Incidence, severity, risk factors and outcomes of SARS-CoV-2 reinfections during the Omicron period: a systematic review and meta-analysis

Background Our previous systematic review estimated the cumulative incidence of SARS-CoV-2 reinfections as 1.16% (95% CI = 1.01-1.33%) during the pre-Omicron period. The Omicron variant that emerged in November 2021 was significantly genetically distinct from the previous SARS-CoV-2 variants and thus, more transmissible and posed an increased risk of SARS-CoV-2 reinfections in the population. We, therefore, conducted a fresh systematic review and meta-analysis to estimate the SARS-CoV-2 reinfection burden during the Omicron period. Methods We searched CINAHL, Medline, Global Health, Embase, and WHO COVID-19 in October 2023 for studies reporting the SARS-CoV-2 reinfection incidence during the Omicron period. The quality of the included studies was assessed using the Joanna Briggs Institute checklists. Random effects meta-analyses were conducted to estimate the incidence, and requirement of hospitalisation of SARS-CoV-2 reinfections. Symptomatic severity of reinfections and case fatality rates were analysed narratively. Results Thirty-six studies were included. The reinfection cumulative incidence during the Omicron period was 3.35% (95% CI = 1.95-5.72%) based on data from 28 studies. The cumulative incidence was higher in 18-59-year-old adults (6.62% (95% CI = 3.22-13.12%)) compared to other age groups and in health care workers (9.88% (95% CI = 5.18-18.03%)) compared to the general population (2.48% (95% CI = 1.34-4.54%)). We estimated about 1.81% (95% CI = 0.18-15.87%) of the reinfected cases required hospitalisation based on limited and highly variable data. Conclusions There was an increased risk of reinfections during the Omicron period compared to the pre-Omicron period. The incidence was higher in 18-59-year-old adults and health care workers and generally less severe during the Omicron period. However, data were limited on disease severity and longterm outcomes. Registration PROSPERO: CRD42023482598.