The follow-up started from the date of the first positive swab and ended the 1st day of reinfection, or was censored on February 18, 2022. Subjects were classified as “vaccinated” if they received ≥1 dose of BNT162b2, ChAdOx1 nCoV-19, mRNA-1273 or JNJ-78436735, ≥14 days before the reinfection. Since the high-quality National Tax Registry has been recently used for fiscal codes generation and input in the main above databases, we used a deterministic linkage.Ī reinfection was defined by the presence of two positive RT-PCR samples detected ≥45 days apart with ≥1 intermediate negative RT-PCR test ( 9, 10). On Febru(to allow ≥45 days of follow-up), we extracted all data of the official vaccination (Regional database “Vaccinazioni Anti-COVID-19”), COVID-19 (“Surveillance COVID-19 Platform”), demographic (Italian “Anagrafica sanitaria”), hospital (Italian “File A - SDO”), and co-pay exemption (Italian “Esenzioni Ticket”) datasets of the National Healthcare System, merging individual information through encrypted fiscal code ( 8). We included all residents in the Abruzzo Region, Italy with ≥1 positive nasopharyngeal swab detected through RT-PCR by the regional-accredited laboratories, from the start of the pandemic (March 2, 2020) up to January 4, 2022. We performed a retrospective cohort study on the entire population of an Italian region in order to estimate the incidence of SARS-CoV-2 reinfections and COVID-19 according to vaccination status, predominant viral strain, and time after primary infection. In fact, some evidence on the degree of protection exerted by the existing vaccines against the BA.1/B.1.1.529 (Omicron) variant is available, but still preliminary ( 3, 7). Although the reported rates have been consistently low, the results differed according to the adopted definition of reinfection, setting, pandemic period, and follow-up duration, and uncertainties remain on the duration of the natural immunity and the impact of vaccination to decrease reinfection rates ( 4– 6). The risk of reinfection was significantly higher among females, unvaccinated subjects, and during the Omicron wave.Īfter the first documented case in August 2020 in Hong Kong ( 1), a number of field studies estimated the rate of SARS-CoV-2 reinfections after a primary episode ( 2, 3). Importantly, the incidence of reinfection did not vary substantially over time: after 18–22 months from the primary infection, the reinfection rate was still 6.7‰, suggesting that protection conferred by natural immunity may last beyond 12 months. After an average of 277 days, we recorded 729 reinfections among 119,266 previously infected subjects (overall rate: 6.1‰), eight COVID-19-related hospitalizations (7/100,000), and two deaths. In this retrospective cohort analysis of the entire population of an Italian region, we followed 1,293,941 subjects from the beginning of the pandemic to the current scenario of Omicron predominance (up to mid-February 2022). Uncertainties remain, however, on the duration of the natural immunity, its protection against Omicron variant, and on the impact of vaccination to reduce reinfection rates. 3Department of Medical and Surgical Sciences, University of Bologna, Bologna, ItalyĬurrent data suggest that SARS-CoV-2 reinfections are rare.2Local Health Unit of Pescara, Pescara, Italy. 1Department of Environmental and Prevention Sciences, University of Ferrara, Ferrara, Italy.Maria Elena Flacco 1 Graziella Soldato 2 Cecilia Acuti Martellucci 1 Giuseppe Di Martino 2 Roberto Carota 2 Antonio Caponetti 2 Lamberto Manzoli 3 *
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