The city of Wuhan, China, was the first place where COVID-19 was recorded worldwide, accounting for nearly two thirds of all COVID-19 cases in China between December 2019 and May 2020. Now, researchers reporting on PLOS Neglected Tropical Diseases have tested more than 60,000 healthy individuals for SARS-CoV-2 antibodies in China and concluded that after the infection appeared to be under control in China, thousands of Wuhan residents were infected with asymptomatic cases of COVID-19.
To diagnose current and past infections with the SARS-CoV-2 virus that causes COVID-19, rapid antibody tests are used; positive IgG antibodies indicate past infection, while IgM antibodies indicate current or recent infection.
A greater understanding of the amount of asymptomatic infections in a population over time can be obtained by detecting both forms of antibodies.
The number of cases of COVID-19 in Wuhan peaked in February 2020, and the city was initially declared disease-free in late April, although there were still minor clusters of cases in the following months.
In the new research, Xue-jie Yu of Wuhan University, China, and colleagues examined the prevalence of IgG and IgM antibodies in blood samples collected between March 6 and May 3, 2020, from 63,107 individuals in China.
Before returning to work, all the people tested were well and were screened.
The percentage of people with positive SARS-CoV-2 antibodies, consistent with a large number of cases occurring in Wuhan, was around 1.68 percent, substantially higher than in other regions of China, where antibody positivity averaged 0.38 percent. Moreover, based on the 0.46 percent IgM-positive incidence in Wuhan, the researchers estimated that between March and May 2020, thousands of people in Wuhan were asymptomatically infected while there were no clinically reported cases of COVID-19.
“We conclude that … a large amount of asymptomatic carriers of SARS-CoV-2 existed after the elimination of clinical cases of COVID-19 in Wuhan City,” say the researchers. “Avirulent SARS-CoV-2 strains may still cause symptoms in extremely susceptible individuals, and it may also revert to a highly virulent strain to reignite the epidemic of COVID-19 in China.”
Reference: “Seroprevalence and asymptomatic carrier status of SARS-CoV-2 in Wuhan City and other places of 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 research was sponsored by the China National Funds for Natural Science (No. 81971939).
The funders had no impact on the design of the report, the collection and analysis of data, the decision to publish, or the manuscript preparation.