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.

The clinical sensitivity of a single SARS-CoV-2 upper respiratory tract RT-PCR test for diagnosing COVID-19 using convalescent antibody as a comparator.

The clinical false negative rate of reverse transcriptase polymerase chain reaction (RT-PCR) testing for SARS-CoV-2 on a single upper respiratory tract sample was calculated using convalescent antibody testing as a comparator. The sensitivity in symptomatic individuals was 86.2% (25/29). Of the missed cases, one (3.5%) was detected by repeat RT-PCR, one by CT thorax and two (7.1%) by convalescent antibody. The clinical false negative rate of a single RT-PCR on an upper respiratory tract sample of 14% in symptomatic patients is reassuring when compared to early reports. This report supports a strategy of combining repeat swabbing, use of acute and convalescent antibody testing and CT thorax for COVID-19 diagnosis.