Wuhan, the largest city in Hubei, China, was the first place to report COVID in the world, with the first case reported in December 2019.

Now, Chinese researchers have tested over 60,000 healthy individuals for the coronavirus (SARS-CoV-2) antibodies and found that thousands of Wuhan residents were infected with the virus but were asymptomatic, making officials believe that the outbreak was under control in China.

The researchers published their findings in the scientific journal PLOS Neglected Tropical Diseases.

Experts use rapid antibody tests to diagnose present as well as past COVID infections. Positive IgG antibodies suggest a previous infection and positive IgM antibodies indicate a current or recent infection.

COVID antibody detection could give a better understanding and a clear picture of the number of asymptomatic cases over time.

In Wuhan, the number of coronavirus cases surged in February 2020. In late April, the city declared to be disease-free after officials reported no new cases; however, a few cases appeared in later months.

In the current study, Dr. Xue-jie Yu of Wuhan University and his team studied the prevalence of IgG and IgM antibodies in blood samples collected from March 6 to May 3, 2020, from more than 63,100 people in China.

The team found that a large number of cases occurred in Wuhan, who were asymptomatic. About 1.68% of the people tested positive for SARS-CoV-2 antibodies, which was significantly higher than in other regions of China.

They also found that the IgM positive rate was 0.46% in Wuhan, estimating that thousands of residents in Wuhan were asymptomatic from March to May 2020, meaning they were not reported during the outbreak.

The researchers said, “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.”

“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,” they added. The article originally appeared on Medical Xpress.