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Risk of SARS-CoV-2 Reinfection by Vaccination Status, Predominant Variant, and Time from Previous Infection: A Cohort Study in Italy
15 Pages Posted: 9 Jun 2022More...
Background: We evaluated the risk of SARS-CoV-2 reinfection by vaccination status, predominant variant, and time from first infection.
Methods: The cohort includes all residents in Reggio Emilia, on December 31, 2019 and has been followed up until February 28, 2022 for SARS-CoV-2 infection, first or re-infection (i.e. new positivity >90 days after a previous infection). Cox model was used to compare the risk of first vs. second infection, adjusting for age, sex, vaccine doses, and comorbidities.
Findings: The cohort included 538516 residents, 121154 first SARS-CoV-2 infections, and 3739 reinfections (3243 with the Omicron period) were diagnosed. In the pre-Omicron period, three doses of vaccine reduced the risk of infection by 89% (87-90), a previous infection reduced the risk by 90% (88-91), while vaccination and infection reduced the risk by 98% (88-100). In the Omicron period, three doses reduced the risk by 54% (53-55) and infection by 9% (3-14); combined immunity of one dose and infection gave a 56% (53-59) while three doses and infection 78% (75-80). In the pre-Omicron period, protection from reinfection remained above 88% for up to 15 months from the first diagnosis. In the Omicron period, the protection decreased from 71% (65-76) to 21% (10-30) if the first infection occurred <5 months or more than 21 months before.
Interpretation: Natural immunity acquired with previous variants showed low protection to Omicron. The combination of vaccination and natural immunity seems to be more protective than the two taken individually.
Funding Information: This study was partially supported by the Italian Ministry of Health – Ricerca Corrente Annual Programme 2023 and partially funded by the Emilia-Romagna Regional Health authority (DGR 396/2021).
Declaration of Interests: The study authors declare no conflict of interest.
Ethics Approval Statement: The collection of data used for this manuscript (surveillance and contact tracing data) is compulsory according to national laws on infectious diseases. The COVID-19 Italian National Working group on Bioethics has stated that consensus for the collection of this data in the context of the COVID-19 emergency is not mandatory, based on Guideline 12 of the WHO on ethical issues in public health surveillance (Rapporto ISS COVID-19 n. 34/2020 https://www.iss.it/documents/20126/0/Rapporto+ISS+COVID-19+34_2020.pdf) The legal ordinance n. 640 of February 27, 2020 (Ordinanza n. 640, Gazzetta Ufficiale Serie Generale n.50 28-02-2020 https://www.gazzettaufficiale.it/eli/id/2020/02/28/20A01348/sg) explicitly declares Istituto Superiore di Sanità as entitled to collect data for COVID-19 surveillance and contact tracing and that such data can be used and shared, upon anonymization, to advance scientific knowledge on this new disease.
Keywords: SARS-CoV-2, Omicron, Reinfection, Immunity, Epidemiology
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