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Nearly a quarter of travelers flying out of China after ‘zero COVID’ lift were positive for the disease, new report reveals

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Nearly a quarter of travelers flying from China to Italy in late December were COVID-positive, according to a new study—one that offers a glimpse at transmission occurring from the eastern superpower to the rest of the world.

Nearly 23% of the 565 passengers on four flights to airports in Rome and Milan from Dec. 26-29 were carrying the highly infectious disease, according to a study published last week in infectious disease journal Eurosurveillance. As many as 42% of passengers on one flight were infected.

The study likely gives a good sense of the number of infected individuals on the flights, experts told Fortune. Some passengers might have been too early in their infection to test positive. That number, however, is likely offset by others who tested positive but were too late in the course of their infection to be contagious.

About 11% of travelers in the study were “highly likely to be infectious to others, but the remainder may not have been,” Dr. Jay Varma, chief medical adviser at the New York-based think tank Kroll Institute, tells Fortune, citing the study. Varma’s two-decade career with the U.S. Centers for Disease Control and Prevention included postings in China.

And that’s the number that matters most to travelers, he says: “What percentage of people boarding the plane are likely to infect me?”

Countries such as the U.S., Britain, France, Australia, Canada, Japan, and others are requiring travelers from China to provide proof of a recent negative COVID test result before entering. This comes as COVID infections in the country explode—an increase that began prior to the lifting of “zero COVID” restrictions in early December. COVID levels began rising in the country during widespread protests of the policy, and they likely served as super-spreader events, Dr. Ali Mokdad, professor at the IHME and a former senior epidemiologist with the U.S. Centers for Disease Control and Prevention, previously told Fortune.

Such restrictions on incoming travelers from China are “understandable,” given the inability of countries to perform a full risk assessment on the situation in China, World Health Organization director of emergencies Michael Ryan said at a news conference last week, emphasizing that such measures are not travel restrictions.

COVID data coming from the country has long been uncertain, and the country stopped releasing daily updates last month. In a major update, Chinese health officials said this weekend that the country has seen nearly 60,000 COVID deaths since Dec. 8—more than an 11-fold increase over its all-time reported COVID death toll prior.

But the number is “almost certainly a dramatic underestimate,” Dr. Stuart Ray,

vice chair of medicine for data integrity and analytics at Johns Hopkins’ Department of Medicine, tells Fortune: “The number could be 10 times what is being reported, given the lack of transparency.”

Virtually all sequenced samples obtained by the study’s authors were Omicron subvariants, mostly off-shots of BA.5, BF.7, and BQ.1.1, lining up with data released by China.

Nearly a quarter of travelers participating in the CDC’s voluntary COVID testing and sequencing program were COVID-positive during Christmas week, the latest period for which data is available on the agency’s website. The CDC did not respond to Fortune’s request as to what percentage of travelers from China are testing positive.

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