In March of 2020 Bill Gates said to expect immunity passports for travel. In April I laid out things we didn’t know enough about the virus for the idea to be viable. We now know, though, that re-infection is rare at least during the first year after recovering from Covid-19 and that vaccinations protect people from both symptomatic and asymptomatic infection to a significant degree. The CEO of Qantas even declared last fall that you’d need proof of vaccination in order to fly.

As a result the E.U. and others are working on the idea of a vaccination passport or ‘certificate’ for those who find the nomenclature more palatable. 62% of Americans even believe vaccination should be required for air travel.

However there are several challenges that make it hard to create such a document, though certainly some countries will put in place vaccination requirements either for entry, for waiting testing requirements, or for avoiding quarantine on arrival.

1. Proof of vaccination isn’t secure Singapore may be using the blockchain to validate vaccination status, but the U.S. hands out easily replicable cardstock. How would Americans actually prove they’re vaccinated?

2. Different levels of efficacy. Different vaccines offer different levels of protection for the individual and against spread, including differences in how prophylactic they are against continued mutations in the virus. Will passports be accepted for some vaccines and not others?

3. Different vaccines are approved in different places would AstraZenecea be accepted, approved in much of the world but not the U.S.? Would a Russian who received the Sputnik V vaccine, or a citizen of the U.A.E. who received China’s Sinopharm vaccine, be eligible for entry into the U.S.? South Africa has suspended use of AstraZeneca out of fears it isn’t as effective against their dominant strain of the virus, would a U.K. citizen who received that jab have their vaccine passport recognized on arrival in Johannesburg?

4. What counts as vaccination? Many experts recommend a regimen of ‘first doses first’ delaying second doses in order to get first jabs into as many arms as possible as quickly as possible. That’s the strategy the U.K. has pursued, because it provides the greatest level of societal protection.

   Does a first dose of Moderna count as vaccinated, when clinical trial data showed greater effectiveness against symptomatic Covid from one dose than the Johnson & Johnson vaccine which is a one-dose vaccine? Would it make sense to require two Moderna shots but only one from J&J for this purpose? What if Sinovac’s 50% effective vaccine counts for vaccination?

5. Booster shots are coming, are they required too? Different vaccines may have different lengths of efficacy, and boosters may be required. How long will a vaccine passport be valid for and will it vary by shot received? At what point does a booster dose become required? And does it matter which one? (Would a Pfizer booster dose be accepted for someone initially vaccinated with Moderna, and would these requirements change on a country-by-country basis?)

Some problems though aren’t so hard to overcome. Tyler Cowen thinks that having different rules in different places is too big a challenge,

How many different passport systems would a flight attendant or gate agent have to read, interpret and render judgment upon?

The likely result of all this: Many foreign visitors to the U.S. would never quite know in advance whether they could board an airplane or attend a public event.

However we have this now. Each country has its own entry rules, and did so prior to the pandemic (passport restrictions and validity rules, visa requirements, fees etc.). The rules are loaded into TIMATIC and each airline employee can look those up based on an individual’s citizenship, residence, transit point, origin and destination.

Things have gotten more complicated during the pandemic with testing and quarantine rules, but they’ve been handled in much the same way. The rules are laid out in a database, and airlines have contracted with third parties to validate an individual’s health credentials where needed.

This doesn’t always work perfectly. Hawaii has rejected valid tests results (and accepted ones they shouldn’t) for instance with passengers flying home before bureaucrats realize their mistake. But in most cases this is manageable up front.

As Cowen observes though vaccine passports would have the benefit of encouraging people to get vaccinated because it would increase their opportunities and would encourage people to get booster doses when needed, to maintain access to those opportunities.

Ultimately though the U.S., with its paper vaccination cards and slowness approving new approaches, may not be able to create a vaccine passport while it still matters. Other countries, hopefully, will accept our cardstock.