In China, where a handful of cases can prompt mass testing, a growing number of public health experts are now favoring a mitigation, rather than zero-tolerance, approach, according to Huang Yanzhong, senior fellow for global health at the Council on Foreign Relations.

"The zero Covid strategy obviously has been successful in some parts of the world over the last 18 months. I don't think anyone wants it to be the future," said Karen A. Grépin, an associate professor at the University of Hong Kong's School of Public Health. "The choice now is: when do you want to start letting people die? It won't be a perfect transition, there will be parts of the population that will get this and will die."

Did China and Australia take the right approach?

"The Asia-Pacific countries, by and large, have had an incredibly successful year and a half responding to Covid," Grépin said. "It would be very difficult to say that the strategies adopted in this region were not good ones."

"I believe that (China and Australia) overrated the integrity of their borders," Fisher said. "It just may not have been such a big problem with the Wuhan version. But then you get something much more transmissible, and then any breach is exposed."

Once Delta arrived in Australia, it exposed a major flaw in the country's strategy -- a slow vaccine rollout. When other countries frantically rolled out vaccines earlier this year, Australia's leader seemed to be in no rush.

"(That) was a huge mistake," said Alexandra Martiniuk, a professor at the University of Sydney school of public health. "So we are stuck in this position (in Australia) where there's very few people vaccinated and a very dangerous variant."

Can a zero Covid approach work?

"For this outbreak, I think they'll be down to zero fairly soon, but it does illustrate the risks of Covid still in a zero Covid strategy," Cowling said. "This won't be the last outbreak -- there will be more outbreaks in the months to come."

For months, the zero Covid strategy has worked well. While other countries have battled overburdened health care systems and high numbers of deaths, China and Australia have reported just 4,848 and 939 deaths respectively. That's allowed them to resume life as normal within their borders, and meant their economies have taken less of a hit.

Longer term, though, many experts think a zero Covid strategy isn't sustainable. Eventually, all countries will want to open up to the world again -- and when they do, they may need to accept that some people would likely get ill, a hard shift in Asia-Pacific countries used to keeping the virus out altogether.

"Unless you're prepared to cut yourself off from society forever, you're going to have Covid in your country. So it's a question of when you let it in, and when you live with it," Fisher said.

That shift could be tough politically.

"This containment-based approach is still popular among the Chinese populous, in a way that's a reflection (of) how this has been so internalized among the Chinese people. They accepted it as the only effective approach in coping with the pandemic," he said. "So we're not talking not just about the shift of the incentive structure of the government officials, but also to change the mindset of the people, to prepare them for a new strategy."

But ditching the zero Covid strategy isn't something Australia and China should necessarily be thinking about right now, said Grépin.

When more than 80% of people are vaccinated, countries can loosen borders, Fisher said.

In China, they may need to add additional shots to increase immunity, Grépin said.

Opening borders too early could mean "the death that they fought so hard to avoid will happen," she added.

It's not over

The collective experience of China and Australia also highlights the risk that other countries with tough border restrictions might not be able to keep out Delta -- or another variant -- forever.

Fisher said Delta outbreaks would likely happen in other countries that had so far not experienced it, such as New Zealand.

"There should be the same urgency to vaccinate when you don't have Covid because it's just a matter of time, and we know the social and economic impact when you have to lock down and mass test as a response," Fisher said.

He recommended maintaining some restrictions -- like wearing masks indoors -- even when a country had sealed off borders, and no local cases were reported.

"Every country should pretend there's cases in their borders, and at least have mask wearing indoors, limit gatherings," he said. "Sure that bothers people, but I can tell you, when you get a case, suddenly life's a lot easier."

Countries needed to keep learning from other countries about how to handle the pandemic, Fisher added.

"If anyone thinks this is over, they're wrong," Fisher said, "Everyone's got to face up to it and live with it someday -- and it's not over for any country yet."