Who Is at Higher Risk of SARS-CoV-2 Reinfection? Results from a Northern Region of Italy

The SARS-CoV-2 pandemic continues to spread worldwide, generating a high impact on healthcare systems. The aim of the study was to examine the epidemiological burden of SARS-CoV-2 reinfections and to identify potential related risk factors. A retrospective observational study was conducted in Liguria Region, combining data from National Vaccines Registry and Regional Chronic Condition Data Warehouse. In the study period (September 2021 to May 2022), 335,117 cases of SARS-CoV-2 infection were recorded in Liguria, of which 15,715 were reinfected once. During the Omicron phase (which predominated from 3 January 2022), the risk of reinfection was 4.89 times higher (p < 0.001) than during the Delta phase. Unvaccinated and vaccinated individuals with at least one dose for more than 120 days were at increased risk of reinfection compared with vaccinated individuals with at least one dose for <= 120 days, respectively (odds ratio (OR) of 1.26, p < 0.001; OR of 1.18, p < 0.001). Healthcare workers were more than twice as likely to be reinfected than non-healthcare workers (OR of 2.38, p < 0.001). Lower ORs were seen among people aged 60 to 79 years. Two doses or more of vaccination were found to be protective against the risk of reinfection rather than a single dose (mRNA vaccines: OR of 0.06, p < 0.0001, and OR of 0.1, p < 0.0001; vector vaccines: OR of 0.05, p < 0.0001). Patients with chronic renal failure, cardiovascular disease, bronchopneumopathy, neuropathy and autoimmune diseases were at increased risk of reinfection (OR of 1.38, p = 0.0003; OR of 1.09, p < 0.0296; OR of 1.14, p = 0.0056; OR of 1.78, p < 0.0001; OR of 1.18, p = 0.0205). Estimating the epidemiological burden of SARS-CoV-2 reinfections and the role played by risk factors in reinfections is relevant for identifying risk-based preventive strategies in a pandemic context characterized by a high circulation of the virus and a high rate of pathogen mutations.