NEW YORK • As Covid-19 vaccines started rolling out late last year, there was a palpable sense of excitement.

People began browsing travel websites and airlines grew optimistic about flying again.

Irish budget airline Ryanair even launched a "Jab & Go" campaign alongside images of 20-somethings on holiday, drinks in hand.

It is not working out that way, however. For a start, it is not clear the vaccines actually stop travellers from spreading the disease, even if they are less likely to catch it themselves. Neither are the shots proven against the more-infectious mutant strains that have startled governments from Australia to Britain into closing, rather than opening, borders.

An ambitious push by carriers for digital health passports to replace the mandatory quarantines killing travel demand is also fraught with challenges and has yet to win over the World Health Organisation (WHO).

This bleak reality has pushed back expectations of any meaningful recovery in global travel to next year.

That may be too late to save the many airlines with only a few months of cash remaining. And the delay threatens to kill the careers of hundreds of thousands of pilots, flight crew and airport workers who have already been out of work for close to a year.

Rather than a return to worldwide connectivity - one of the economic miracles of the jet era - prolonged international isolation appears unavoidable.

"It's very important for people to understand that at the moment, all we know about the vaccines is that they will very effectively reduce your risk of severe disease," said Dr Margaret Harris, a WHO spokesman in Geneva. "We haven't seen any evidence yet indicating whether or not they stop transmission."

To be sure, it is possible a travel rebound will happen on its own, without the need for vaccine passports.

Should jabs start to drive down infection and death rates, governments might gain enough confidence to roll back quarantines and other border curbs, and rely more on passengers' pre-flight Covid-19 tests.

The United Arab Emirates, for example, has largely done away with entry restrictions, other than the need for a negative test.

For now though, most governments broadly remain skittish about welcoming international visitors and rules change at the slightest hint of trouble.

Witness Australia, which slammed shut its borders with New Zealand last month after New Zealand reported one Covid-19 case in the community.

New Zealand and Australia, which have pursued a successful approach aimed at eliminating the virus, have both said their borders will not fully open this year.

Travel bubbles, meanwhile, such as one proposed between the Asian financial hubs of Singapore and Hong Kong, have yet to take hold. France on Sunday tightened rules on international travel, while Canada is preparing to impose tougher quarantine measures.

"Air traffic and aviation is really way down the priority list for governments," said Mr Phil Seymour, president and head of advisory at British-based aviation services firm IBA Group.

"It's going to be a long haul out of this," he added.

The pace of vaccine roll-outs is another sticking point. While the rate of vaccinations has improved in the United States - the world's largest air travel market before the virus struck - inoculation programmes have been far from aviation's panacea.

In some places, they are just one more thing for people to squabble about. Vaccine nationalism in Europe has dissolved into rows over supply and who should be protected first. The region is also fractured over whether a jab should be a ticket to unrestricted travel.

According to Mr Joshua Ng, Singapore-based director at Alton Aviation Consultancy, it all means a rebound in passenger air traffic "is probably a 2022 thing".

Long-haul travel may not properly resume until 2023 or 2024, he predicts. The International Air Transport Association said this week that in a worst-case scenario, passenger traffic may improve by only 13 per cent this year. Its official forecast for a 50 per cent rebound was issued in December last year.

The lack of progress is clear in the skies. American Airlines Group on Wednesday warned that 13,000 employees could be laid off, many of them for the second time in six months.

Global commercial flights as at Feb 1 wallowed at less than half of pre-pandemic levels, according to travel data provider OAG Aviation Worldwide.

Quarantines that lock up passengers upon arrival for weeks on end remain the great enemy of a real travel rebound. And the doubts around vaccines mean that aviation's top priority should be a standardised testing regime, said the IBA's Mr Seymour.

This might involve a coronavirus test 72 hours before departure, 24 hours of isolation on arrival, and then another test before being released.

"If this was the world standard, then I think we would all be prepared to start picking holidays and fly away," he said.

BLOOMBERG