Starting Monday in the UK, passengers must have a negative Covid-19 test within three days of boarding a flight to the United States.

“Today, President Trump is taking another step to protect the health of the American people,” according to the statement.

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But according to researchers on the CDC’s Covid-19 response team, testing three days before a flight might not accomplish very much — reducing the risk of spreading the virus by just 5 to 9%.

Dr. Paul Offit, an infectious disease specialist at the University of Pennsylvania, said the new testing requirement “is like putting up a chain link fence to keep out a mosquito.”
The CDC’s order acknowledges that “testing does not eliminate all risk,” but when combined with other measures like self-monitoring for symptoms, masks and distancing, “it can make travel safer.”

Covid-19 tests often miss recent infections, so someone can test negative and actually be infected because they’re not shedding enough virus to be detected.

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“Testing 72 hours before a flight is going to miss a lot of people,” said Dr. Peter Hotez, an infectious disease specialist at the Baylor College of Medicine, who pointed out that the new variant appeared in the UK three months ago and is likely already in the United States.

On the other hand, testing on the day of departure may be more effective, reducing the risk of transmission by 37 to 61%, according to the CDC scientists’ November report, which is based on mathematical modeling and has not been peer reviewed.

Health officials have been anxious to curtail the spread the UK variant, which appears to transmit more easily than other coronavirus strains.

The CDC did not respond to requests for comment for this story.

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