As officials around the world try and get a handle on the coronavirus pandemic, some UK businesses and politicians have suggested introducing a "passport" that proves someone has been vaccinated against the infection.
This may be required before an individual can travel, get a job or even enter a pub.
Certain countries have a similar system for yellow fever, with travellers only allowed in if they carry a card proving they are immunised against the viral disease.
With the UK government not enforcing coronavirus vaccinations, some have argued introducing a passport system could be unlawful.
As the debate rages on, scientists from The Royal Society have outlined 12 criteria a coronavirus vaccine passport must fulfil in order for a programme to be implemented effectively.
"An effective vaccine passport system that would allow the return to pre-COVID-19 activities, including travel, without compromising personal or public health, must meet a set of demanding criteria, but it is feasible," said co-lead author Professor Christopher Dye, from the University of Oxford.
"First there is the science of immunity, then the challenges of something working across the world that is durable, reliable and secure.
"There are the legal and ethical issues, and if you can crack all that, you have to have the trust of the people.
"Huge progress has been made in many of these areas, but we are not there yet.
"At the most basic level, we are still gathering data on exactly how effective each vaccine is in preventing infection and transmission, and on how long the immunity will last."
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"Immunity risk certification" was introduced in April 2020 as the sixth pillar of the UK government's contract tracing plan.
Uncertainties around antibody test results, which indicate immunity, "resulted in a stepping back from this position".
On 24 January 2021, it was reported the innovation agency Innovate UK had granted eight projects a total of £450,000 ($629,912) to carry out feasibility studies into developing vaccine passports and coronavirus-status apps.
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A vaccine passport is a form of identification "consisting of data related to the vaccination", including its date, the type of jab and place of immunisation, which is linked "to the identity of the holder".
Negative coronavirus tests are being used as a form of passport for international travel amid the pandemic, coupled with other non-pharmaceutical interventions like contact tracing, quarantining and face coverings.
Three coronavirus vaccines are approved in the UK; Pfizer-BioNTech, University of Oxford-AstraZeneca and Moderna.
All three jabs are known to protect against severe COVID-19, the disease caused by the coronavirus.
Only the Oxford-AstraZeneca vaccine has shown signs of stemming transmission, however.
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Experts are unanimously optimistic all three jabs will cut the spread of the coronavirus to some extent, however, this is yet to be backed up by hard clinical trial data.
The emergence of new variants also raises questions as to how effective the vaccines will be against coronavirus mutations, with studies producing mixed results.
The length of immunity a jab offers is also unknown, which may mean a passport is only valid for a set length of time. With yellow fever, a single vaccine dose "provides lifelong protection for most people".
Writing in the report "Twelve criteria for the development and use of COVID-19 vaccine passports", the scientists urge such a programme must:
Meet benchmarks for coronavirus immunity – whether it be individuals are protected from the coronavirus so they "can carry out the activities for which the passport is needed and avoid additional burdens on health services" or proof the person "cannot become infectious and transmit" the infection
Accommodate for differences between the vaccines' efficacy and how their effectiveness may change against new variants
Be internationally standardised – like yellow fever's International Certificate of Vaccination or Prophylaxis
Have verifiable credentials – the Common Pass and COVID-19 Credentials Initiative are working towards app-based solutions that use a quick response (QR) code and do not release personal sensitive information
Have defined uses – will the passport discriminate between potential employees when it comes to hiring? Will it influence entry to sports events? Or the ability to take out insurance?
Be built on a platform of "interoperable technologies" – passports "must meet certain standards for interoperability"; allowing different systems to work together across boundaries
Be secure around personal data – general data protection regulation (GDPR) laws must be considered. The passport must not be used to track populations or place "additional scrutiny of already marginalised groups", for example by police
Be portable – there must be "clarity" across QR codes, card readers or paper copies of a passport
Be affordable for individuals and governments – a potential charge to obtain the passport could be "unfair". Having to print the documentation out or display it on a smartphone would also "result in some inequality"
Meet legal standards – includes laws related to human rights, data protection, and equality and discrimination
Meet ethical standards – the passport must be inclusive, have defined uses, and avoid "discrimination and exacerbating existing inequalities", like among those who are "vaccine hesitant" or have a "digital divide"
Have conditions of use that are understood and accepted by the passport user – "unintended behavioural responses and resistance could arise if uses are not transparent"
Based on existing evidence, the scientists concluded a coronavirus vaccine passport system is "feasible, but not all criteria have yet been satisfied and consideration should be given to what longer term precedents this may create".
Co-lead author Professor Melinda Mills from the University of Oxford added: "International standardisation is one of the criteria we believe essential, but we have already seen some countries introducing vaccine certificates related to travel, or linked to quarantine or attending events.
"We need a broader discussion about multiple aspects of a vaccine passport, from the science of immunity through to data privacy, technical challenges and the ethics and legality of how it might be used."
Outside of the UK, Greece's prime minister has urged the European Commission to introduce a coronavirus vaccine certificate that allows "the freedom of movement of persons who have been vaccinated".
Estonia and the United Nation's health agency are creating a "smart yellow card" that acts as an e-vaccination passport.
A digital passport is also being developed in Denmark, allowing residents to prove they have been immunised against the coronavirus.
Spain is also compiling a database of people who have turned down the jab, which it will share with the European Union.
"Understanding what a vaccine passport could be used for is a fundamental question," said Professor Mills.
"Is it literally a passport to allow international travel or could it be used domestically to allow holders greater freedoms?
"The intended use will have significant implications across a wide range of legal and ethical issues that need to be fully explored and could inadvertently discriminate or exacerbate existing inequalities."
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