China on Thursday locked down two cities at the epicentre of a coronavirus outbreak that has killed 17 people and infected nearly 600, as health authorities around the world took action to prevent a global pandemic.
Health officials fear the transmission rate will accelerate as hundreds of millions of Chinese travel at home and abroad during week-long holidays for the Lunar New Year, which begins on Saturday.
The previously unknown virus strain is believed to have emerged late last year from illegally traded wildlife at an animal market in the central Chinese city of Wuhan.
Most transport in Wuhan, a city of 11 million people, was suspended on Thursday morning and people were told not to leave. Hours later, neighbouring Huanggang, a city of about 7 million people, announced a similar lockdown.
"The lockdown of 11 million people is unprecedented in public health history, so it is certainly not a recommendation the WHO has made," Gauden Galea, the World Health Organization's representative in Beijing, told Reuters.
Other cities were also taking steps to restrict movement and contact. Nearby Ezhou shut its train stations. The capital Beijing cancelled major public events, including two well-known Lunar New Year temple fairs, the state-run Beijing News said.
Airports worldwide were screening passengers arriving from China.
There is no vaccine for the virus, which can spread through respiratory transmission. Symptoms include fever, difficulty in breathing and cough, similar to many other respiratory illnesses.
Preliminary research suggested it was passed on to humans from snakes, but government medical adviser Zhong Nanshan has also identified badgers and rats as possible sources.
The WHO has said it will decide on Thursday whether to declare the outbreak a global health emergency, which would step up the international response.
If it does so, it will be the sixth international public health emergency to be declared in the last decade. A WHO news conference is expected some time after 1800 GMT.
Chinese authorities gave no new details on the numbers of virus infections but it has been reported in Beijing, Shanghai and Hong Kong. Of eight known cases worldwide, Thailand has confirmed four, while Japan, South Korea, Taiwan and the United States have reported one each.
Authorities had confirmed 571 cases and 17 deaths by the end of Wednesday, China's National Health Commission said. Earlier, it said another 393 suspected cases had been reported.
In a report on Wednesday, Imperial College London said it estimated a total of 4,000 cases of the coronavirus in Wuhan alone as of Jan. 18, an infection rate based on the number of cases reported in China and elsewhere.
Wuhan shut down all urban transport networks and suspended outgoing flights from 10 a.m. (0200 GMT). Domestic media said some airlines were operating after the deadline, however.
Wuhan's Hankou rail station was nearly deserted, with gates blocked, state broadcasts showed. The government urged citizens not to leave the city.
State media reported highway toll booths around Wuhan were closing down, which would effectively cut off road exits. Guards were patrolling highways, one resident told Reuters.
As the city slipped into isolation, residents thronged into hospitals for checks and scrambled for supplies, clearing out supermarket shelves and queuing for petrol.
Authorities in Huanggang ordered indoor entertainment venues including cinemas and internet cafes to close.
In contrast with its secrecy over the 2002-03 Severe Acute Respiratory Syndrome (SARS) that killed nearly 800 people, China's Communist Party government has provided regular updates to avoid panic ahead of the holidays.
During a visit to Wuhan, Vice Premier Sun Chunlan said authorities needed to be open about the virus and efforts to contain it, the official Xinhua news agency said.
Some experts believe the new virus is not as dangerous as previous coronaviruses such as SARS and Middle East Respiratory Syndrome (MERS), which has killed more than 700 people since 2012.
"The early evidence at this stage would suggest it's not as severe," Australia's Chief Medical Officer Brendan Murphy told reporters.
Despite China's response, world shares fell on Thursday, led by the biggest tumble in Chinese stocks in more than eight months, as concern mounted about the outbreak. China's yuan fell to a two-week low.
The economic impact of such outbreaks are hard to quantify but a 2006 estimate by the International Air Transport Association (IATA) calculated that SARS shaved just over 1 percentage point off the GDP of China in 2003.
InterContinental Hotels and Hyatt are allowing guests to change or cancel stays at most Chinese hotels.
Many Chinese were cancelling trips, buying face masks and avoiding shopping centres.
The release of seven movies over the Lunar New Year has been postponed. The holiday is the high season for distributors and cinemas attract huge crowds.
Airports globally, including in Britain, stepped up screening of passengers from China and the European Centre for Disease Control and Prevention said the further global spread of the virus was likely.
"All the fatalities have so far been contained to mainland China, however, this is a rapidly developing situation and the number of deaths and the number of cases is likely to be higher than those that have been confirmed so far and I expect them to rise further," Health Secretary Matt Hancock told the British parliament.