If accurate, the findings are significant given that Chinese officials declared the city free of the virus and lifted lockdown restrictions there in April.
“We conclude that … a large amount of asymptomatic carriers of [the new coronavirus] existed after elimination of clinical cases of COVID-19 in Wuhan City,” the researchers wrote.
The global pandemic is believed to have started in the city, and it accounted for nearly two-thirds of the more than 96,000 cases of infection reported in China, according to figures from Johns Hopkins University.
However, as of Thursday, World Health Organization officials attempting to investigate the origins of the virus have yet to be allowed access to the city, the agency said.
“I’m very disappointed with this news, given that two members have already begun their journeys, and others were not able to travel at the last minute,” WHOs director general Tedros Adhanom Ghebreyesus told the media on Wednesday.
“But I have been in contact with senior Chinese officials [and] I have once again made made it clear that the mission is a priority for WHO and the international team,” he said.
Wuhan was the first place in the world to report cases of COVID-19, in December 2019, the authors of the PLOS Neglected Tropical Diseases study said.
In Wuhan, the number of COVID-19 cases peaked last February and the city was initially declared virus-free in late April, although small clusters of cases appeared in later months, according to the researchers.
For this study, the researchers, from the Wuhan University School of Health Sciences, tested blood collected from more than 63,000 healthy individuals in China for COVID-19 antibodies between March 6 and May 3.
Antibodies are cells produced by the human immune system to fight off invading pathogens, such as viruses.
The presence of immunoglobulin G antibodies in blood is a sign of past COVID-19 infection, while blood with immunoglobulin M suggest a current or recent infection, according to the researchers.
The percentage of Wuhan with virus antibodies, about 1.7%, was significantly higher than in other regions of China, where antibody positivity averaged 0.38%, the researchers said.
Of the tested blood samples from Wuhan, roughly 0.5% were positive for immunoglobulin M as late as May, they said.
Given that the population of the city is close to 11 million, this indicates that thousands of residents were infected with asymptomatic cases of COVID-19, and their cases went unreported, after the infection was believed to be under control in China, the researchers said.
“Virulent [coronavirus] 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 wrote.