COVID-19 was circulating silently in Wuhan even after the city reported no cases

China has lifted the lockdown in Wuhan, China, where the outbreak first began.
China has lifted the lockdown in Wuhan, China, where the outbreak first began. (Image credit: Shutterstock)

COVID-19 may have continued to spread silently in Wuhan, China, during the spring of 2020, even after official government tallies had suggested the coronavirus had been stamped out, a new study suggests.

SARS-CoV-2, the virus that causes COVID-19, was first discovered in Wuhan in December 2019, and the city soon became the epicenter of what would become the COVID-19 pandemic. Cases peaked in Wuhan in February 2020 but soon declined rapidly, with just a few cases reported in late March. By early April, the city's lockdown had ended, and later that month, Wuhan was declared coronavirus-free.

But the new study, published Thursday (Jan. 7) in the journal PLOS Neglected Tropical Diseases, tells a different story. The researchers, from Wuhan University, analyzed more than 63,000 blood samples collected in China — mainly in Wuhan — between March 6 and May 3, 2020. All of these participants were healthy and were undergoing screening before returning to work, the researchers said.

The blood samples were tested for antibodies against SARS-CoV-2. Specifically, the researchers looked for both IgG antibodies, a type of long-lasting antibody that suggests a previous infection with SARS-CoV-2, and IgM antibodies, a relatively short-lived antibody that suggests a current or recent infection with the virus.

In Wuhan, the percentage of participants with either of these antibodies was 1.7%. That's much higher than the percentage seen in areas outside Hubei province (which includes Wuhan), which was about 0.4%.

What's more, the researchers found that the IgM positivity rate — indicating an active or recent infection — in Wuhan was nearly 0.5%, compared with 0.07% in other parts of China.

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Based on the level of IgM antibodies seen in Wuhan in the spring of 2020, the researchers estimated that thousands of people were infected asymptomatically during this period.

"We conclude that ... a large amount of asymptomatic carriers of SARS-CoV-2 existed after elimination of clinical cases of COVID-19 in Wuhan City," the researchers wrote.

Based on the antibody numbers from the study, the researchers estimated that in Wuhan, a city of about 10 million people, roughly 168,000 people had been infected overall in Wuhan at that time — higher than the approximately 50,000 cases that had been reported.

The authors noted that from May 14 through June 1, officials in Wuhan conducted mass COVID-19 testing of 9.9 million people and found a rate of asymptomatic infection of just 0.3 per 10,000 people based on PCR testing for the genetic material of SARS-CoV-2. 

But the rate found in the current study, based on the IgM testing, was hundreds of times higher, the researchers said. This discrepancy could be due to several factors, including a greater sensitivity of blood antibody tests compared with PCR testing and the earlier dates of collection in the current study compared with the surveillance testing by city officials, the researchers said. 

Originally published on Live Science. 

Rachael Rettner

Rachael is a Live Science contributor, and was a former channel editor and senior writer for Live Science between 2010 and 2022. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.