Wuhan City in China was the first place to report COVID-19 in the world and – between December 2019 and May 2020 – caused almost two-thirds of all COVID-19 cases in China. Now researchers report in PLOS neglected tropical diseases tested more than 60,000 healthy individuals in China for EARS-CoV-2 antibodies and concluded that thousands of Wuhan residents were infected with asymptomatic cases of COVID-19 after the infection was reportedly under control in China.
Rapid antibody tests are used to diagnose current and past infections with the SARS-CoV-2 virus that causes COVID-19; positive IgG antibodies indicate a previous infection, while IgM antibodies indicate a current or recent infection. Detection of both types of antibodies can provide a better understanding of the number of asymptomatic infections in a population over time. In Wuhan, the number of COVID-19 cases peaked in February 2020, and the city was initially declared disease-free in late April, although small groups of cases appeared later.
In the new work, Xue-jie Yu of Wuhan University, China, and colleagues studied the occurrence of IgG and IgM antibodies in blood samples collected between March 6 and May 3, 2020 from 63,107 individuals in China. All the people tested were healthy and did research before going back to work.
Consistent with a large number of cases occurring in Wuhan, the percentage of people with positive SARS-CoV-2 antibodies, about 1.68%, was significantly higher than in other regions of China, where the antibody positivity averaged 0.38 % was. According to the IgM-positive rate of 0.46% in Wuhan, the researchers estimated that thousands of people in Wuhan were asymptomatically infected between March and May 2020, when there were no clinically reported cases of COVID-19.
“We conclude that … a large number of asymptomatic carriers of SARS-CoV-2 existed after clinical cases of COVID-19 in Wuhan City were eliminated,” the researchers said. “Avirulent SARS-CoV-2 strains can still cause symptoms in extremely susceptible individuals and it can also return to a very virulent strain to flare up the epidemic of COVID-19 in China.”
Reference: “Seroprevalence and asymptomatic carrier status of SARS-CoV-2 in Wuhan City and other places in China” by Duan S, Zhou M, Zhang W, Shen J, Qi R, Qin X, et al, January 7, 2021, PLOS neglected tropical diseases.
Funding: This study was supported by the National Natural Science Funds of China (No. 81971939). The funders played no role in the design of the study, the collection and analysis of data, the decision to publish or compile the manuscript.