Disease severity of unvaccinated SARS-CoV-2 positive adults less than 65 years old without comorbidity, in the Omicron period and pre-Omicron periods (preprint)

ABSTRACT

Background: The reduced severity and burden of COVID-19 in 2022 can largely be attributable to vaccination and a shift to Omicron predominance. However, millions of individuals remain unvaccinated. In the present study, we aimed to study disease severity in unvaccinated individuals without risk factors during the Omicron period, compared to pre-Omicron periods. Methods: This register-based study included all unvaccinated individuals in Sweden aged 18-64 years without comorbidity or care dependency who were SARS-CoV-2 positive between week 45 of 2020 and week 5 of 2022. Variant of concern (VOC) periods were periods with certain VOCs identified in [≥]92% of sequenced cases nationwide. Outcomes were hospitalization with a main discharge code of COVID-19; severe illness, defined as high-flow nasal oxygen treatment or intensive care unit admission; and death with COVID-19 as the underlying cause of death on the death certificate. Results: Among 788,895 individuals in the overall SARS-CoV-2 positive cohort, both hospitalization and death increased stepwise from the pre-VOC period to the Alpha and Delta periods, and decreased in the Omicron period. Among 15,179 patients hospitalized for COVID-19, the proportions with severe illness and death increased to the Delta period, but in the Omicron period, these outcomes returned to the level of the pre-VOC period. Conclusion: In the Omicron period, compared to pre-Omicron periods, unvaccinated SARS-CoV-2 positive adults <65 years old without comorbidity had reduced proportions of hospitalization and death overall, but similar proportion of severe illness among patients hospitalized for COVID-19. These results support continuous efforts to prevent hospitalizations for COVID-19.