56-73%). For the Omicron BA.2 subvariant, stronger protection was suggested during early (85%, 95% CI 75-91%) than later BA.5 dominance (66%, 95% CI 48-78%). Lower protection was observed from the previous BA.1 subvariant (30%; 95% CI -4 to 53%). These findings suggest that natural infection from the Omicron subvariants contributes to short-term population protection against reinfection with the subvariant BA.5 among vaccinated, but wanes considerably 5-6 months after infection.