Various groups and countries are working on developing so-called vaccine passports aimed at allowing more travel. But airlines fear that a smattering of regional credentials will cause confusion and none will be widely accepted.

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“It is crucial to establish uniform guidance" and “the U.S. must be a leader in this development,” more than two dozen groups said in a letter Monday to White House coronavirus-response coordinator Jeff Zients. However, the groups said that vaccination should not be a requirement for domestic or international travel.

The groups include the main U.S. and international airline trade organizations, airline labor unions and the U.S. Chamber of Commerce. The White House did not immediately comment.

The World Health Organization and the United Nations' aviation arm are working on the type of information to include in a credential. The airline industry groups are particularly interested in having the U.S. Centers for Disease Control and Prevention take a leading role, believing that would increase certainty that information in the credentials is legitimate.

The CDC issued new guidelines Monday for fully vaccinated people, saying they can — without face masks — meet other vaccinated people and visit unvaccinated people in a single household who are at low risk for severe disease. However, the health agency still recommends against travel.

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“Every time that there is a surge in travel, we have a surge in cases in this country,” said CDC Director Dr. Rochelle Walensky.

Walensky noted that many variants of COVID-19 now spreading in the U.S. started in other countries. Still, she held out the possibility that with more data CDC might soon approve of travel by vaccinated people.

Airlines have been particularly hard hit by the pandemic. Despite a partial recovery, U.S. airlines are still losing $150 million a day, according to the Airlines for America trade group.

In the U.S., the number of people going through airports remains down nearly 60% so far this year compared to 2019, the last normal, pre-pandemic year. Most of those people are flying within the United States.

Airlines are counting on widespread vaccinations to boost travel, and for vaccine passports to give a boost to highly lucrative international flying.

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