The Centers for Disease Control and Prevention announced last week that the United States will require a negative coronavirus test result taken within 72 hours before arrival for all international arrivals, beginning next week. The new rule is an effort to keep out a fast-moving new variant of the coronavirus, but in the rollout of the new health protocols, some travel groups saw an opportunity to restart some banned international travel.

Airlines for America, a trade group that lobbies for the biggest airlines in the United States, called for the new testing rule to replace bans on international travelers from countries with cases of the new strain.

If it were not for a new administration taking office, the bans would have been lifted: Trump signed a proclamation that was swiftly ruled out by the Biden administration. Trump called removing travel restrictions on Europe and Brazil (while leaving in place restrictions on travel from China and Iran) “the best way to continue protecting Americans from covid-19 while enabling travel to resume safely.”

But even with the travel bans staying, experts say that the new U.S. entry requirement, which allows for less-accurate rapid antigen tests, is not as robust as other countries’ testing protocols — and coronavirus-infected travelers could make it past testing barriers and onto planes undetected. There have been such cases in New Zealand, Hawaii and, most recently, Australia.

The new system is being implemented as the nation’s health officials attempt to prevent a new, highly contagious variant of the coronavirus, which has been detected in both Europe and Brazil, from becoming dominant in the United States, although some experts fear rapid antigen tests may be even less effective on the new strains.

Carlos Acuna-Villaorduna, an epidemiologist with expertise on modeling of infectious diseases at Boston Medical Center, says travel bans have value in times when certain regions are experiencing a high volume of a contagious disease. He also notes that allowing rapid antigen tests (which are 20 percent less effective than lab tests), is particularly risky for the new strain, which some experts think is less detectable by those kinds of tests.

“We don’t know how accurate the rapid tests are for the new strain. There is some concern some can be false negative” because of the make up of the new strain, Acuna-Villaorduna says. “It doesn’t mean that we need to completely block all travel, but we as a society should manage the risk of traveling. From a modeling standpoint allowing travel from those high-infection areas, you are going to see an increase in cases regardless of measures.”

If rapid tests are as effective on the new strain, doctors say, antigen tests are still about 20 percent less effective in detecting positive cases.

“The message needs to be gotten across that [the new system] is not going to make airplanes 100 percent safe,” says David Freedman, an infectious-disease specialist at the University of Alabama at Birmingham. “About 90 percent of countries requiring pre-departure testing require PCR [tests]. It’s only a small number, which now includes the U.S., that will allow an antigen test.”

Even if the United States stopped allowing for those less-accurate antigen tests to be used, cases could get through. Infectious-disease doctors like Freedman have long said that coronavirus tests taken during a patient’s virus incubation period will be negative, even though the person can become highly contagious hours later.

Over the weekend, a cluster of U.S.-linked coronavirus cases were found on fully pretested flights to Australia (which requires PCR tests) from Los Angeles.

Three passengers on the 18-hour journey tested positive for the virus on arrival in Australia despite testing negative for the virus within 72 hours before the flight, resulting in quarantines for dozens of people and athletes heading to Melbourne for next month’s Australian Open.

And other examples of flight-linked coronavirus cases make it clear that you can be tested and still bring the virus into a country: New Zealand, which has similarly strict rules to Australia (requiring multiple tests and quarantines for entry), prevented a cluster of flight-linked cases from entering the country this summer despite the original case testing negative for the virus as required.

The incoming Biden administration agreed with that sentiment, saying Monday that it will not allow those travel restrictions to be immediately lifted. The policy requiring a coronavirus test three days before departure also will stand.

“With the pandemic worsening, and more contagious variants emerging around the world, this is not the time to be lifting restrictions on international travel,” Biden spokesperson Jen Psaki said on Twitter. “On the advice of our medical team, the [Biden] Administration does not intend to lift these restrictions on 1/26. In fact, we plan to strengthen public health measures around international travel in order to further mitigate the spread of COVID-19.”

Medical experts also say the 72-hour timeline for the tests, which requires swabs be taken no more than three days before flying, is a relatively broad window of time which could allow the virus to develop in a person’s system undetected before a flight.

A report by the International Civil Aviation Organization recently estimated that testing passengers three days out via a PCR lab (not the less-effective rapid testing) is about 75 percent effective in keeping infected people off the plane.

“In other words, 25% of cases could be getting onboard,” Freedman says.

ICAO estimated that 48 hours would be a safer window of time, but Freedman notes that shortening the time frame would “present issues of practicality” by leaving travelers with little time to acquire a test result from the time of their appointment. PCR labs typically take two to three days to return results.

A quarantine or follow-up testing requirement like Australia’s, Acuna-Villaorduna says, would be a helpful additional layer in catching those cases. But it would not catch them all.

“Doing all interventions, like PCR [tests], quarantines, symptom screening, that certainly would decrease the transmission,” Acuna-Villorduna says. “But it would not eliminate it completely.”

“A big part of this is just keeping most of those cases of dangerous strains out of the U.S., but people shouldn’t think that it’s going to keep every case out of the U.S.,” Freedman says, “And it won’t keep every infected person off a plane.”

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