Patterns in Repeat Reinfections: Pre and Post Omicron Emergence

Our understanding of SARS-CoV-2 reinfection patterns remains limited. We conducted a longitudinal study using Qatars national SARS-CoV-2 data from February 28, 2020 to June 11, 2023 to investigate incidence of reinfections both prior to and after omicron emergence. The latter analysis excluded individuals with pre-omicron infections. Before omicron introduction, the proportion of incident infections classified as reinfections gradually increased but remained minimal, reaching 1.8% just before omicron emerged. During the first omicron wave, this proportion reached 9.0%, a 5-fold increase. After the conclusion of the first omicron wave, the proportion of incident infections identified as reinfections rapidly increased, reaching 43.3% towards the end of the study. In the pre-omicron era, a total of 3,131 reinfections were documented, of which 99.6% were first reinfections and 0.4% were second reinfections. Meanwhile, a total of 20,962 reinfections were documented after an omicron primary infection of which 99.0% were first reinfections, 1.0% were second reinfections, and 0.01% were third reinfections. Reinfections were rare before omicrons emergence but became widespread during the omicron era, including among individuals previously infected with omicron. Our findings may indicate accelerated viral evolution in the omicron era aimed at evading population immunity, but with minimal impact on COVID-19 severity, or potentially suggest immune imprinting effects that require further investigation.