The protective effect of previous COVID-19 infection in a high-prevalence hospital setting.

Objective: To assess the protective effect of previous COVID-19 infection for healthcare workers in a high-prevalence setting.

Method: The COVID-19 antibody and PCR results of 538 healthcare workers on wards with COVID-19 outbreaks from 1 March 2020 to 31 July 2020 were evaluated. Infection rates of the 'previously infected' and 'no evidence of previous infection' groups were compared during second-wave outbreaks between 29 September 2020 and 20 November 2020.

Results: One out of 115 individuals previously infected developed infection compared with 104 out of 423 individuals with no evidence of previous infection. Attack rates in staff previously infected was reduced significantly from 24.59% to 0.87% (odds ratio 0.027, 95% CI 0.004-0.195, p<0.001) when compared to the 'no evidence of previous infection' group with the same exposure risk.

Conclusion: Prior SARS-CoV-2 infection offers significant protection against reinfection and this protection lasts 4 months for the majority of individuals.