It's some day in the near future and you've just arrived in another country.
The Customs officer checks your passport while you use your phone to scan a QR code that instantly relays all of your health information.
A green tick flashes to show you've been vaccinated and aren't carrying Covid-19, and the official stamps and hands your passport back.
Welcome back to the rest of the world.
Sounds simple, right?
What is a vaccine passport?
With the roll-out of vaccines around the globe, countries are under mounting pressure from the airline and business sectors to reopen international travel.
And "vaccine passports" are touted as the key to unlock the world.
Broadly, we can think of these as a certificate to prove we've been vaccinated, much like a regular passport certifies our identity and nationality.
That's a similar but separate concept from infection or "immunity" passports, which refer to tests demonstrating that we're either not infected with, or are immune to, Covid-19.
Along with other airlines, Air New Zealand is preparing to trial the International Air Transport Association (IATA)'s Travel Pass app, beginning with Auckland-Sydney flights at some point next month.
At the same time, New Zealand was working with the World Health Organisation on the development of global standards for vaccine certification.
The Government was also closely following the work of the International Civil Aviation Organisation (ICAO) - the UN's air travel policy arm - which last week said vaccinations shouldn't be a pre-requisite for global travel.
"As 99 per cent of overseas travel to New Zealand is by air, how the aviation system interacts with the international vaccine roll-out will be of critical importance to us," Covid-19 Response Minister Chris Hipkins told the Herald.
"Whatever system New Zealand travellers use, it will likely be a digitally based health passport which stores and shares all vaccination and testing information, in a secure fashion, with the health and border entry authorities of the countries people travel to."
How will it work?
The problems that swirled around early efforts to deliver a tracing app for New Zealand showed just how challenging it can be to rapidly build and roll out any new system at scale.
So, it's not hard to imagine the Gordian knot the developers of the Travel Pass have faced in trying to create a single, end-to-end solution that can be used from one jurisdiction to another.
Not only has the IATA had to overcome the Byzantine complexities inherent in requirements for entry and exit - with a range of different types of tests to consider – it's also had to make its app simple and secure.
Ultimately, the association, whose 290 airlines make up some 82 per cent of total air traffic, singled out the four key ingredients needed to make it work.
That was governments with the means to verify tests and travellers' identities; airlines that could also verify passengers; recognised laboratories able to issue certificates; and travellers who knew where and how to get tested.
The IATA also needed to ensure its platform was safe and protected users' privacy.
As such, its app - successfully trialled last week at London's Heathrow Airport - didn't use any central data repository and was built to the highest standards of data protection laws.
It also used the "Secure Enclave" features of Apple devices, and a similar security encryption technology for Android phones, which made it extremely difficult for hackers to remotely decrypt any sensitive information.
Verifying users' identities was also relatively straightforward.
After downloading the app, travellers took a selfie and then scanned biometric data in their passports, before the Travel Pass matched the two.
Things got more complicated when it came to building in test results, which the IATA was focusing on before vaccinations became widely available to the public.
While the WHO was developing digital standards to make test result data secure, the IATA was partnering with selected labs to link their test results with the verified identity of the pass holder.
When vaccination became the basis for a pass, similar requirements from trusted providers would apply.
While a global front runner, Travel Pass is far from the only vaccine passport in the works; apps like Clear's Health Pass and The Commons Project's CommonPass are also being trialled.
Would they prove as secure and reliable as standard passports?
University of Auckland data ethics expert Professor Tim Dare felt the only way was to have "genuinely international standards".
That was backed by a recent IATA poll of travellers that found 89 per cent agreed with the need for global standards.
Dare said these would have to be administered at state level, and allow for countries to stop issuing them unless they satisfied an agreed set of conditions.
"Obvious conditions would include certainty that a passport holder had had a recognised and effective vaccine or vaccine course and that the passport holder's identity could be confirmed."
The myriad problems this catch-all approach threw up illustrated why international standards ordinarily took such a long time to develop.
"There's an inherent trust that is required to make sure that everybody is going to play ball," said Dr Andrew Chen, a research fellow at University of Auckland-based Koi Tū: The Centre for Informed Futures.
"And inevitably, some country is going to slightly not apply the standard correctly and all the other countries will be suspicious.
"But, at the moment, it just seems to be in everyone's interest to make sure they're contributing to the system. Otherwise, people are restricted from moving."
Will it be reliable?
Given that the science around vaccines was still emerging, Dare was surprised passport proposals were so advanced.
"Any uncertainty about the effectiveness of vaccines will flow on to the passports people who have had the vaccines produce," he said.
