Estimation of COVID-19 outbreak size in Harbin, China

Abstract Background: Since the first level response to public health emergencies (FLRPHE) was launched on January 25, 2020 in Heilongjiang province, China, the outbreak of COVID-19 seems to be under control. However, an outbreak of COVID-19 caused by imported case developed in Harbin during April 2020. Here, we provide an estimate of the COVID-19 outbreak size in Harbin in April based on the number of found infected cases, assess the proportion of found and unfound infected cases in infected cases and evaluate the effective reproduction number which shows the transmission risk of COVID-19. Methods: We used data from April 9 to April 30, 2020, on the number of found infected cases from the outbreak in Harbin to infer the number of infections in Harbin in the outbreak of COVID-19 and give the proportion of found and unfound infected cases in infected cases. Data on found infected cases were obtained from the the reports of Health Commission of Heilongjiang Province. A Susceptible-Unfound infected-Found infected-Removed model was used to fit the data on found infected cases of COVID-19 in Harbin using the least square method and simulate the transmission of COVID-19. The effective reproduction number was estimated. Results: The COVID-19 outbreak size estimated in Harbin in April reaches 174, where 54% of infected cases were found and 46% of infected cases were not found out. Our findings suggest that the effective reproduction number decreased drastically in contrast with the value of 3.6 on April 9 after that the effective interventions were implemented by Heilongjiang province government. Finally, the effective reproduction number arrived the value of 0.04 which is immensely below the threshold value 1, which means that Heilongjiang province government got the outbreak of COVID-19 in Harbin under control. Conclusions: The COVID-19 outbreak size in Harbin based on the assumptions that infected people with COVID-19 in the incubation period have the same infectivity with infectious people with COVID-19 could have been overestimated. As an increasing number of imported infected cases got into China and a growing number of asymptomatic infected people were found, our study provides evidence that unfound infected cases would increase the risk of local outbreak of COVID-19 in China.