Previous COVID infection may confer 56% protection against reinfectionA study in Qatar estimates that previous COVID-19 infection imparts 56% protection against future symptomatic infection caused by the highly transmissible Omicron variant, down from about 90% for other SARS-CoV-2 strains.The study, published yesterday in the New England Journal of Medicine (NEJM), was led by researchers at Weill Cornell Medicine–Qatar in Doha.The team extracted data on COVID-19 testing, vaccination, clinical infections, outcomes, and demographics from national databases amid the Omicron surge.The median interval between initial COVID-19 infection and polymerase chain reaction (PCR) testing of 5,696 positive cases and 10,673 negative controls was 279 days for analysis of the Alpha variant, 285 days for the Beta variant, 254 days for the Delta variant, and 314 days for Omicron.Estimated effectiveness of a previous infection in preventing reinfection was 90.2% against Alpha, 85.7% against Beta, 92.0% against Delta, and 56.0% against Omicron.
Sensitivity analyses that excluded vaccinated patients confirmed the results.Among reinfected patients, one patient with the Alpha variant became severely ill, as did two each with Beta and Omicron, but no patients with Delta did so.
No reinfection was critical or fatal. Estimated effectiveness against severe, critical, or fatal COVID-19 was 69.4% for Alpha, 88.0% for Beta, 100% with Delta, and 87.8% with Omicron."We found that the effectiveness of previous infection in preventing reinfection with the alpha, beta, and delta variants of SARS-CoV-2 was robust (at approximately 90%), findings that confirmed earlier estimates," the researchers concluded."Such protection against reinfection with the