Estimation of the time-varying effective reproductive number of COVID-19 based on multivariate time-series of severe health outcomes.

The time-varying effective reproduction number (Rt at time t) measures the transmissibility of SARS-CoV-2 and is conventionally based on daily case counts which may suffer from time-varying ascertainment. We analyzed Rt estimates from case counts, and severe COVID-19 (ICU admissions, severe or critical cases, and mortality) across 2022 in Hong Kong's fifth and sixth waves of infection. Within the fifth wave, the severe disease-based Rt (3.5) was significantly higher than the case-based Rt (2.4), but not in the sixth wave. During periods with fluctuating underreporting, data based on severe diseases may provide more reliable Rt estimates.