One of New Zealand's top infectious diseases experts says travel bubbles with other nations are at risk of being delayed if too many people fail to get vaccinated.
So far New Zealand has quarantine-free travel arrangements with just two other countries: Australia and the Cook Islands, which has never had a single case of COVID-19.
The bubble with Australia is partially suspended, with travel to and from Victoria on pause until at least Friday night after a cluster of cases in Melbourne.
There has been talk of setting up travel bubbles with Asian nations which have also handled the pandemic well.
"For a while we were thinking the next country might be Taiwan, which has been extremely successful at eliminating this virus," University of Otago epidemiologist Michael Baker told The AM Show on Wednesday.
"But of course now they've got quite a large outbreak there."
At the start of May, Taiwan had only reported 1132 cases in total and 12 deaths. In the past three-and-a-half weeks, there have been more than 4000 new cases and 17 deaths.
"It's a big reminder that you absolutely have to be very conscientious with maintaining all of these measures, because you can go backwards very quickly with this virus," said Dr Baker.
Local experts told BBC News Taiwan's outbreak happened because doctors became complacent and testing rates dropped, failing to pick up the virus' spread at adult entertainment venues known as 'tea houses'. Many who tested positive were reluctant to admit they'd been to the brothels, which are illegal, hampering contact tracing efforts.
Dr Baker says it shows how important vaccines will be to reopening the world's borders. Asked by host Duncan Garner when a bubble with the US could be set up, Dr Baker said it was "very hard to speculate".
"I think certainly by the end of the year we should see the effects of vaccination. The problem is in some countries they're not getting sufficiently high vaccine coverage to interrupt transmission. That is a concern with the US in particular, because there are some groups that are very anti-vaccination, or have a lot of hesitancy. I think gradually we'll see coverage go up."
The US has fully vaccinated about half its adult population, and has just recorded its lowest daily death toll since March last year. Just under half of all Americans - under-18s included - have received at least one dose. President Joe Biden's goal is for 70 percent of all Americans to have at least one dose by July 4.
But around a quarter of Americans say they won't get vaccinated, even if it's free, according to a recent Gallup poll.
Just what level of coverage will be needed for herd immunity isn't clear yet. Highly infectious diseases like measles need around 95 percent of the population to be immune to stop wider outbreaks. COVID-19 isn't quite as infectious, with most estimates putting the figure at around 70 percent.
Dr Baker said it's likely we won't know until later in the year, but the UK appears to be approaching it. Seventy percent of UK adults have now had at least one dose, and about 43 percent their second.
They're currently reporting about 2500 new cases a day, down from more than 60,000 in January. Daily deaths, above 1600 in early January, have fallen to single-digits.
"We're seeing that effect very dramatically now in the UK... There's much higher vaccine coverage," said Dr Baker.
But despite the outbreaks in Taiwan, Vietnam and Singapore, he still thinks southeast Asia is our next likeliest option for travel bubbles. They're in range for direct flights, reducing the risk of passengers picking up the virus at international travel hubs.