SYDNEY — New Zealand has suspended a travel bubble with Australia for two months as an outbreak of a hyper-transmissible variant of the novel coronavirus threatens the nation’s earlier success in containing the pandemic.

More than 200,000 people have flown between New Zealand and Australia since the quarantine-free travel corridor opened in April. But a delta variant outbreak that started in Sydney in mid-June has “materially changed the risk profile,” New Zealand Prime Minister Jacinda Ardern told reporters Friday. The small island nation has worked hard to eliminate the virus from its shores, Ardern said, “and we will not risk that.”

When the bubble started, “Australia had seen only a handful of community cases since January, and delta was a blip on the global radar,” said Michael Plank, a professor at the University of Canterbury in New Zealand, who has led mathematical modeling of the country’s pandemic response. He called delta a “game-changer.”

Travel bubbles have been key to reopening skies across the Asia-Pacific region, where border closures and quarantine restrictions are most prevalent, even as other places around the world open up. But logistics and shifting patterns of virus spread have complicated such plans.

Bubbles have proved especially fragile in countries such as Australia and New Zealand that, unlike the United States and Europe, have pursued a strategy of zero transmission — with a low tolerance for even single-digit daily cases.

Travel from individual states has been paused several times since the bubble began, in response to small clusters of cases. But this is the first time it has been suspended Australia-wide, even for states that have gone as long as a year without a case of the virus.

More than half a million New Zealanders live in Australia, and many were distraught at Friday’s decision. The two countries normally are closely linked by visa-free travel, and citizens of each country have working rights in the other.

The prime minister said Friday that any New Zealanders visiting Australia should return within the next seven days. Those traveling from New South Wales, the state surrounding Sydney, will be required to isolate in hotel quarantine for 14 days, even if fully vaccinated.

Both countries handled the pandemic well last year, but officials have struggled with the transmissibility of the delta variant, which makes it harder to stamp out even with contact tracing and lockdowns. About half of Australia’s 26 million residents are in lockdown in an attempt to control the virus’s spread, only allowed to leave their homes to exercise outdoors or to shop for groceries.

The worst outbreak, in Sydney, has grown to nearly 1,800 cases since the first case — an airport limousine driver — was recorded June 16. That is a small outbreak by global standards, but officials are concerned about how easily the virus spreads through fleeting contact, especially given Australia’s relatively low rates of vaccination.

“It’s quite possible that Australia’s lockdown strategy — that’s worked so well with all the previous outbreaks we’ve had — is simply not strong enough, not fast enough, to deal with delta,” the president of the Australian Medical Association, Omar Khorshid, told reporters. He is calling for an overhaul of the country’s vaccination program, which has been plagued by mixed messages and supply delays.

Only about 12 percent of Australia’s population is fully vaccinated, compared with about 49 percent in the United States, according to Johns Hopkins University data.

Top Biden administration officials on Thursday said that the delta variant is posing new challenges for the U.S. health system, and they urged the millions of unvaccinated Americans to get shots to protect themselves and their communities.

The way that Australia’s early success has rapidly unraveled during the winter months in the Southern Hemisphere could offer a reality check for Europe and the United States as they enjoy a summer of freedom.

“When the virus changes,” Plank said, “we must adapt our response or it will come back to bite us.”

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