Population serology for SARS-CoV-2 is essential to regional and global preparedness.

The emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus that causes COVID-19, poses a tremendous threat to human health.1, 2, 3 There were more than 4 million confirmed cases and 300 000 reported deaths worldwide as of May 11, 2020. There have been warnings of a major coronavirus pandemic since the SARS outbreak in 2002–04, and in 2020 that threat has been realised. Now we need to consider the challenges ahead; the current prevalence, the incidence of asymptomatic infections, and the true mortality remains unclear. We also need to ascertain the probable effectiveness of current control measures so future strategies can be prioritised accordingly.

At the end of 2019, the COVID-19 outbreak was first reported in the city of Wuhan, China, and has since spread worldwide. To contain the spread of the disease, a cordon sanitaire was imposed on Wuhan city on Jan 23, 2020, and travel restrictions were subsequently imposed on other cities across Hubei province the next day. After 61 days of lockdown in Hubei province, the province reopened again on March 25, 2020, and after 76 days of lockdown in Wuhan, the city reopened again on April 8, 2020. The screening of individuals in these areas provides essential information on how immunity, and potentially herd immunity, is shaped in the community that has so far had the longest chain of community transmission but also some of the strongest physical distancing measures.

During early-2020, many studies attempted to estimate the reporting rate (or ascertainment rate) of COVID-19 in Wuhan.6, 7, 8 Seroprevalence surveys of the general population are key to understanding reporting rates, the underlying number of infections, the build-up of immunity, and to reconstruct chains of transmission of SARS-CoV-2. In The Lancet Microbe, Kelvin To and colleagues present the results of a seroepidemiological survey of SARS-CoV-2 in the general population in Hong Kong and returnees evacuated from Hubei, China. The investigators enrolled 1938 individuals before and after the COVID-19 pandemic between 2018 and 2020, and 452 asymptomatic Hubei returnees in March, 2020. The study showed that as an emergent virus, SARS-CoV-2 had a seroprevalence of 3·8% among Hubei returnees (17 of 452 returnees); very far from any plausible level of herd immunity.

Although the study assessed a small sample—452 of approximately 60 million people in Hubei province—it provides an essential baseline for public health authorities when evaluating the effect of current and future interventions. Judging by patterns of circulation of endemic coronaviruses (HCoV-NL63, HCoV-HKU1, HCoV-229E, and HCoV-OC4), it is probable that broader immunity might build up in the years ahead. However, To and colleagues' study shows that the current level of immunity is far below the herd immunity threshold and will not appreciably slow future spread, so testing, screening, and contact tracing from symptomatic and asymptomatic infections are still key to stopping further infections.