Intrinsic and effective severity of COVID-19 cases infected with the ancestral strain and Omicron BA.2 variant in Hong Kong

Abstract

Background: Understanding severity of infections with SARS-CoV-2 and its variants is crucial to inform public health measures. Here we used COVID-19 patient data from Hong Kong to characterise the severity profile of COVID-19 and to examine factors associated with fatality of infection. Methods: Time-varying and age-specific effective severity measured by case-hospitalization risk and hospitalization risk was estimated with all individual COVID-19 case data collected in Hong Kong from 23 January 2020 through to 26 October 2022 over six epidemic waves, in comparison with estimates of influenza A(H1N1)pdm09 during the 2009 pandemic. 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. Factors potentially associated with the fatality risk of hospitalized Omicron patients were also examined. Results: With 32,222 COVID-19 hospitalizations and 9,669 deaths confirmed over 6 epidemic waves in Hong Kong, the time-varying hospitalization fatality risk dramatically increased from below 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, the highest fatality risk was amongst unvaccinated patients aged [≥]80 years and the risk was inversely associated with the number of vaccination doses received. Conclusions: Omicron has comparable intrinsic severity to the ancestral Wuhan strain although the effective severity is substantially lower in Omicron cases due to vaccination. With a moderate-to-high coverage of vaccination, hospitalized COVID-19 patients caused by Omicron subvariants appeared to have similar age-specific risks of fatality to patients hospitalized with influenza A(H1N1)pdm09.