COVID-19 likely lingered longer than reported in Wuhan

Researchers reporting in PLOS Neglected Tropical Diseases have tested more than 60,000 healthy individuals in China for SARS-CoV-2 antibodies and concluded that thousands of Wuhan residents were infected with asymptomatic cases of COVID-19 after the infection was believed to be under control in China.

Rapid antibody tests are used to diagnose present and past infections with the SARS-CoV-2 virus that causes COVID-19; positive IgG antibodies suggests a previous infection while IgM antibodies mean a current or recent infection. Detection of both types of antibodies can give 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 to be free of disease in late April, although small clusters of cases appeared in later months.

Xue-jie Yu of Wuhan University, China, and colleagues studied the prevalence of IgG and IgM antibodies in blood samples collected between March 6 and May 3, 2020, from 63,107 individuals in China. All people tested were healthy and were undergoing screening before returning to work.

Consistent with a large number of cases having occurred 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 antibody positivity averaged 0.38%. Moreover, according to the IgM positive rate of 0.46% in Wuhan, the researchers estimated that thousands of people in Wuhan were infected asymptomatically between March and May 2020, when there were no clinically reported cases of COVID-19.