Seroprevalence of SARS-CoV-2 in Hong Kong returnees – Authors' reply

We are grateful for the useful comments of William P Hanage and colleagues. We admitted the limitations of our study, in which we used a relatively small and geographically non-random cohort of 452 Hong Kong Special Administration Region (HKSAR) residents evacuated from Hubei Province as a representation of the situation in Hubei Province in March, 2020. Therefore, we can only prudently include the possible implications of our findings in the discussion. Although we acknowledge that Hanage and colleagues estimated a higher case fatality rate on the basis of data from Wuhan city, the number of deaths stated in their letter, 3689, is much higher than those reported in early March when the blood samples were collected from the returnees. According to data from the Health Commission of Hubei Province, there were only 2227 deaths in Wuhan city as of March 1, 2020, and 2469 deaths in Wuhan city as of March 15, 2020.1, 2 Therefore, the case fatality rate in Wuhan would probably be lower than that estimated by Hanage and colleagues.

In the discussion of our study, we estimated the number of seropositive individuals in Hubei province, and, in particular, Wuhan city on the basis of seroprevalence rates among evacuees returning to HKSAR. Limited by the relatively small sample size, the 95% CI of the seroprevalence rate is large, and therefore we did not do a further statistical analysis on the mortality rate on the basis of our seroprevalence data, and did not calculate a separate mortality rate on Wuhan city. Rather, our estimate aimed to provide the reader with a sense of the magnitude of the mortality when compared with that of pandemic influenza. Furthermore, since only 88 individuals returning from outside Wuhan city were included, we did not separate the analysis for other cities. Further studies in other parts of Hubei province will be needed to better understand the seroprevalence rates in different geographical areas.

Compared with regions in other countries affected by the ongoing COVID-19 pandemic, our study found that Hubei Province and HKSAR have relatively low seroprevalence rates which were probably the result of successfully implemented stringent control measures in these localities. As Hanage and colleagues have pointed out, our study showed that the first wave of COVID-19 did not result in population immunity. This low herd immunity among our population in Hubei Province and HKSAR might allow a second wave of COVID-19 if control measures were overly loosened before an effective vaccine is available.