Risk of sars-cov-2 reinfection in sicily: A population-based evaluation of covid-19 events using current health databases
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University of Palermo Department of Health Promotion, Mother and Child Care, Internal Medicine, and Medical Specialties "G. D'Alessandro" Via del Vespro, 133 - Palermo (PA) Italy
University of Palermo
Publication date: 2023-04-26
Popul. Med. 2023;5(Supplement):A465
Background and Objective:
Worldwide, the COVID-19 outbreak resulted in more than 640 million cases as of 15 December 2022. As immunity to natural infection deteriorates with time and SARS-CoV-2 strains mutate, reinfections began to occur. The current study sought to assess the probability of SARS-CoV-2 reinfection among residents of Sicily, Italy, in relation to sex, age, number of mRNA COVID-19 vaccine doses administered, and occurrence of hospitalization during a previous infection.

A population-based retrospective cohort analysis was designed using the Sicilian COVID-19 monitoring system run by the National Institute of Health and vaccination flows acquired through a regional registry in Sicily. Only Sicilian adults were included in the study, and hazard ratios were calculated using Cox regression.

After an average of 198 days, there were 39,509 reinfections among 1,089,782 previously infected people (overall cumulative incidence: 3.62%) from the start of the pandemic to 3 September 2022. Almost all reinfections (37,084; 93.9%) occurred during the Omicron variant period, with the remaining 6.1% attributed almost entirely to the period in which Delta and Omicron variants co-circulated (2,011; 5.1%). The risk of reinfection was significantly lower among males than females (HR:0.74, 95%C.I.:0.73-0.76), decreased with age (from HR:1.24, 95%C.I.:1.20-1.28 for 30-39 years old people to HR:0.36, 95%C.I.:0.33-0.38 for 80+ individuals compared to 18-29 group), and lower for people receiving two or more mRNA vaccine doses (HR:0.39, 95%C.I.:0.38-0.40 compared to the individuals who did not receive vaccination). Interestingly, those who were hospitalized during their previous SARS-CoV-2 infection exhibited a significantly lower chance of reinfection (HR:0.74, 95%C.I.:0.70-0.78).

SARS-CoV-2 reinfection was found to be a relatively common occurrence, predominantly caused by the Omicron strain. COVID-19 vaccination provides significant protection against reinfection. Furthermore, the lower risk observed in the elderly and previously hospitalized persons reflects more stringent adherence to the use of personal protective equipment.