Millions left Wuhan before quarantine – where did they go?

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For weeks after the first reports of a mysterious new virus in Wuhan, millions of people poured out of the central Chinese city, cramming onto buses, trains and planes as the first wave of China’s great Lunar New Year migration broke across the nation. Some carried with them the new virus that has since claimed over 8 00 lives and sickened more than 37,000 people.

Officials finally began to seal the borders on Jan. 23. But it was too late. Speaking to reporters a few days after the the city was put under quarantine, the mayor estimated that 5 million people had already left.

Where did they go?

An Associated Press analysis of domestic travel patterns using map location data from Chinese tech giant Baidu shows that in the two weeks before Wuhan’s lockdown, nearly 70% of trips out of the central Chinese city were within Hubei province. Baidu has a map app that is similar to Google Maps, which is blocked in China.

Another 14% of the trips went to the neighboring provinces of Henan, Hunan, Anhui and Jiangxi. Nearly 2% slipped down to Guangdong province, the coastal manufacturing powerhouse across from Hong Kong, and the rest fanned out across China. The cities outside Hubei province that were top destinations for trips from Wuhan between Jan. 10 and Jan. 24 were Chongqing, a municipality next to Hubei province, Beijing and Shanghai.

The travel patterns broadly track with the early spread of the virus. The majority of confirmed cases and deaths have occurred in China, within Hubei province, followed by high numbers of cases in central China, with pockets of infections in Chongqing, Shanghai and Beijing as well.

“It’s definitely too late,” said Jin Dong-Yan, a molecular virologist at Hong Kong University’s School of Biomedical Sciences. “Five million out. That’s a big challenge. Many of them may not come back to Wuhan but hang around somewhere else. To control this outbreak, we have to deal with this. On one hand, we need to identify them. On the other hand, we need to address the issue of stigma and discrimination.”

He added that the initial spread of travelers to provinces in central China with large pools of migrant workers and relatively weaker health care systems “puts a big burden on the hospitals … of these resource-limited provinces.”

Baidu gathers travel data based on more than 120 billion daily location requests from its map app and other apps that use Baidu’s location services. Only data from users who agree to share their location is recorded and the company says data is masked to protect privacy. Baidu’s publicly available data shows proportional travel, not absolute numbers of recorded trips, and does not include trips by people who don’t use mobile phones or apps that rely on Baidu’s popular location services.

Public health officials and academics have been using this kind of mapping data for years to track the potential spread of disease.

A group of researchers from Southampton University’s WorldPop research group, which studies population dynamics, used 2013-2015 data from Baidu’s location services and international flight itineraries to make a predictive global risk map for the likely spread of the virus from Wuhan.

It’s important to understand the population movements out of Wuhan before the city’s lock down, said Lai Shengjie, a WorldPop researcher who used to work at China’s Center for Disease Control and Prevention.

“Maybe they hadn’t developed symptoms but could transmit the virus. We need to look at destinations across China and the world and focus on the main destinations and try to prepare for disease control and prevention,” he said.

The last trains left Wuhan the morning of Jan. 23, cutting off a surge of outbound travel that had begun three days earlier, Baidu data shows. Nearby cities rushed to impose travel restrictions of their own. From Jan. 23 to Jan. 26, the 15 cities that Baidu data shows received the most travelers from Wuhan – a combined 70% – all imposed some level of travel restrictions.