"Of course, there is always some uncertainty around vaccines: even well-established vaccines, backed by trials and by years of use in large communities, fail in some cases."
The key question was whether there was significantly greater uncertainty around Covid vaccines.
"And I would have thought there was," he said, pointing to the evolution of new strains like a South African variant that has dealt a blow to AstraZeneca's vaccine.
It was also still likely that some vaccinated travellers would be able to pass the virus on to others, even if they didn't get sick themselves.
"Even under an optimistic scenario where the vaccine prevents 90 per cent of people getting infected with Covid-19, that means 10 per cent of people are still susceptible," Covid-19 modeller Professor Michael Plank said.
Currently, about 20 cases were being reported each week at the border.
If New Zealand allowed vaccinated people to enter without quarantine, we could expect to see two cases going straight into the community each week.
"That number would be even higher if travel volumes increased back towards pre-Covid levels," said Plank, of Te Pūnaha Matatini and Canterbury University.
"Our vaccination rates at present are still very low, so this would rapidly lead to a major community outbreak."
New Zealand would essentially need to get through its own vaccine roll-out - set to take most of the year - before the country could start to allow quarantine-free travel for people from countries with community transmission, even if they had been vaccinated.
"Once we have vaccinated a high proportion of New Zealanders, the risk posed by overseas cases to the community will decrease," Plank said.
"Vaccine passports will then become a useful tool in gradually reopening borders. But the ongoing risk of community outbreaks will still need to be actively managed."
Will it be fair?
If a workable vaccine passport did fly, it could have the potential to drive further inequality.
But the scale of that inequality depended on how - or where - the passports were used.
"If they are used only for international travel, we may think the inequality is tolerable," Dare said.
"It may be true that some people cannot obtain a passport, but perhaps many of those people would not have wished to travel internationally in any event.
"However, if vaccine passports are used to control access to a wider range of services – perhaps affecting a person's ability to work, or gather with others, or to participate in educational service, or the like, unequal access to vaccines – and so passports – will be a much more serious issue."
Those are just the contentious questions that have surrounded the advent of domestic passports like Israel's highly-publicised green pass app.
Coming on the back of one of the world's earliest mass-vaccinations, the app granted immunised Israelis exclusive access to places like gyms, hotels, theatres and concerts.
China's own QR-encrypted digital vaccine passport - embedded within social messaging app WeChat - similarly enabled citizens to move freely around the country and visit stores.
Similar to the track-and-trace system China developed last year to successfully quash outbreaks, the app carried a user's vaccine and test history - all collated by China's National Health Commission.
The US, UK and European Union are considering their own passes - albeit with more caution about the implications.
While privacy groups have flagged big worries over what they consider further state surveillance, human rights advocates have pointed to the potential for the rich and privileged to get easier and quicker access.
Whole sections of society might be shut out not because they'd refused a Covid-19 shot, but couldn't get one.
"It seems inevitable that there will also be people who should not have the vaccine on medical grounds – as we see with every other vaccine," Dare said.
Chen expected New Zealand wouldn't follow China and Israel's model.
But he added that might not stop employers and businesses here from enacting their own policies.
"An employer doesn't have a right to ask whether you are vaccinated or not, but they probably have a stronger health and safety argument for wanting to know."
But there were plenty of pluses to consider as well.
Dare could see why countries might like to have passports used for a wide range of important activities – and these might also help lift coverage rates.
Director general of health Dr Ashley Bloomfield has indicated a vaccination target of 70 per cent of New Zealand's adult population, which experts say would be at the lower end of the threshold to achieve herd immunity.
That will prove a challenge, with surveys indicating that a quarter of Kiwis are hesitant about Covid-19 vaccines and that one in 10 is against them altogether.
"Not only would passports allow those activities to continue or recommence but also, appreciation that one needed a passport to participate would be a powerful incentive to vaccinate," Dare said.
"It happens that I am keen to go to Australia in July, but many people who were reluctant to have a vaccine might not be motivated by the possibility of transtasman travel."
Is it here to stay?
Whatever the post-pandemic world looks like, many commentators expect Covid-19 passports, of some kind, will be a part of it.
"I think the most positive future we'd be looking at is what you'd call progressive elimination," Otago University epidemiologist Professor Michael Baker said.
"We've now got almost 25 per cent of the world's population protected by governments that simply use public health measures to eliminate the virus, effectively.
"With vaccines, that number of countries will increase, as will the ability to actually limit transmission of the virus between countries with vaccine passports."
Despite a widely held view among scientists that the SARS-CoV-2 will become endemic, Baker was hopeful the world may have a shot at global eradication.
"That's a much cheaper option than having to live with the virus."