The scientists also estimated nearly 7 per cent of Wuhan’s population had virus antibodies, a standard measure of previous Covid-19 infection, by last April. While various factors can impact such estimates, this suggests more than 600,000 of the 9 million people thought to be in Wuhan during the peak of its outbreak could have been infected – more than 10 times the 50,000 officially reported cases from the time.

Despite this, the researchers, who are affiliated with several institutions including the Wuhan Centre for Disease Control and Prevention, said their findings meant most of the population remained vulnerable to Covid-19.

“Even at the epicentre of the pandemic in China … the estimated seroprevalence in Wuhan remains low, and around 40 per cent of people with [traces of past infection] developed neutralising antibodies, suggesting there is still lack of immunity in the population,” said co-author Wang Chen of Peking Union Medical College.

China is seeking to ramp up its domestic vaccination programme, with an ambitious goal to vaccinate 40 per cent of the population – some 560 million people – by July.

Chinese experts have expressed concern over immunity in the country, where there has been limited spread of Covid-19 since last spring. Meanwhile, countries like the US and Britain, with both high levels of infection and vaccination per capita, are speeding towards the threshold needed to reach so-called “herd immunity” and curb future spread.

Richard Strugnell, a professor of microbiology at the Doherty Institute in Melbourne, said the latest findings could mean less than 3 per cent of Wuhan’s population is likely to be protected as a result of prior infection.

He credited the halt in infections to the disease control measures enacted in Wuhan. These included citywide lockdowns and isolating patients with mild to moderate Covid-19 in makeshift hospitals to prevent them from spreading the disease.

“The only – very small – penalty is that, at a population level, the community would now be very susceptible to a reintroduction of the virus,” he said, adding this could be dealt with by a vaccination programme.

The authors of The Lancet report also found that 82 per cent of the 532 people who had traces of past infection may have caught the virus without showing symptoms, higher than other estimates from outside China of around 40 per cent. Some of these people also developed neutralising antibodies.

Such cases could lead to disparity between the number of reported and estimated cases in Wuhan. Other recent studies have estimated around 4 per cent of Wuhan’s total population had been exposed to Covid-19.

Some 4,600 households across 13 districts of Wuhan were randomly chosen to participate in the study, with around 3,500 families made up of 9,542 people ultimately taking part. Of those individuals, 532 showed traces of prior Covid-19 infection, measured through the presence of one of several types of antibodies, when their blood was tested in April.

How the Covid-19 vaccines compare and who can get them

Research published in the journal Nature in November estimated that 1.7 million New Yorkers, or about 20 per cent of the city’s population, had been infected with the virus, by mid-May. At the time, the city reported around 190,000 cases, about a ninth of the researchers’ estimated infections.

Other factors that could account for the difference between estimated and reported cases may include limitations to the system for reporting cases, as University of Hong Kong experts have suggested, and the strain on the health care system.

The authors noted limitations to their study, including that they could not confirm when participants were infected and produced antibodies.