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Preprint in English | medRxiv | ID: ppmedrxiv-22282113


Since the emergence of Omicron, reinfections with SARS-CoV-2 have been rising. We estimated the risk of SARS-CoV-2 reinfection in the widely vaccinated French population, from January to August 2022. At nine weeks post-infection, the relative risk of reinfection, primary infection with pre-Delta variants being the reference group, was estimated at 0.43 [95%CI 0.40-0.47] if the primary infection was attributed to Delta, 0.21 [95%CI 0.19-0.24] with BA.1 and 0.17 [95% CI 0.15-0.18] with BA.2, and rapidly waned overtime. After a BA.1 primary infection the protection was similar against BA.2 or BA.4/5 reinfection.

Preprint in English | medRxiv | ID: ppmedrxiv-22269952


BackgroundA rapid increase in incidence of the SARS-CoV-2 Omicron variant occurred in France in December 2021, while the Delta variant was prevailing since July 2021. We aimed to determine whether the risk of a severe hospital event following symptomatic SARS-CoV-2 infection differs for Omicron versus Delta. MethodsWe conducted a retrospective cohort study to compare severe hospital events (admission to intensive care unit or death) between Omicron and Delta symptomatic cases matched according to week of virological diagnosis and age. The analysis was adjusted for age, sex, vaccination status, presence of comorbidities and region of residence, using Cox proportional hazards model. FindingsBetween 06/12/2021-28/01/2022, 184 364 cases were included, of which 931 had a severe hospital event (822 Delta, 109 Omicron). The risk of severe event was lower among Omicron versus Delta cases; the difference in severity between the two variants decreased with age (aHR=0{middle dot}11 95%CI: 0{middle dot}07-0{middle dot}17 among 40-64 years, aHR=0{middle dot}51 95%CI: 0{middle dot}26-1{middle dot}01 among 80+ years). The risk of severe event increased with the presence of comorbidities (for very-high-risk comorbidity, aHR=4{middle dot}18 95%CI: 2{middle dot}88-6{middle dot}06 among 40-64 years) and in males (aHR=2{middle dot}29 95%CI: 1{middle dot}83-2{middle dot}86 among 40-64 years) and was higher in unvaccinated compared to primo-vaccinated (aHR=6{middle dot}90 95%CI: 5{middle dot}26-9{middle dot}05 among 40-64 years). A booster dose reduced the risk of severe hospital event in 80+ years infected with Omicron (aHR=0{middle dot}27; 95%CI: 0{middle dot}11-0{middle dot}65). InterpretationThis study confirms the lower severity of Omicron compared to Delta. However, the difference in disease severity is less marked in the elderly.