The number of people who have been infected with COVID-19 in the Chinese city of Wuhan, where the virus was first identified, could be around three times the official figure, according to a study by Chinese researchers based in the city.

The paper, published by the PLOS Neglected Tropical Diseases journal on Thursday, analysed blood samples from more than 60,000 healthy individuals taken from locations across China from March to May 2020.

It found that 1.68% of those from Wuhan contained antibodies for SARS-CoV-2, the virus that causes COVID-19, compared to 0.59% in surrounding Hubei province and 0.38% in the rest of China.

With the city’s total population at more than 10 million, the researchers estimated that as many as 168,000 Wuhan residents were infected with the virus, compared to the official number of 50,340 hospitalised cases.

The study suggested at least two thirds of the total number were asymptomatic, and thousands could have been infected after the “elimination” of clinical cases, raising the possibility the virus could exist in a community for a long period without causing hospitalisations.

A separate study published by the Chinese Center for Disease Control and Prevention (CDC) late last month put the “seroprevalence” rate in Wuhan, the percentage of the population with antibodies, even higher at 4.43%, implying that around half a million people in the city could have been infected.

COVID-19 was identified in Wuhan at the end of 2019, with the first outbreak associated with a seafood market in the city. China finally locked down Wuhan and other cities in Hubei province on Jan. 23, 2020, but critics say it should have acted sooner.

China has dismissed criticism of its early handling of the virus, and officials now point to overseas studies suggesting it was circulating in Europe several months before the Wuhan outbreak.

A 10-strong team from the World Health Organization was due to arrive in China this week to investigate the origins of COVID-19, but they have yet to be been given authorisation to enter the country.

Total number of confirmed COVID-19 cases to date in mainland China now stands at 87,331, while the death toll remained unchanged at 4,634.