LONDON (AP) — Boarding pass, suitcase, passport and ... digital vaccination certificate?

Keen to avoid losing another summer of holiday revenue to the coronavirus pandemic, the European Union, some Asian governments and the airline industry are scrambling to develop so-called COVID-19 vaccine passports to help kickstart international travel.

They're working on systems that would allow travelers to use mobile phone apps to prove they've been vaccinated, which could help them avoid onerous quarantine requirements at their destinations.

But the multiple efforts underscore the lack of one central international system to electronically verify vaccination status. The projects also face technical challenges in working together, while questions about privacy and vaccine inequality linger.

Vaccination passports would add another digital layer to the multitude of existing coronavirus health and contact tracing apps many countries and U.S. states have rolled out. Their use domestically to reopen local economies has been hotly debated, with many opposed to requiring them for pubs, concerts and sporting events. However, there's more momentum to use them for international travel, especially as countries like Iceland  open their borders to vaccinated visitors and others like Saudi Arabia start allowing vaccinated citizens to travel abroad. The EU's decision last week  to open its borders to fully vaccinated travelers adds even more urgency.

Here's a look at how vaccine passports work:

OFFICIAL EFFORTS

The first part of a vaccination passport is the user's official or approved electronic immunization record.

The European Union, China  and Japan are all working on their own digital vaccination certificates for cross-border travel. The U.K., meanwhile, updated its National Health Service app last week to let fully vaccinated users prove their status when traveling abroad, coinciding with an easing of travel rules.

Testing is under way for the EU's digital certificate, which will also confirm COVID-19 test results or recovery from the virus and is set to go live by the end of June, allowing residents to reunite with friends and relatives living across 30 European countries. It's still unclear where and how exactly travelers in the EU, which doesn't have internal border checkpoints, will have their certificates checked. Officials in Brussels say that will be up to individual countries. The idea is that travelers will flash a QR code on their phones so it can be scanned at, say, an airport or train station, using an official verification app that checks with national databases, via an EU technical "gateway."

The World Health Organization doesn't recommend vaccination proof as a requirement for international travel, citing unequal distribution of vaccines, even as it consults on interim guidance  for developing a "Smart Vaccination Certificate."

TRAVEL APPS

Travelers also need a smartphone app to carry any eventual official vaccination certificates.

The EU's project includes open source technology European countries can use to build their own official mobile wallets.

The International Air Transport Association, an airline industry group, has its smartphone IATA Travel Pass, which airlines including Qantas, Japan Airlines, Emirates, British Airways and Virgin Atlantic have signed up to. A rival effort, CommonPass, has gained traction with carriers like Cathay Pacific, JetBlue, United and Lufthansa.

Travelers can already use the apps to verify that their COVID-19 test results are accepted at their destination. Travel Pass and CommonPass are so far only available to travelers on airlines that are using them. Both can also be integrated into airline travel apps so users can verify their vaccine status when they check in online. Both are also expected to work with the EU certificates. CommonPass says users will be able to import vaccine credentials by mid-June.

Amid a pandemic-dimmed travel outlook, vaccine passports will only become more widespread, said Paul Meyer, CEO of The Commons Project, the nonprofit behind CommonPass. "Our expectation is it will remain a requirement for international travel."

WHAT TRAVELERS WANT

Business travelers like British public relations executive Richard Fogg welcome vaccine passports. Fogg's firm scaled back plans to attend a major telecom trade show in Barcelona next month, given quarantine rules for people returning to the U.K.

"Those 10 days of quarantine will have negative business implications – there's no way around it," Fogg said, while acknowledging tradeoffs including concerns about data privacy.

Eymeric Segard, CEO of Geneva-based private jet broker Lunajets, noted travelers already hand over passports with personal data on arrival.

"Personally, you know, I would be happy to tell anybody, yes, I am vaccinated or no I'm not vaccinated," he said, adding that vaccine passports would help avoid the "logistical nightmare" of multiple COVID-19 tests Europeans face when visiting other EU countries.

WHAT ABOUT FAKES?

Phony paper COVID-19 document s sold by fraudsters have been a problem during the pandemic but developers say digital versions have safeguards that make them hard to fake.

IATA says it doesn't verify test results or vaccination status but acts as the conduit for registered labs to securely send those details to travelers whose identity the app can match to the person who took the test or vaccination. The app scans a traveler's face using the phone camera and matches it to passport biometric details, and there are checks to prevent someone else using their identity.

SECURITY AND PRIVACY

Vaccination passports are a polarizing topic, with online discussion highlighting unfounded fears that they'll be used to control people, restrict freedom and erode privacy. Developers stress that minimal personal data is kept on phones, and the only thing that gets transmitted are encryption keys allowing information to be exchanged securely.

"If done correctly, this doesn't bring an additional level of privacy risk because you're just putting in a credential status of yes or no," said Kevin Trilli, chief product officer at ID verification company Onfido, which is working on vaccination cards technology.

There's also the question of how well various vaccine credential systems will work together and whether countries will recognize each others' certificates. The U.K. government has warned that  not many countries currently accept proof of vaccination from travelers.

"You can't have an interoperable system on day zero," but over time the kinks will be worked out, which helps lay the groundwork for the next pandemic, Trilli said.

What about people who don't have smartphones? Or families that don't have a device for each member? IATA and EU officials say they're are working on solutions, including paper-based options.

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Follow Kelvin Chan at www.twitter.com/chanman