Intrinsic and Effective Severity of Coronavirus Disease 2019 Cases Infected With the Ancestral Strain and Omicron BA.2 Variant in Hong Kong

Evaluating changes in COVID-19 severity over time is crucial when novel variants emerge. Intrinsic fatality risk was similar between Omicron and ancestral strains. Risk was inversely proportional to number of doses received, and higher risk was observed in older ages. Background Understanding severity of infections with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and its variants is crucial to inform public health measures. Here we used coronavirus disease 2019 (COVID-19) patient data from Hong Kong to characterize the severity profile of COVID-19. Methods Time-varying and age-specific effective severity measured by case hospitalization risk and hospitalization fatality risk was estimated with all individual COVID-19 case data collected in Hong Kong from 23 January 2020 through 26 October 2022 over 6 epidemic waves. The intrinsic severity of Omicron BA.2 was compared with the estimate for the ancestral strain with the data from unvaccinated patients without previous infections. Results With 32 222 COVID-19 hospitalizations and 9669 deaths confirmed over 6 epidemic waves, the time-varying hospitalization fatality risk dramatically increased from <10% before the largest fifth wave of Omicron BA.2 to 41% during the peak of the fifth wave when hospital resources were severely constrained. The age-specific fatality risk in unvaccinated hospitalized Omicron cases was comparable to the estimates for unvaccinated cases with the ancestral strain. During epidemics predominated by Omicron BA.2, fatality risk was highest among older unvaccinated patients. Conclusions Omicron has comparable intrinsic severity to the ancestral Wuhan strain, although the effective severity is substantially lower in Omicron cases due to vaccination.