SYDNEY, Australia — Three days after the emergence of a rare Covid-19 case in Sydney, around 40 friends gathered for a birthday party. Along with cake and laughter, there was a hidden threat: One of the guests had unknowingly crossed paths with that single Covid case, an airport driver who had caught the Delta variant from an American aircrew.
Two weeks later, 27 people from the party have tested positive, including a 2-year-old child, along with 14 close contacts. And the seven people at the gathering who were not infected? They were all vaccinated.
The party points to the immense challenge Australia now faces to its wildly successful policy of total Covid suppression. In a simple suburban setting, the vaccines and the highly contagious Delta variant went head-to-head, and because too few Australians have been immunized, the virus spread.
For Australia and every other nation pursuing a so-called “Covid zero” approach, including China and New Zealand, the gathering in western Sydney amounts to a warning: Absent blanket vaccinations, the fortress cannot hold without ever more painful restrictions.
“This is the beginning of the end of Covid zero” said Catherine Bennett, the chair of epidemiology at Deakin University in Melbourne. “We may be able to get it under control this time, but it’s just going to be harder and harder.”
The Delta mutation has already raced from Sydney across Australia, carried on flights and by people visiting schools, hospitals, hair salons and a mass vaccination hub. Half of the country’s 25 million people have been ordered to stay home as the caseload, now at around 200, grows every day. State borders are closed, and exasperation — another lockdown 16 months into the pandemic? — is intensifying.
It’s a sudden turn in a country that has spent most of the past year celebrating a remarkable achievement. With closed borders, widespread testing and efficient tracing, Australia has quashed every previous outbreak, even as almost every other country has lived with the virus’s unceasing presence, often catastrophically.
In Australia, no one has died from Covid-19 in all of 2021. While New York and London sheltered last year from a viral onslaught, Sydney and most of the country enjoyed full stadiums, restaurants, classrooms and theaters with “Hamilton.”
That experience of normalcy — diminished only by a lack of overseas travel, occasional mask mandates and snap lockdowns — is what Australian politicians, from Prime Minister Scott Morrison to local officials, are so desperate to defend. To them, keeping Covid out, whatever it takes, remains a winning policy.
On Friday, Australia doubled down on this approach, announcing that the trickle of a few thousand international arrivals allowed each week (and quarantined) would be cut by half.
It’s an old playbook. During the 1918 flu pandemic, Australia shut its borders to international arrivals for a year, and opened up later than the rest of the world. This time around, most Australians were willing to accept isolation again, assuming it would keep them safe. Until Delta.
Now, public officials are scrambling to counter a variant they have labeled a formidable foe, as if it were a Marvel villain.
Contact tracers have found video footage showing one case of transmission in a Sydney department store, when the man who started the outbreak simply walked by someone else. Delivery drivers have also passed on the virus with brief interactions, and health officials have warned that, in most households, one person with Delta typically leads to infection for everyone.
The variant has forced officials to move faster and harder with restrictions than before.
New South Wales avoided a full lockdown during previous Covid outbreaks, including a cluster last December that was curbed with three weeks of suburb-specific restrictions. This time, Gladys Berejiklian, the state premier, tried a similar tactic, but found that Delta moved too quickly to be contained.
Across the world, it’s a similar story. The Delta variant has been found in at least 85 countries. It is now the dominant strain in England and India, where it first emerged, and it was the source of the outbreak in southern China last month that brought a ferocious response from the authorities.
Many countries anticipate a lengthy battle. On Monday, Chinese officials announced that they planned to build a giant quarantine center in Guangzhou with 5,000 rooms to hold international travelers.
Australia, too, has indicated that the reduced quota for international arrivals will last through the end of the year or longer, depending on how quickly mass vaccination can be achieved.
Officials and economists now worry that the social costs of these severe measures will only increase. The 34,000 Australians still waiting to come home will have to wait longer. The businesses that were starting to revive face many months of further uncertainty.
Melbourne, which has endured on and off lockdowns more often than any other Australian city, may offer a peek at what’s to come. The city’s central business district is already marked by empty storefronts. Some people there are still so scarred by fear that they rarely leave their homes, even when there are no current cases of community transmission.
Even economists who note the economic benefits of Australia’s approach argue that policymakers have become too reliant on border control and locking down at the first sign of trouble. Throughout Sydney’s current outbreak, there have never been more than three people in intensive care, while 12 million Australians have been locked down.
Richard Holden, an economist at the University of New South Wales Business School, said the measurable economy — which has kept employment high thanks to continued trade and government support — masks incalculable costs.
“It’s the weddings and funerals that can never be replaced; it’s the people who couldn’t be by someone’s bedside when they die,” he said. “It’s hard to put a dollar value on that.”
What is especially galling for Australians, he added, is that the country should be further along. Australia, after mastering Covid testing, made the mistake of betting too heavily on two vaccine options, the AstraZeneca shot and one proposed by the University of Queensland. The latter failed in early trials; the former has been caught in a debate in Australia over whether the low risk of blood clots should keep it from being used by anyone under 60.
As a result, the country has been late to obtain the Pfizer and Moderna vaccines, and light on the planning and promotion of its inoculation campaign. Less than 8 percent of Australians are fully vaccinated.
And yet, the birthday party shows that the best vaccines do more than reduce serious illness; they also appear to suppress Delta transmission.
The challenge of the next few months, for Australia and many other countries, involves making sure that most people are vaccinated and only a handful are not.
When that happens, epidemiologists said, deaths, not infections, should become the measurement for policy.
“It used to be that Covid would kill one person for every 100 or 200 cases,” said Peter Collignon, a physician and microbiologist at the Australian National University. “Once you have enough people vaccinated it becomes 1 in 1,000.”
Even Australia’s prime minister, who has been slow to take responsibility for his government’s vaccine failures, acknowledged on Friday that Australians would eventually need to stop aiming for zero Covid.
“Our mind-set on managing Covid-19 has to change once you move from pre-vaccination to post-vaccination,” Mr. Morrison said. The end goal, he added, is that “we should treat it like the flu, and that means no lockdowns.”