Authorities in China raced Friday to lock down more cities in an extraordinary bid to halt the spread of the deadly coronavirus, which has left at least 26 people dead in the country and sickened hundreds more. The move is unprecedented and affects more than 30 million people.

The U.S. has confirmed one case, and several others have popped up in Asian countries — all among people who visited Wuhan, China, the city at the epicenter of the outbreak.

Authorities believe the virus, which causes flu-like symptoms, moved into the human population from an infected animal at a market in Wuhan.

Chinese health officials and the World Health Organization confirmed this week that the virus has been transmitted person-to-person, but it remains unclear how easy it is to contract it from another infected individual. The WHO on Thursday declined to declare the virus a global public health emergency, saying it was still too soon to take that measure.

China’s bid to contain the deadly new coronavirus by placing cities of millions under quarantine is an unprecedented undertaking, but it is unlikely to stop the disease spreading, a public health expert has warned.

“I think we have passed the golden period of control and prevention,” said Guan Yi, an expert on viruses at Hong Kong University.

Yi, who returned to Hong Kong from Wuhan just before the lock-down, pointed out that huge numbers of people would have already left ahead of the Lunar New Year holiday, which began Friday.

They could have been incubating the virus “on their way out of Wuhan,” he said.

On the eve of the country’s biggest holiday, the Lunar New Year, transportation was shut down in at least 10 cities, affecting about 33 million people in total.

Wuhan, the metropolis at the center of the outbreak and where the majority of the cases have been reported, was the first to lock down all public transport and prevent people leaving. Nine neighboring cities in the central province of Hubei had followed suit by Friday morning, restricting at least most public transport.

Chinese authorities had also canceled all major public events in the capital Beijing indefinitely, including the massive “temple fairs” that normally stand out among New Year celebrations. Beijing’s famed Forbidden City and Disneyland in the business hub of Shanghai also said they would be closed from Saturday.