Those infected with the Wuhan coronavirus are passing it on to two to three people on average, but that number could change quickly depending on factors including the success of China's efforts to contain the outbreak through quarantines and increased public awareness.
This was the consensus found in separate scientific analyses released in recent days.
The researchers offered slightly different views on the transmission rate, however, and stressed that the figure was a moving target and that there were large gaps in information needed to come up with a definitive number.
Among the unknowns is the so-called zoonotic rate: the number of people infected through contact with an animal.
One study led by British infectious disease specialist Neil Ferguson put the basic reproduction number, known as the R0 (R naught), for the virus at 2.6.
A second British study, by researchers at Lancaster University, put the figure between 3.6 and 4.0.
Another analysis by researchers at Guangzhou Provincial Centre for Disease Control and Prevention brought an estimate of 2.9.
"Whether transmission continues at the same rate now critically depends on the effectiveness of the intense control effort now underway in Wuhan and across China," said the report by Ferguson's team at London's Imperial College.
The R0 of an infection is the number of average secondary transmissions a patient can cause when there has been no previous occurrence of the illness and a vaccine is unavailable.
A value below one indicates that each infection will cause less than one new infection, leading to its decline. A value of one indicates that each infection will cause exactly one new infection, meaning the outbreak will be stable but will not result in an epidemic.
A value higher than that is cause for alarm as it indicates an epidemic is possible if control measures are not put in place.
In a briefing on Monday, a top US health official said there was nothing surprising about the researchers' different R0 interpretations.
"Most of the articles have had an interpretation that the R0 is somewhere between 1.5 and 3," said Nancy Messonnier of the US Centres for Disease Control and Prevention.
That range was "not really a dramatic difference", said Messonnier, director of the agency's National Centre for Immunisation and Respiratory Diseases. She compared the figures with the R0 for measles, which stood between 12 and 18.
In all three studies, the researchers acknowledged that their estimates came with significant uncertainties.
Apart from the rate of animal-sourced infections, the R0 may differ from place to place depending on the control measures put in place by local authorities.
Calculations are also affected by a lack of data about people who contracted the virus from carriers who do not show symptoms.
The Guangzhou researchers said their initial findings suggested that the outbreak of the Wuhan virus " officially known as 2019-nCOV " could be much higher than the 2003 Sars epidemic.
This was partly because of the possible spread of the coronavirus from asymptomatic carriers, which was uncommon during the Sars outbreak.
"Our findings indicate that more rigorous control and prevention measures on early detection, diagnosis and treatment of cases infected with 2019-nCoV are needed to contain its further spread," the Chinese researchers said.
Among their recommendations were the shortening of the period between when patients are found to have coronavirus symptoms and when they are put in isolation, and a reduction in public gatherings and movement.
The Imperial College researchers said transmission rates could drop significantly if the public became "fully aware of the threat", as was the case during the Sars crisis.
"If a similar change to contact patterns is occurring in this outbreak, rates of transmission are likely to be lower now than during the period for which these estimates were made, due to control measures and risk avoidance in the population," they said.
The Imperial College report was based on cases reported through January 18. The Lancaster University study examined cases up to January 21, and the study by the Guangzhou-based researchers examined those through January 23.
As of Monday, at least 2,863 cases have been confirmed in mainland China, with the vast majority coming from Wuhan, where the contagion first broke out.
International cases are also on the rise, with dozens confirmed across East and Southeast Asia. Sri Lanka recorded its first case late on Monday after confirming that a mainland Chinese woman had tested positive.
Additional reporting by Owen Churchill
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