The spread of Coronavirus Disease 2019 (COVID-19) had been substantially controlled in China. As the capital of China, Beijing took a series of strict containment measures during the past several months. However, the seroprevalence of COVID-19 in Beijing has not been evaluated.
During April 15-18, 2020, residents in Beijing were enrolled using a multi-stage cluster random sampling technique in four districts with top COVID-19 incidence and one district without case reported. Communities and households in each district were randomly selected based on probability proportional to size. Blood samples were collected and antibodies to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) were tested by two kinds of colloidal gold kits. All colloidal gold positive serums and 20 randomly selected negative serums were then tested by Micro-neutralization assay. A likelihood ratio test was used to estimate 95% confidence intervals of seroprevalence with the Clopper-Pearson exact method.
A total of 2,184 residents participated in this survey, among which 13 were tested positive by colloidal gold tests. Of those who tested positive, 8 were IgM positive, 3 were IgG positive, 1 was total antibody positive, 1 was both IgM and total antibody positive. Among the 13 seropositive samples and 20 randomly selected seronegative samples, no positive sample was detected by micro-neutralization assay. The seroprevalence of COVID-19 in Beijing was estimated no higher than 0.17%.
The seroprevalence of COVID-19 was low in April suggests that the comprehensive control measures to prevent and control further spread in Beijing was effective. However, the majority of the residents in Beijing were still susceptible to infection; the risk of rebound should be noted due to low population-level immunity.