Australia already has a vaccine certificate - it's just that it has no force at the moment. It doesn't get you easier travel or access to restaurants.
When and if such certificates, on phones or on paper, are a condition of employment or travel or entry to a concert or restaurant, they would become, in effect, "vaccine passports".
At the moment, when you get your vaccination, it's registered on a central government database and you can download a "COVID-19 digital certificate". "You'll be able to get your digital certificate after you've had all required doses of a vaccine as approved for use in Australia," the government says.
Australians have a series of rights - the right to free movement, for example. But there are also other laws which may constrain those rights, like the need to protect public health.
Imagine that someone hadn't got vaccinated for a good reason, say because they were pregnant or allergic to the vaccine or being treated for cancer. It would still be legal to bar them from working in an aged care home.
Supermarkets have conditions of entry - they can insist on searching your bags - so they could insist that a certificate of vaccination was a condition of entry.
Sports and music venues have conditions of entry - like not taking outside drinks in - and they could make a vaccine passport such a condition.
An American music venue gave much cheaper tickets ($18) to those who were vaccinated but hyper-expensive ones ($999) for those who weren't. That was a condition of entry.
The legality of demanding a proof of vaccination depends on context - the particular conditions of the business. It is not clear, for example, if a college could legally insist on proof of vaccination for all students and staff.
But insisting on a vaccine certificate before entry or employment is certainly within the law in lots of cases, according to Maria O'Sullivan of the Castan Centre for Human Rights Law at Monash University.
"Some states and territories have adopted a mandatory vaccination policy for staff working in close contact with patients or infectious materials. In the ACT, for example, all ACT Health staff are subject to an 'occupational assessment, screening and vaccination procedure', which requires them to be immunised against diseases including influenza, diphtheria and hepatitis B," Dr O'Sullivan said.
How are other countries doing it?
Australia doesn't have to make the hard choices yet because so few Australians are vaccinated - just over 5 per cent with the full two jabs on the latest count.
Other countries are further down the road and they are starting to think about how they live with Covid.
In Israel, where more than half the population is fully vaccinated, residents can get a "green pass" which allows them into places like swimming pools, gyms, concert venues and restaurants.
"Upon entering a place that complies with the green pass restrictions, you are required to present your green pass, along with identification (identification document, driver's license, passport)," the Health Ministry says.
China has also launched a similar scheme and the Israeli government wants to link up so the pass becomes an international document.
The European Union is working towards an electronic vaccination certificate from July 1. Britain has also started testing a certificate system. Japan plans one in the coming months, modelled on the EU system.
For an internal system of vaccination certificates to turn into an international passport, some sort of coordination would be needed so countries accepted each other's certificates.
Some airlines have started using an app - Common Pass - to verify a passenger's COVID-19 test results before they board flights.
The International Air Transport Association's health pass is being used by more than 20 airlines. It is not, though, essential to travel - but it could easily be developed to make it more like a passport.
There are privacy concerns. If you have to register your vaccine pass before entering a venue and the venue then registers it with a central data base, the owner of that data base knows where you have been.
One way round this is to minimise the amount of information. All the venue needs to know is that the holder of the passport at the door really is the owner and that the information is up to date.
Passports would need to be forger-proof and there would need to be safeguards for those who couldn't get vaccinated for good reason.
Experts see technical hurdles but believe they can be crossed.
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