Travelers from China, Hong Kong and Macau will have to show a negative Covid-19 test before entering the United States, according to the Biden administration. There will be a requirement on January 5.

Amid growing concern over a surge of cases in China and the country's lack of transparency about the outbreak there, the Centers for Disease Control and Prevention announced.

The requirement for testing will apply to air passengers regardless of their nationalities. Travelers from China who enter the United States through a third country and those who connect through the United States to other destinations will be affected.

Hundreds of thousands of infections a day are reported by local governments in China. The videos were obtained by The New York Times. China doesn't release reliable Covid data so it's hard to track the situation.

After three years of insistence on a zero Covid policy, China made an abrupt turnabout in December after mass protests threatened the ruling Communist Party. There have been many cases in Beijing since that time.

According to scientists in Hong Kong, the Beijing outbreak was caused by an Omicron sub variant. The sublineage of BA.5 has been dominant in the US. There are other versions of Omicron that have shown better results in the US.

According to the C.D.C., 4 percent of cases were made up of BF.7 in late December. The United States is currently dominated by other Omicron subvariants that may be able to evade immune responses.

The variant that fuels China's outbreak may not be the one that most effectively sidestepped immune responses according to scientists. Since so few people in China have been exposed to versions of Omicron, any of the highly contagious Omicron subvariants could take off there.

The dominant variant may not be the one that is best at getting around the immune responses, but the one that catches fire, they said.

James Wood is an infectious disease expert at the University of New South Wales.

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