The renamed quarantine-free travel corridor plan between Hong Kong and Singapore has hit another snag after months of delay.

An announcement on the arrangement, previously known as the air travel bubble, was expected yesterday but is understood to have been scrapped at the last minute.

Under the updated deal, both Singapore and Hong Kong would make it compulsory for passengers on designated flights to be fully vaccinated against Covid-19.

But the plan is now up in the air after some Hong Kong lawmakers on Friday pushed for it to be pulled.

Legislators, including Ms Alice Mak of the Federation of Trade Unions and Mr Michael Tien, had urged the government to scrap the plan as Singapore shifts from a zero-infection strategy, which Hong Kong places great importance on.

They reasoned that having zero coronavirus cases is a requirement for the territory to reopen borders with the mainland.

Under Singapore's transition from dealing with Covid-19 as a pandemic to managing it as endemic, its Government aims to fully inoculate two-thirds of its 5.7 million population by early next month.

As at last Saturday, about 69 per cent of Singapore's residents had received at least one dose of a Covid-19 vaccine, and 40 per cent were fully vaccinated.

Hong Kong's Commerce and Economic Development Bureau said in a reply to The Straits Times that "the two governments have been maintaining communication and reviewing the epidemic situation of the two places". It said an announcement may be made in due course.

A spokesman for Singapore's Ministry of Transport, in response to media queries, also said yesterday: "Both Singapore and Hong Kong are continuing discussions and will provide an update in due course."

Respiratory diseases expert Leung Chi Chiu of the Hong Kong Medical Association told ST that Hong Kong cannot afford to shut its borders forever, but the key question is when and how to reopen.

"Whether we can reopen with Singapore depends on whether there is a net importation risk into Hong Kong. If Singapore is going to abandon the zero transmission policy, and at risk of having local Delta variant transmission, the answer may be obvious," he said.

With the more transmissible Delta variant behind the rise in cases in a number of countries, Hong Kong is not primed to reopen its borders, given its slow and low vaccination take-up rate.

"With the low vaccination coverage among the elderly in Hong Kong, we do not have the necessary conditions to reopen to areas with Covid-19 incidence that are hundreds-or tens of thousands-fold that of Hong Kong," said Dr Leung.

"However, as we have largely cleared local transmission in the last month, we are ready to explore reopening borders with areas that can keep zero transmission."

So far, about 35 per cent of Hong Kong's 7.5 million residents have had at least the first dose of a Covid-19 vaccine, while 24 per cent have received two jabs.

Epidemiologist Ben Cowling, a professor at The University of Hong Kong, said it was likely that Hong Kong would tailor its exit strategy to the Chinese mainland's.

"Quarantine-free cross-border travel with the mainland would stimulate the local economy in Hong Kong and I can envisage we might synchronise our Covid-19 strategy with the strategy in the mainland," he said.

"In other words, perhaps Hong Kong will continue with its zero-Covid-19 strategy and plan to eventually drop quarantines and Covid-19 measures at the same time as the mainland."

But pressure has been mounting on businesses, particularly in the hard-hit aviation, hospitality and retail sectors.

Urging people to get vaccinated, Hong Kong General Chamber of Commerce head George Leung told local media last month that as a small and externally oriented economy, Hong Kong will suffer more than other cities if its borders remain closed and normal business activities cannot resume.

In an interview with Bloomberg TV last week, Singapore Health Minister Ong Ye Kung expressed optimism that the travel plan with Hong Kong would materialise.

"With very low Covid-19 case counts, or none at all on many days, the cities are well positioned to open their borders again," Mr Ong said. "That gives us common ground to talk again about restarting the air travel bubble."

The Hong Kong-Singapore travel scheme has been postponed several times.

• Additional reporting by Toh Ting Wei