The words "vaccine passport" have a reassuring ring to them, perhaps conjuring up the image of a sleek, embossed document with watermarked pages and official stamps of approval. Flourished at border controls, it would open travel doors that, for so many of us, have been closed by Covid-19.
Some destinations -- including the Seychelles, Cyprus and Romania -- have already lifted quarantine requirements to visitors able to prove they're vaccinated. Others, such as Iceland and Hungary, have opened up to people who've recovered from Covid-19.
This raises the prospect that proof of inoculation or immunity could be the golden ticket to rebooting travel and seems good news for people eager to book summer vacations after months of lockdown, particularly as vaccine rollouts gather pace.
They could open up the restaurants, bars, cinemas and other leisure and entertainment facilities whose closure over the past year have left many teetering on the edge of -- or already victim to -- financial ruin.
Tech companies such as IBM are also trying to get in on the act, developing smartphone apps or digital wallets into which individuals can upload details of Covid-19 tests and vaccinations. These are gaining support from major travel industry players.
This week, Zurab Pololikashvili, the secretary general of the United Nations World Tourism Organization, called for the global adoption of vaccination passports as part of wider measures he said were essential to get the world in motion once again.
"The rollout of vaccines is a step in the right direction, but the restart of tourism cannot wait," he told a meeting of the UNWTO's Global Tourism Crisis Committee in Madrid. "Vaccines must be part of a wider, coordinated approach that includes certificates and passes for safe cross-border travel."
But the concept of immunity passports remains deeply contentious, and anyone banking on it for a 2021 summer vacation could be disappointed.
While there's strong argument that globally recognized inoculation documentation could help reconnect the planet, fears remain over what protection they actually afford, how they might be abused and what it means for those still awaiting jabs.
Questions also remain over whether they would become mandatory for any travel and how personal data would be shared safely.
Calls from several European countries to create an internationally recognized vaccination certificate this week prompted the European Union to debate the measure, even as deadly Covid-19 waves continue to ravage the continent.
Greek Prime Minister Kyriakos Mitsotakis wrote to the European Commission President Ursula von der Leyen earlier in January stressing that the need for such universally accepted documentation was a "fundamental priority for us all."
"While we are not going to make vaccination compulsory or a prerequisite for travel, persons who have been vaccinated should be free to travel," Mitsotakis wrote. "This will provide a positive incentive for ensuring citizens are encouraged to undergo vaccination, which is the only way to ensure a return to normality."
During late summer 2020, some borders opened within the EU, allowing vacationers to seek a dose of sunshine and tourism-dependent countries to recoup some losses. There are fears that, without free movement in 2021, the economic impact could be disastrous.
Such pleas have been met with caution by other EU members.
Discussing the issue in Brussels on Thursday, they agreed the need for cross-border cooperation on vaccine certifications, but worried that using them to enable travel may result in the unvaccinated being treated as second-class citizens.
That could lead to scenarios where restaurants or bars require proof of vaccination from customers seeking a glass of wine or see travel companies barring the nonvaccinated from accessing their services.
It's something already being seen, with airlines such as Australia's Qantas and companies such as the UK's Saga Cruises insisting that only vaccinated passengers will be allowed to make international trips.
Von der Leyen told the EU parliament on Thursday that there were concerns over vaccine unknowns, such as whether those inoculated could still carry and transmit the coronavirus and how long protection lasts.
"And then the political question," she added. "How do you ensure that you respect the rights of those who have not had access to a vaccine, and what alternatives do you offer to those who have legitimate reasons for not getting the vaccine?"
Worries about balancing the need to reopen borders with the fairness of allowing those inoculated to trot the world while others continue to endure lockdown and the risk of infection were expressed earlier this month by the World Health Organization.
Tedros Adhanom Ghebreyesus, the WHO's director general, said last week he was skeptical about the divisive effect vaccine passports could have.
"There are two urgent issues which need particular attention, and for which we seek your advice today," he told a meeting of the WHO's emergency committee. "The first is the recent emergence of new variants of the SARS-CoV-2 virus; and the second is the potential use of vaccination and testing certificates for international travel.
"One theme ties both issues together: solidarity. We cannot afford to prioritize or punish certain groups or countries.
'Too early' to book
Such inequalities, of course, have already been a feature of current travel restrictions.
The exclusive resorts of the Maldives have, for instance, been open to everyone for several months, although the high price tag and current premium price of getting there restricts access to all but the wealthy.
But, even as the debate continues on how to implement, control and apply immunity certification -- if at all -- the issue may remain moot for those hoping to plan a summer 2021 getaway.
In the UK, the first country to begin vaccinations, infection rates are continuing to gallop despite enhanced restrictions in place since late December, prompting government officials to warn that international vacations remain uncertain this summer.
Matt Hancock, UK health minister, said on Monday that it was "too early" to book travel before September, by which time the majority of the population should be vaccinated. He advised UK citizens to plan, instead, for a vacation on home soil.
Even this might be optimistic. With major UK calendar events such as the 2021 Glastonbury Festival having already been called off because of ongoing uncertainties, there's the real possibility that a summer holiday at home might also be canceled.