Transmission dynamics, serial interval and epidemiology of COVID-19 diseases in Hong Kong under different control measures

<ns4:p><ns4:bold>Background:</ns4:bold> The outbreak of coronavirus disease 2019 (COVID-19) started in Wuhan, China in late December 2019, and subsequently became a pandemic. Hong Kong had implemented a series of control measures since January 2020, including enhanced surveillance, isolation and quarantine, border control and social distancing. Hong Kong recorded its first case on 23 January 2020, who was a visitor from Wuhan. We analysed the surveillance data of COVID-19 to understand the transmission dynamics and epidemiology in Hong Kong.</ns4:p><ns4:p> <ns4:bold>Methods:</ns4:bold> We constructed the epidemic curve of daily COVID-19 incidence from 23 January to 6 April 2020 and estimated the time-varying reproduction number (<ns4:italic>R</ns4:italic><ns4:sub>t</ns4:sub>) with the R package EpiEstim, with serial interval computed from local data. We described the demographic and epidemiological characteristics of reported cases. We computed weekly incidence by age and residential district to understand the spatial and temporal transmission of the disease.</ns4:p><ns4:p> <ns4:bold>Results:</ns4:bold> COVID-19 disease in Hong Kong was characterised with local cases and clusters detected after two waves of importations, first in late January (week 4 to 6) and the second one in early March (week 9 to 10). The <ns4:italic>R</ns4:italic><ns4:sub>t</ns4:sub> increased to approximately 2 95% credible interval (CI): 0.3-3.3) and approximately 1 (95%CI: 0.2-1.7), respectively, following these importations; it decreased to below 1 afterwards from weeks 11 to 13, which coincided with the implementation, modification and intensification of different control measures. Compared to local cases, imported cases were younger (mean age: 52 years among local cases vs 35 years among imported cases), had a lower proportion of underlying disease (9% vs 5%) and severe outcome (13% vs 5%). Cases were recorded in all districts but the incidence was highest in those in the Hong Kong Island region.</ns4:p><ns4:p> <ns4:bold>Conclusions:</ns4:bold> Stringent and sustained public health measures at population level could contain the COVID-19 disease at a relatively low level.</ns4:p>