Comparison of control and transmission of COVID-19 across epidemic waves in Hong Kong: an observational study (preprint)

Background: Hong Kong contained COVID-19 for two years, but experienced a large epidemic of Omicron BA.2 in early 2022 and endemic transmission of Omicron subvariants thereafter. Methods: We examined the use and impact of pandemic controls in Hong Kong by analysing data on more than 1.7 million confirmed COVID-19 cases and characterizing non-pharmaceutical and pharmaceutical interventions implemented from January 2020 through to 30 December 2022. We estimated the daily effective reproductive number (Rt) to track changes in transmissibility and effectiveness of community-based measures against infection over time. We examined the temporal changes of pharmaceutical interventions, mortality rate and case-fatality risks (CFRs), particularly among older adults. Findings: Hong Kong experienced four local epidemic waves predominated by the ancestral strain in 2020 and early 2021 and prevented multiple SARS-CoV-2 variants from spreading in the community before 2022. Strict travel-related, case-based, and community-based measures were increasingly tightened in Hong Kong over the first two years of the pandemic. However, even very stringent measures were unable to contain the spread of Omicron BA.2 in Hong Kong. Despite high overall vaccination uptake (>70% with at least two doses), high mortality was observed during the Omicron BA.2 wave due to lower vaccine coverage (42%) among adults [≥]65 years of age. Increases in antiviral usage and vaccination uptake over time through 2022 was associated with decreased case fatality risks. Interpretation: Integrated strict measures were able to reduce importation risks and interrupt local transmission to contain COVID-19 transmission and disease burden while awaiting vaccine development and rollout. Increasing coverage of pharmaceutical interventions among high-risk groups reduced infection-related mortality and mitigated the adverse health impact of the pandemic.

Resurgence of Omicron BA.2 in SARS-CoV-2 infection-naive Hong Kong (preprint)

After keeping infections at bay for two years, Hong Kong experienced a surge of Omicron BA.2 infections in early 2022 that overwhelmed the health care system, isolation facilities, and contact tracing capacity, leading to one of the highest per-capita death rates of COVID-19 in early 2022. The outbreak occurred against a backdrop of a dense population with low immunity towards natural SARS-CoV-2 infection, high vaccine hesitancy in vulnerable populations, comprehensive disease surveillance and the capacity for stringent public health and social measures. Using genome sequences and epidemiological data from this time, we reconstruct the epidemic trajectory of the BA.2 wave, estimate transmission and incidence rates, and evaluate the effectiveness of policy changes. We identify an increase in the effective reproductive rate (Re) to 9.5 in mid-January 2022, which preceded real-time estimates of transmission (Rt), revealing that BA.2 community transmission was under-ascertained weeks before the epidemic appeared to surge in mid-February 2022. Due to this, public health measures were relaxed in early February (Spring Festival) while Re increased and remained > 1 throughout February. An independent estimation of point prevalence and incidence using phylodynamics also indicates extensive superspreading at this time, which likely contributed to the rapid expansion of the epidemic. This study demonstrates that relying on Rt estimation methods dependent on case reporting can misinform epidemic response planning, sometimes with substantial consequences. There is a need for future research and implementation of improved estimates of epidemic growth in near real-time that combine multiple disparate data sources to better inform outbreak response policy.