• Airlines are already offering digital platforms that contain passenger health information.
• Several governments are opening borders if travelers can prove they've been vaccinated or tested negative for Covid-19 but it's unclear if digital credentials will be required.

Italy, Iceland, Greece and Spain now allow or are opening their borders to people who've been vaccinated or who recently tested negative for Covid-19. The European Union has agreed to open its borders to more vaccinated tourists, including from the U.S.

The question is: How will individuals prove their vaccine or Covid status?

As of Wednesday, almost half of the total U.S. population had received at least one Covid-19 vaccine dose. New Covid infections in the country continue to drop. As of Thursday, the seven-day average of daily new Covid infections is at its lowest level since June 22, according to data compiled by Johns Hopkins University.

That trend is helping people return to pre-pandemic activities — from concerts to indoor dining to live sporting events and even international travel. Vaccinated people no longer need to wear masks or physically distance indoors or out, the Centers for Disease Control and Prevention said earlier this month.

Retailers like Walmart and Costco and hotel chains including Hyatt dropped their mask requirements for vaccinated customers this month, unless it's required in local rules. U.S. officials have said they are largely relying on people being honest about their vaccine status, and retailers and hotel chains have said they don't plan to check for a proof of a vaccine.

Officials in charge of overseeing international travel require more than the honor system. Federal officials require U.S.-bound international travelers, including U.S. citizens, to show proof of a negative Covid test result to board flights.

A spokeswoman for the Department of Homeland Security said DHS officials don't check passengers' Covid test results upon entry but — that is up to the airlines.

As airlines like United, Delta and American ramp up trans-Atlantic service, they are encouraging travelers to upload health documents online.

The travel industry has urged the Biden administration to create a set of standards for digital health credentials in an effort to lift travel restrictions that have devastated demand for leisure and business trips abroad for more than a year. The administration has said it will leave the task of developing digital health credentials up to the private sector. Federal officials also said they don't intend to keep a database of vaccination records; that will be left to states.

Enter digital health apps. Sometimes referred to as vaccine passports, several of these platforms are already in development and some in use, including partnerships with airlines and local governments.

Here is where things stand on vaccine passports in the U.S.:

What is a health passport?

Digital health passports — also called vaccine passports — are platforms for smartphones that allow access to an individual's health data, such as Covid test results or vaccination status. Israel and Denmark have already made platforms available and other countries are working on their own.

When individuals are vaccinated in the U.S., they are given proof in the form of a CDC-issued card. Airlines, which don't want staff to have another piece of traveler information to check, are eyeing digitized versions that could easily be scanned.

Do I need one?

The health passports are not mandatory but might become more useful as more countries and attractions reopen.

Countries that have recently opened or plan to open their borders to foreign tourists like Iceland and Greece say travelers must show proof that they are vaccinated against Covid-19 or proof of a recent negative Covid-19 test.

But so far, digital health certificates are not required.

"Certificates may be in paper or electronic format," Sveinn Gudmarsson, communications director at Iceland's Ministry for Foreign Affairs, said by email. "Border control will evaluate whether a certificate is valid and will consult a representative of the Chief Epidemiologist [health care worker] as needed."

The U.S. since January has required all inbound travelers from abroad, including citizens, to show proof of a negative Covid-19 test, even if they are vaccinated.

In California, venues can allow more people in if the establishment verifies that they have been vaccinated.

Who makes them?

There are several platforms already out there. IBM has developed New York State's Excelsior Pass, which was tested at a New York Nets game in February. The app uses blockchain to communicate with state vaccination records or with health providers. The display shows a simple go or no-go sign, not the actual test result.

The International Air Transport Association, a trade group that represents nearly 300 airlines worldwide, has rolled out its own digital health passport. Singapore started accepting Covid-19 test results on that platform this month.

Some airlines like JetBlue Airways have announced trials of another digital health passport by The Commons Project Foundation, which is called Common Pass.

United Airlines recently said it would expand its own app to allow travelers to book Covid-19 test appointments online, results of which will upload automatically and let customers know whether they can travel to their destination.

The European Union and Israel are developing their own digital health certificates.

Why are they controversial?

Digital health certificates have drummed up concerns over how secure customers' data will be with third-party apps communicating with databases containing sensitive health information. It has also raised concerns about inequality, since the platforms mainly work on individuals' smartphones.

The governors of Florida, Texas and Arizona have moved to stop businesses from requiring proof of vaccination from customers. Arizona Gov. Doug Ducey signed an executive order last month that bans these certificates as a requirement to receive services and said that vaccinations are the choice of individuals not the government.

Even the World Health Organization said it is against requiring proof of a vaccine to enter another country "given the limited (although growing) evidence about the performance of vaccines in reducing transmission and the persistent inequity in the global vaccine distribution."

Will I need to verify vaccination forever?

That is still unclear, just as it's not clear when other Covid-era rules like masks on planes or public transportation will last. Even the travel industry, one of the most devastated by the pandemic, has its reservations.

Willie Walsh, former CEO of British Airways' parent International Consolidated Airlines Group, and current director-general of IATA, has said he doesn't want proof of vaccines to become a permanent fixture.

"These are measures that may be necessary as temporary arrangements while we go through this crisis, but once we're through it, we want to see these restrictions permanently removed so people can get back to traveling as they experienced back in 2019," he said earlier this month.

-CNBC's Nate Rattner contributed to this article.