“What’s key to all of this is good infection prevention and control in hospital settings. If health workers continue to get infected now that they know this virus is present, they can inadvertently infect other patients,” he said.

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China’s National Health Commission (NHC) said on Wednesday that tougher measures introduced this week authorised hospitals nationwide to quarantine any suspected carriers of the virus and their close contacts, even against their will. The coronavirus has killed 17 people – all of them in Hubei province – since it first caught the attention of medical authorities a little over three weeks ago. As of 11pm Wednesday, 541 people in mainland China had been infected, according to official reports.

“The basic [measures besides quarantine] are contact tracing, identifying people who might have been exposed, and making sure that they understand that if they develop a fever, they must immediately report to a health facility because that’s the time they become infectious,” said Heymann, who is chair of an advisory body to the WHO.

He said another measure was to isolate patients with the disease and to take care of them until the immune system could rid the body of the virus. “There’s no other medicine right now that can treat it.”

Wuhan, in Hubei province, has remained the ground zero of fatalities and infections since the outbreak was first reported on December 31 and with no drugs available to fight the virus, containment and quarantine are the tools China is turning to try and control it.

That had now reached a “most critical stage”, Li Bin, the director of the NHC, said on Wednesday.

The challenge for the authorities is how and to what extent to quarantine a city such as Wuhan, which has a larger population than New York and is a major hub in China’s rail network, with an average of 700,000 passengers a day passing through its three main stations. It also has an international airport with direct flights to the United States and the rest of Asia.

China is also this week experiencing its largest single annual mass migration of people as hundreds of millions travel across the country for the Lunar New Year holiday.

Li, of the NHC, said there was a risk of the virus mutating and spreading further during the country’s annual peak travel period.

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“The virus is mainly transmitted through the respiratory tract,” Li said. “Now, during Chinese New Year, the surge [in people moving around the country] increases the risk of the epidemic’s spread and the difficulty of prevention and control. We must not take it lightly.”

The virus, which has pneumonia-like symptoms, has most recently been discovered in travellers in Hong Kong and the US for the first time. Infected individuals have also been found in Taiwan, Thailand, Japan, South Korea and Macau, with most of the cases involving people travelling from Wuhan.

China’s stricter quarantine measures came as the provincial government in Hubei, where Wuhan is located, announced a “grade II public health emergency” – the second-highest level of emergency response – in the early hours of Wednesday morning.

“This means that various local authorities, government departments and medical institutions can legally screen for fever patients, and conduct isolated treatment of confirmed or suspected cases, as well as isolated treatment and medical observations of others in close contact with those people,” Xu Shuqiang, director of the health emergency response office at China’s Ministry of Health, said.

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On Tuesday, the NHC upgraded the coronavirus to a Class B infectious disease but said it would use the stricter control measures for a Class A disease to handle the outbreak, meaning any infection nationwide must be reported within two hours and monitored.

A Class B classification gives the government the power to stop travel to, from and within a city, and to take other emergency measures that would effectively shut down a city.

Closing down a city the size of Wuhan would be unprecedented in China, but the country’s top infectious disease expert said quarantine was the only way to stop the disease at present.

“As soon as it spreads from human to human, quarantine must be the first priority,” Zhong Nanshan said at a briefing on Tuesday. “At the moment, I don’t think quarantine has been implemented thoroughly enough.”

The disease is believed to have first appeared in a seafood and animal meat market in Wuhan.

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The first cluster of infections in December involved people who worked at the market, suggesting that the virus had jumped from an animal host in the same way that the severe acute respiratory syndrome (Sars) epidemic began.

Gao Fu, director of China’s disease control and prevention centre, said at the briefing that the source of the disease was a wild animal sold at the seafood market, but did not elaborate on what species it was, or how the disease spread from animals to humans.

As many as 15 medical staff in Wuhan have been infected with the virus, which exposed “weak links” in preventive measures in medical institutions, according to Jiao Yahui, deputy director in charge of medical affairs with the NHC.

“The fact that our medical staff have been infected is evidence that the virus is transmissible from human-to-human,” she said at the NHC briefing.

“We are in a gradual process of learning the development stages and characteristics of the virus,” said Jiao, who also paid tribute to front-line medical staff confronting the outbreak.

Heymann said it was not possible to make predictions of how the virus would proceed from now.

“Influenza viruses can’t be stopped, they run their course and they stop. It would be wrong to make any type of estimate of what might happen in future because not enough is known to base that estimate on. The WHO will be looking at all the evidence and making recommendations for the world to follow.”

Additional reporting by Elizabeth Cheung