| Summary: | Background: Understanding the association of prior SARS-CoV-2 infection with subsequent reinfection has public health relevance. Objective: To explore COVID-19 severity and SARS-CoV-2 infection and reinfection rates. Design: Retrospective cohort study. Setting: Boston, Massachusetts, during the first COVID-19 surge (01/01/2020–05/31/2020; Period-1) and after the first surge (06/01/2020–02/28/2021; Period-2); Period-2 included the second surge (11/01/2020–02/28/2021). Participants: Patients in an academic medical center and six community health centers who received a clinical diagnosis of COVID-19 between 01/01/2020 and 05/31/2020 or SARS-CoV-2 testing between 01/01/2020 and 02/28/2021. Measurements: COVID-19 severity was compared between Period-1 and Period-2. Poisson regression models adjusted for demographic variables, medical comorbidities, and census tract were used to assess reinfection risk among patients with COVID-19 diagnoses or SARS-CoV-2 testing during Period-1 and additional SARS-CoV-2 testing during Period-2. Results: Among 142,047 individuals receiving SARS-CoV-2 testing or clinical diagnoses during the study period, 15.8% were infected. Among COVID-19 patients, 22.5% visited the emergency department, 13% were hospitalized, and 4% received critical care. Healthcare utilization was higher during Period-1 than Period-2 (22.9% vs. 18.9% emergency department use, 14.7% vs. 9.9% hospitalization, 5.5% vs. 2.5% critical care; p < 0.001). Reinfection was assessed among 8961 patients with a SARS-CoV-2 test or COVID-19 diagnosis in Period-1 who underwent additional testing in Period-2. A total of 2.7% (n = 65/2431) with SARS-CoV-2 in Period-1 tested positive in Period-2, compared with 12.6% (n = 821/6530) of those who initially tested negative (IRR of reinfection = 0.19, 95% CI: 0.15–0.25). Conclusions: Prior SARS-CoV-2 infection among this observational cohort was associated with an 81% lower reinfection rate.
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