Infection with a pre-Omicron SARS-CoV-2 variant offered 35.5% protection against symptomatic Omicron BA.4 or BA.5 reinfection, while an Omicron infection was 76.2% protective, according to a Qatari test-negative, case-control study published yesterday in a letter to the New England Journal of Medicine.Led by researchers from Weill Cornell Medicine-Qatar in Doha, the team analyzed data on COVID-19 lab testing, clinical infection, vaccination, and demographic factors from an integrated nationwide digital-health information platform, which includes all results of polymerase-chain-reaction (PCR) and rapid antigen testing results from healthcare facilities.The study spanned May 7 to Jul 28, 2022, a period of BA.4/BA.5 dominance in Qatar.
The researchers said that Qatar has an unusually young and diverse population, with only 9% of its residents aged 50 years and older, and 89% expatriates from more than 150 countries.
Each case-patient was matched with an uninfected control.Protection waned over timePre-Omicron infection at least 90 days before reinfection was 35.5% (95% confidence interval [CI], 12.1 to 52.7) effective against symptomatic BA.4/BA.5 reinfection and 27.7% (95% CI, 19.3 to 35.2) effective against symptomatic or asymptomatic BA.4/BA.5 reinfection.Post-Omicron infection offered greater protection (76.2%; 95% CI, 66.4 to 83.1) against symptomatic BA.4/BA.5 reinfection and 78.0% (95% CI, 75.0 to 80.7) protection against symptomatic or asymptomatic infection.Results were consistent in the different primary, sensitivity, and additional analyses.
An analysis of effectiveness stratified according to time since previous infection revealed waning protection over time, and analyses classified by COVID-19 vaccination