Due to a new strain of COVID-19 in the United Kingdom that appears to be up to 70 percent more contagious, travelers from the UK will now be required to test negative before entering the United States.

Starting December 28, travelers entering the US will have to take a COVID-19 PCR or antigen test no more than 72 hours before their flight to the US, and provide written documentation of their lab results to the airline before boarding.

According to the CDC’s testing order, “Airlines must confirm the negative test result for all passengers before they board. If a passenger chooses not to take a test, the airline must deny boarding to the passenger.”

The CDC also recommends getting tested three to five days after you arrive in the US, and self-quarantining for seven days.

Whether this new rule will have any significant impact, however, is another matter. According to CNN, researchers on the CDC’s COVID-19 response team say that testing three days before a flight will only reduce viral transmission by just five to nine percent, while testing the day of departure “reduces the risk of transmission by 37 to 61%, according to the CDC scientists’ November report.” Dr. Paul Offit, an infectious disease specialist at the University of Pennsylvania, compared it to “putting up a chain link fence to keep out a mosquito.”