CHICAGO (WGN) — All passengers flying from the United Kingdom now have to show they’ve tested negative for COVID-19 to come to the United States.

The order from the Centers for Disease Control and Prevention was announced Christmas Day and went into effect Monday.

Passengers have to get tested within three days of their departure from the U.K. and provide documentation of their results to their airline.

Chicago-based United said it’s making same-day, pre-flight rapid tests available to travelers at London’s Heathrow Airport.

Last weekend, Britain’s Prime Minister Boris Johnson said the new variant of the coronavirus seemed to spread more easily than earlier ones and was moving rapidly through England. But Johnson stressed “there’s no evidence to suggest it is more lethal or causes more severe illness,” or that vaccines will be less effective against it.

Britain has expanded lockdowns because of this variant and dozens of other countries have banned travel from Britain.

That is not the case in the US. Passengers are just required to take that extra step of testing negative for COVID-19 before boarding.