This is particularly true for the Omicron variant, which is the predominant viral strain globally as of October 2022, causing mostly milder infections compared to preceding strains. Yet, weak or waning humoral response in tandem with the emergence of new viral variants, capable of potentially escaping immune response, lead to a continued risk of reinfection with heterologous SARS-CoV-2 strains. A still lower reinfection risk has been reported for individuals with hybrid immunity, i.e., SARS-CoV-2 infection plus at least one vaccination. Concerning (re)infection however, previous infection is associated with 85% protection over at least 9 months also, infected individuals might be even better protected than only vaccinated individuals. Although natural infection with SARS-CoV-2 elicits a broader humoral response than mRNA vaccine administration, levels of neutralizing antibodies are lower after natural infection. Studies from the pre-Omicron era show that specific neutralizing antibodies as marker for humoral immunity reduce the risk of symptomatic (re)infection and thus disease recurrence upon reexposure to a homologous viral strain. Mitigation of Coronavirus Disease 2019 (COVID-19) relies on establishing an ideally long-lasting immune barrier against Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2). Severe Acute Respiratory Syndrome Coronavirus 2 Funders played no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript.Ĭompeting interests: The authors have declared that no competing interests exist. įunding: This work was supported by the Swiss National Sciences Foundation (grant number 31CA30_196544 to PK and CRK grant number PZ00P3_179919 to PK), the Federal Office of Public Health (grant number 20.008218/421-28/1), and the Health Department of the Canton of St. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.ĭata Availability: Data are available at. Received: Accepted: OctoPublished: November 7, 2022Ĭopyright: © 2022 Babouee Flury et al. PLoS Med 19(11):Īcademic Editor: James G. (2022) Risk and symptoms of COVID-19 in health professionals according to baseline immune status and booster vaccination during the Delta and Omicron waves in Switzerland-A multicentre cohort study. Important limitations are that SARS-CoV-2 swab results were self-reported and that results on viral variants were inferred from the predominating strain circulating in the community at that time, rather than sequencing.Ĭitation: Babouee Flury B, Güsewell S, Egger T, Leal O, Brucher A, Lemmenmeier E, et al. Group H (versus N, risk ratio (RR) 0.80, 95% CI 0.66 to 0.97, p = 0.021) and participants with booster vaccination (versus nonboosted, RR 0.79, 95% CI 0.71 to 0.88, p < 0.001) reported less symptoms during infection. HRs substantially increased during the Omicron period for all groups in multivariable analyses, only belonging to group H was associated with protection (adjusted HR 0.52, 95% CI 0.35 to 0.77, p = 0.001) booster vaccination was associated with reduction of breakthrough infection risk in groups V (aHR 0.68, 95% CI 0.54 to 0.85, p = 0.001) and H (aHR 0.67, 95% CI 0.45 to 1.00, p = 0.048), largely observed in the early Omicron period. We compared the time to positive SARS-CoV-2 swab and number of symptoms according to immune status, viral variant (i.e., Delta-dominant before DecemOmicron-dominant on/after this date), and subsequent vaccinations, adjusting for exposure/behavior variables.Īmong 2,595 participants (median follow-up 171 days), we observed 764 (29%) (re)infections, thereof 591 during the Omicron period. Date and symptoms of (re)infections and subsequent (booster) vaccinations were recorded until March 2022. We determined immune status in September 2021 based on serology and previous SARS-CoV-2 infections/vaccinations: Group N (no immunity) Group V (twice vaccinated, uninfected) Group I (infected, unvaccinated) Group H (hybrid: infected and ≥1 vaccination). In this prospective, multicentre cohort performed between August 2020 and March 2022, we recruited hospital employees from ten acute/nonacute healthcare networks in Eastern/Northern Switzerland.
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