More than 232,000 people may have been infected in the first wave of Covid-19 in mainland China, four times the official figures, according to a study by Hong Kong researchers.

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China reported more than 55,000 cases as of 20 February but, according to research by academics at Hong Kong University’s school of public health, published in the Lancet, the true number would have been far greater if the definition of a Covid-19 case that was later used had been applied from the outset.

China has now reported more than 83,000 cases. Globally, the death toll from the coronavirus has exceeded 184,000, with the number of cases worldwide standing at more than 2.6m.

The new estimates come amid a mounting clamour for an international inquiry into the coronavirus outbreak’s origins, led by the United States and Australia, although the estimates appear far from constituting the proof of a coverup sought by some on the political right. The US and Australia have called for an international investigation into the handling of the outbreak.

The initial diagnostic criteria for identifying the disease, as the paper’s authors make clear, was very narrow, and was revisited seven times between 15 January and 3 March by China’s national health commission as new information about the disease became available and as laboratory testing capacity was expanded.

The Hong Kong study analysed data up to 20 February taken from the World Health Organization’s mission to Wuhan. It estimated that each of the first four changes increased the proportion of cases detected and counted, by between 2.8 and 7.1 times.

“If the fifth version of the case definition had been applied throughout the outbreak with sufficient testing capacity, we estimated that by 20 February 2020, there would have been 232,000 … confirmed cases in China as opposed to the 55,508 confirmed cases reported,” the study said.

Based on the two factors the authors went back to re-examine how many cases could have been missed if the later criteria had been applied at the beginning of the outbreak.

The researchers point in particular to the very narrow scope of the first diagnostic guidelines, which required six specific criteria to be met for a patient to be a confirmed case of Covid-19. The criteria included a patient’s epidemiological link to Wuhan or a wet market in Wuhan, and a full genome sequencing test of a patient’s respiratory specimen showing a close homology with Covid-19.

“In China, broadening the case definitions over time allowed a greater proportion of infections to be detected as cases,” the researchers wrote.

“The true number of infections could still be higher than that currently estimated, considering the possibility of under-detection of some infections, particularly those that were mild and asymptomatic, even under the broadest case definitions.”

There have been controversies in many countries about the counting of deaths and infections, but China in particular has been accused of a lack of transparency over the reporting of its figures. Last week it said that the death toll in Wuhan, where the virus is believed to have originated, was in fact 50% higher than first reported.

In the UK there has been a disparity between the government’s daily figures, which only count deaths in hospitals, and the figures published weekly by the Office of National Statistics, which include deaths in the community.

An analysis by the Financial Times this week suggested the true death toll in the UK could be 41,000 – twice what has been recorded.

Equally it is assumed that official figures for infections in the UK are much lower than the true figures, owing to a lack of testing.

Similar issues were highlighted in Spain on Thursday, when the federal government and Madrid’s regional government reported conflicting figures for deaths.

The Trump administration’s public accusations against China of a lack of transparency over the origins of the virus and the reporting of its figures have provoked anger in Beijing.

On Wednesday China’s ambassador to the US, Cui Tiankai, said there needed to be “a serious rethink of the foundations” of US-China relations. He also criticised politicians in the US for ignoring scientists and making “groundless” accusations.

The same day, the US secretary of state, Mike Pompeo, said the US believed that China failed to report the outbreak of the virus in a timely manner.

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The US, mainly via president Donald Trump, has amplified theories that the virus escaped from a Chinese lab, without evidence.

On Wednesday evening, Trump rebuked a state governor and Republican ally over his decision to reopen bowling alleys, hair salons and other businesses on Friday “in violation” of the phased federal guidelines.

Despite having voiced support for US citizens protesting against lockdowns, Trump said of the Georgia governor, Brian Kemp: “I want him to do what he thinks is right, but I disagree with him on what he’s doing.”

The US’s most senior infectious diseases expert, Dr Anthony Fauci, also urged against Kemp’s decision.

At the same press conference however there was complete contradiction between Trump and his experts over the risk of a virus resurgence later in the year.

Trump said the director of the Centers for Disease Control and Prevention, Dr Robert Redfield, had been “totally misquoted” in an article about the dangers of the virus during flu season. Redfield, standing by Trump at the podium, told reporters: “I’m accurately quoted in the Washington Post”.

Fauci said he was “convinced” of the risk of resurgence, adding: “We will have coronavirus in the fall.”