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medrxiv; 2023.
Preprint in English | medRxiv | ID: ppzbmed-10.1101.2023.01.11.23284137


In face of evidence of rapid waning of vaccine effectiveness against Omicron and its sub-lineages, a second booster with mRNA vaccines was recommended for the most vulnerable in France. We used a test negative design to estimate the effectiveness of the second booster relative to the first booster and the protection conferred by a previous SARS-CoV-2 infection, against symptomatic Omicron BA.2 or BA.4/5. We included symptomatic ≥60 years old individuals tested for SARS-CoV-2 in March 21-October 30, 2022. Compared to a 181-210 days old first booster, a second booster restored protection with an effectiveness of 39% [95%CI: 38% - 41%], 7-30 days post-vaccination This gain in protection was lower than the one observed with the first booster, at equal time points since vaccination. High levels of protection were associated to previous SARS-CoV-2 infection, especially if the infection was recent and occurred when an antigenic-related variant was dominant.

medrxiv; 2022.
Preprint in English | medRxiv | ID: ppzbmed-10.1101.2022.11.09.22282113


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.

medrxiv; 2022.
Preprint in English | medRxiv | ID: ppzbmed-10.1101.2022.02.02.22269952


Background: A rapid increase in incidence of the SARS-CoV-2 Omicron variant in France in December 2021, while the Delta variant was prevailing since July 2021. Aim: To determine whether the risk of occurrence of a serious hospital event in adults following symptomatic SARS-CoV-2 infection differs for Omicron versus Delta. Methods: A retrospective cohort study from 06/12/2021 to 07/01/2022. The outcome was a serious hospital event (admission to intensive care unit OR admission to critical care unit OR death). Omicron and Delta symptomatic cases were matched on the week of virological diagnosis and on age. Risk was adjusted for age, sex, vaccination status, presence of comorbidity and region of residence using Cox proportional-hazards model. Results: 149,064 cases were included of which 497 had a serious hospital event (447 in the Delta arm, 50 in the Omicron arm). The risk of serious event was lower among Omicron versus Delta cases (adjusted Hazard Ratio, aHR=0.13 CI95 0.09-0.18 in 18 to 79 yo, aHR=0.30 CI95 0.17-0.54 in 80+ yo). The risk increased sharply with age and was lower in vaccinated compared to unvaccinated, without interaction between variant and vaccination status (aHR=0.15 CI95 0.11-0.19 for 18-79 yo with primary vaccination versus unvaccinated), was higher in cases with comorbidities (aHR = 3.70 CI95 2.66-5.13 fort 18-79 yo with very-high-risk comorbidity versus no comorbidity) and in males. Conclusion: This study confirms the lower severity of Omicron. The vaccine protection is essential in the elderly as they have a high risk of severe hospital events following infection with Omicron, even if much this risk is lower than with Delta.