US public health officials are considering tightening restrictions at their borders. They are weighing stricter testing requirements for all travelers.

Right now, visitors are required to prove they were fully vaccinated at least 14 days prior to boarding a flight. They have to show a negative test for the virus within three days of departure.

Officials with the Centers for Disease Control and Prevention released a statement on Tuesday. They said they are working toward requiring all air travelers to be tested within a day before boarding.

CDC officials are expanding their surveillance at four of the busiest international hubs in the US. They will be monitoring and testing travelers coming through airports in New York, New Jersey, Atlanta, and San Francisco.