Five years after the virus that causes COVID emerged in China it still holds some mysteries. The disease has killed an estimated 20 million people globally, according to the World Health Organization, and thousands are still dying every year. But scientists still aren't clear where it came from, and they are still trying to understand what causes long-term symptoms called long COVID. And while vaccines have helped dramatically reduce severe disease and death from COVID, the virus mutates so quickly researchers have struggled to find a vaccine that stops the spread.

As COVID-19 surges in China, the U.S. is expanding its traveler surveillance program, an early warning system for detecting new variants. With the addition of Los Angeles and Seattle, there are now seven airports where volunteer passengers can swab themselves on arrival. The Centers for Disease Control and Prevention program now covers about 500 flights, including more than half from the China region. The CDC on Thursday also began requiring travelers from China, Hong Kong and Macao to test negative for COVID-19 before flying to the U.S. Meanwhile, scientists want to see more screening of wastewater from arriving airplanes.