Following a stretch of more than 10 days without Covid-19 community cases here and positive updates on vaccine developments internationally, the Hong Kong-Singapore two-way travel bubble - heralded as the world's first and set to take flight last Sunday - looked to be a welcome step in the resumption of leisure travel by air. People would be able to fly between the two cities without being quarantined, subject to conditions such as testing negative for Covid-19 more than once. Alas, it was not to be. The plan has been deferred due to the rise in Covid-19 cases in Hong Kong. Under the arrangement, a two-week suspension would kick in if the seven-day moving average of the daily number of unlinked local cases is more than five for either Singapore or Hong Kong.

The latest development shows just how delicate and cautious the process of re-opening borders needs to be. The Hong Kong authorities said on Sunday that the epidemic in the city was worsening rapidly, with new cases every day in the double digits. This is not unique to Hong Kong. Infections are rising in 69 countries, and the number of deaths due to Covid-19 has passed 250,000 in the United States. South Korea - widely praised for bringing the disease under control, tightened social distancing measures after reporting hundreds of fresh cases over consecutive days. With the pandemic showing no sign of slowing down, tightly-controlled air travel seems likely to be the modus operandi for some time to come.

While Hong Kong's new unlinked cases reported over the weekend took the rolling seven-day average to 3.86 - which was below the trigger point - a judgment call was still made to invoke the suspension. The decision to err on the side of caution is a prudent one, given that there is the risk of silent transmission chains. The decision is meant to protect public health on both sides, not just in Hong Kong. Singapore, too, has on its part imposed stricter restrictions on travellers coming from Malaysia and Japan, following the rising number of cases in the two countries. Until a vaccine is found that is safe and can be widely distributed, it is likely that travel will be subject to such start-stop spurts.

Travellers who had their plans deferred are understandably disappointed. But even so, there should be no undue rush to get the flights off the ground again. Far better to do so only when the situation is safe, and likely to boost confidence that the air travel bubble concept can work. Meanwhile, travel for leisure should be seen as a bonus, not a necessity, given the widely varying levels of infection and risk in the region and elsewhere. Measures still have to be calibrated and applied country by country, which will take time. If travel is to have any hope of resuming among more jurisdictions, it is better to be safe than sorry.