The threat posed by the Delta variant has become clearer over the past month, as Australia's struggles with it have deepened.
About half the population of Australia is now under coronavirus restrictions.
New South Wales has struggled to make any headway against its outbreak, with 124 Covid cases on Thursday - the most since the crisis began in mid-June.
That's even with weeks of lockdown and extensive testing and tracing. Instead of suppressing infections, case numbers are on an upward trajectory, and the virus has spread.
The risk to New Zealand of quarantine-free travel with Australia is much higher now than when flights began in April. Even back then, it was clear the bubble should have followed New Zealand's vaccination rollout rather than preceded it.
With the rollout still operating in random fashion, albeit with improved vaccine supplies, a pause in the bubble closes an obvious danger while people wait for jabs.
Yesterday, the New Zealand Government shut the transtasman travel bubble for at least the next eight weeks.
The two countries now have a chance to resume transtasman travel when far more of the populations are vaccinated. It's also an opportunity to assess what has worked and what hasn't.
With Delta surging around the world, the pandemic's toll is now falling heavily on the unvaccinated or partially vaccinated.
The variant is far more contagious and carries a much higher viral load than the original virus. But the nature of the pandemic has changed with the addition of vaccines.
The United States government's chief adviser on infectious diseases, Dr Anthony Fauci, says 99.5 per cent of deaths from Covid-19 in that country are among the unvaccinated.
In countries where vaccine rollouts are well advanced, the vaccines are doing their job of protecting most people against severe outcomes of hospitalisation and death. There are also common problems such as insufficient percentages of populations vaccinated; large pockets of people unwilling to get jabbed; and a general weariness among the public over the pandemic.
All these factors are helping to push Covid cases up, especially among young and healthy people, who were previously considered lower priority for vaccinations than older age groups.
Dr Scott Gottlieb, a former US Food and Drug Administration commissioner, put it this way to CBS: "Most people will either get vaccinated, or have been previously infected, or they will get this Delta variant. And for most people who get this Delta variant, it's going to be the most serious virus that they get in their lifetime in terms of the risk of putting them in the hospital."
Gottlieb makes a key point when he adds that the "quality of mask is going to make a difference with a variant that spreads more aggressively, like Delta does, where people are more contagious and exude more virus".
He was referring to how people not fully vaccinated could protect themselves. But Delta's contagiousness suggests that people trying to avoid Covid altogether should take that advice onboard, even once fully vaccinated.
New Zealand's two-shot vaccine, from Pfizer/BioNTech, is estimated to be 95 per cent effective against Covid-19 overall, and 88 per cent effective at preventing Delta from causing symptoms.
Vaccines do not come with a 100 per cent guarantee. They shoulder the lion's share of the work in protecting people, but can't always stop infections. When so-called "breakthrough" cases happen, they have tended to be "mild" in a medical sense - although mild cases can still be unpleasant.
This week such cases hit headlines in the US with infections among White House and congressional staff.
Severe breakthrough cases appear to be rare. The US Centres for Disease Control and Prevention says that as of mid-July, 5492 vaccinated people who were in hospital or died in the US had also tested positive for coronavirus. That's in the context of about 160 million Americans being fully vaccinated. Deaths from Covid in the UK are dramatically down compared to earlier in the pandemic.
But ultimately, there is insufficient data as yet about how common breakthrough cases are or the numbers of vaccinated people transmitting the virus or developing chronic long-Covid. About 6.2 per cent of adults in the UK - 3.2 million people - are suffering long-term impacts of the coronavirus.
Overseas issues with Covid may seem very different to New Zealand's situation, but with Delta raging, extra precautions and attention to risks have to be taken everywhere.