Airlines and others in the travel industry are throwing their support behind so-called vaccine passports to boost pandemic-depressed travel, and authorities in Europe could embrace the idea quickly enough for the peak summer vacation season.
Technology companies and travel-related trade groups are developing and testing various versions of the vaccine passports, also called health certificates or travel passes.
It is not clear, however, whether any of the passports under development will be accepted broadly around the world, and the result could be confusion among travelers and disappointment for the travel industry.
Here are some key questions about the health credentials.
What is a vaccine passport?
It is documentation that shows a traveler has been vaccinated against COVID-19 or recently tested negative for the virus that causes it.
The information is stored on a phone or other mobile device that the user shows to airline employees and border officers. The Biden administration and others want a paper version available, too.
Who is designing them?
The trade group for global airlines, the International Air Transport Association, is testing a version it calls Travel Pass. IBM is developing another, called a Digital Health Pass. There are several other private-sector initiatives.
Some countries are getting involved and using the passports beyond air travel. Israel is using a new "green passport" to ensure that only people who have been vaccinated or recovered from COVID-19 can attend public events such as concerts. Denmark expects to launch a pass that will let vaccinated people travel with fewer restrictions.
Why do travel companies want them?
International air travel has collapsed during the pandemic, as countries impose restrictions such as quarantines or outright bans to curb the spread of the virus. Airlines are counting on vaccine passports to convince governments to drop some of those restrictions that discourage visitors.
"The significance of this to re-starting international aviation cannot be overstated," said Alexandre de Juniac, the CEO of the airline trade group.
Operators of hotels that depend on international visitors are also eager to see the passes adopted.
The airline trade group tested its app Wednesday on a Singapore Airlines flight to London. A passenger put a digital version of his passport, coronavirus test results, and travel restrictions at his destination on a mobile device.
Where would these passes be required?
Vaccine passports will be most common on international flights. Some countries already require proof of vaccination for diseases such as yellow fever, and the United States now requires a negative test for COVID-19 to enter the country, so a digital health passport isn't much of a leap.
What are the risks?
The available vaccines are most effective at preventing serious illness, but that doesn't rule out the possibility that vaccinated travelers could still spread the virus.
"I think we have enough evidence right now to say that these vaccines cut transmission, that vaccinated people are much less likely to transmit the disease," says Ashish Jha, dean of the public health school at Brown University. "How much? We don't know." He guesses it's around 80%.
The U.S. Centers for Disease Control and Prevention still recommends against travel even as the agency has relaxed other guidelines for people who have been vaccinated.
What about fairness?
Other critics say the certificates will primarily benefit people in wealthier countries and relatively affluent people within each country — those who are mostly likely to be vaccinated quickly, and most likely to have smartphones.
"It's going to be the wealthy, the privileged, who are going to get to fly around, and other people won't have access to that," says Lisa Eckenwiler, who teaches health ethics at George Mason University. She sees a particular potential for unfairness if health passes expand to workplaces and schools.
What about privacy?
Consumers will be nervous about sharing health information that might get hacked or exposed in a breach, says Stephen Beck of management consultancy cg42.
"When it comes down to it, people are going to ask themselves, is sharing sensitive information worth the trade-off for a leisure trip?" he says, "and for many, the answer will be no."
IATA and IBM say their passes use blockchain technology and the information won't be stored in a central place.
What role will the U.S. government play?
Airline and business groups are lobbying the White House to take the lead in setting standards for health passes. They believe that would avoid a hodge-podge of regional credentials that could cause confusion among travelers and prevent any single health certificate from being widely accepted.
But the Biden administration says it is up to the private sector and nonprofits to figure out how Americans can demonstrate that they have been vaccinated or tested.
"It's not the role of the government to hold that data and to do that," Andy Slavitt, a White House virus-response adviser, said this week. "It needs to be private, the data should be secure, the access to it should be free, it should be available both digitally and in paper and in multiple languages."
Other governments, like those in Israel and Denmark, are taking a more active role.