The new estimation by Los Alamos National Laboratory in New Mexico is that those who carried the coronavirus in Wuhan were passing it on to 5.7 people on average.

The finding could help public health experts to refine their containment and vaccination strategies.

In their study, published last week in the journal Emerging Infectious Diseases, the researchers, led by Steven Sanche and Lin Yen-ting, wrote: “Unavailability of diagnostic reagents early in the outbreak, changes in surveillance intensity and case definitions, and overwhelmed health care systems confound estimates of the growth of the outbreak based on data.”

The Los Alamos research analysed about 140 early patients outside Hubei province, of which Wuhan is the capital, to project how intensely the coronavirus was spreading from the epicentre. Most of the initial cases in other provinces had epidemiological links or exposure to Wuhan.

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“By the time cases were confirmed in provinces outside Hubei, all of the provinces of China had access to diagnostic kits and were engaging in active surveillance of travellers out of Wuhan,” the researchers said.

“The health care systems outside Hubei were not yet overwhelmed with cases and were actively searching for [their] first positive case, leading to much lower bias in the reporting.”

Many provinces’ health commission or local Centre for Disease Control and Prevention (CDC) would also release basic epidemiological information on how each patient could have been related to another, or where they had shared exposure.

The US researchers also used mobile phone data to estimate the numbers of daily travellers in and out of Wuhan.

Their projection was then compared back with the death rate pattern in Wuhan, which was more clearly defined and consistent than the city’s other data on the outbreak.

They found that instead of taking six to seven days for the number of infected people to double, as was previously thought, it took only 2.3 to 3.3 days to do so.

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In late January, researchers from mainland China and Hong Kong, including China CDC chief George Gao, had estimated that a Covid-19 patient could infect an average of 2.2 people, based on studying 425 patients in Wuhan.

A more recent estimate of this reproduction number, by Imperial College London last month, found the figure for 11 European countries including Britain to be 3.87 people.

Also last month, research at Payame Noor University in Tehran, which was subject to peer review, found that an average of 4.86 people could be infected per patient in the first week of the outbreak in Iran.

The reproduction number for the 2002-03 severe acute respiratory syndrome (Sars) outbreak – which infected over 8,000 people and killed nearly 800, mostly in mainland China and Hong Kong – was estimated by the World Health Organisation to be 3.

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The authors of the new Los Alamos study added that a higher level of infectiousness meant that if asymptomatic carriers accounted for a substantial proportion of transmission, then quarantine and tracing of contacts of those showing symptoms would not be enough to halt the virus’ spread.

“When 20 per cent of transmission is driven by unidentified infected persons, high levels of social distancing efforts will be needed to contain the virus,” they said.

“The decline in newly confirmed cases in [mainland] China and South Korea in March 2020 and the stably low incidences in Taiwan, Hong Kong and Singapore, strongly suggest that the spread of the virus can be contained with early and appropriate measures.”