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 900 lives and sickened more than 37,000 people. Officials finally began to seal the borders on January 23. But it was too late. Speaking to reporters a few days after 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 per cent 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 per cent of the trips went to the neighbouring provinces of Henan, Hunan, Anhui and Jiangxi. Nearly 2 per cent 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 January 10 and January 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 the 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 travellers 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 lockdown, 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 January 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 January 23 to January 26, the 15 cities that Baidu data shows received the most travellers from Wuhan a combined 70 per cent all imposed some level of travel restrictions.
Other nations soon followed suit, including the United States, Australia, Singapore, New Zealand and the Philippines, all of which have sharply restricted entry for people coming from China. Others, like Italy and Indonesia, have barred flights. WorldPop researchers found that travel out of Wuhan has historically ramped up in the weeks before Lunar New Year's Day. Based on historical travel patterns, they identified 18 high-risk cities within China that received the most travellers from Wuhan during this period. They then used 2018 flight itineraries from the International Air Transport Association to map the global connectivity of those cities.
They note that travel patterns after restrictions started rolling out on January 23 will not match historical norms and that the cities they identified are initial ports of landing; travellers could have subsequently moved elsewhere. The top 10 global destinations for travellers from high-risk Chinese cities around Lunar New Year, according to their analysis, were Thailand, Japan, Hong Kong, Taiwan, South Korea, the United States, Malaysia, Singapore, Vietnam and Australia. In Africa, Egypt, South Africa, Ethiopia, Mauritius, Morocco, Nigeria and Kenya topped the list.
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