It might seem counter-intuitive but the latest case of COVID-19 at the border could be proof the vaccine works, experts say.
An airplane cleaner was reported positive for the SARS-CoV-2 virus on Tuesday, after their infection was picked up during routine testing. Airside cleaning workers are required to get tested at least once every 14 days, and anyone who spends more than 15 minutes on board an affected aircraft, every seven days.
Genomic testing overnight linked the case to an infected person who arrived in the country on April 10, COVID-19 Response Minister Chris Hipkins said on Wednesday.
"This person has followed all the rules and done all the right things," he told The AM Show.
In addition to being tested and following hygiene protocols, that included being fully vaccinated. So just how did they catch the virus, which has killed more than 3 million people to date?
Firstly, vaccines aren't foolproof. The one being used here, made by Pfizer and BioNTech, is about 95 percent effective according to trials and real-world data from Israel's speedy rollout.
"There's a 5 percent failure rate, one in 20," medical director of Royal NZ College of GPs Bryan Betty told Newshub.
"So we know... a number of people could still contract COVID. We're not sure the implications of that - whether they can still spread COVID - but it's something we need to be aware of."
But there's also a chance the airport case is one of the lucky 95 percent and the vaccine is doing its job, says University of Otago professor of public health Michael Baker.
"This would be consistent with the idea that the vaccine, it's mainly designed and the trials are set up to determine if the vaccine stops you getting symptomatic illness, and it looks like that may be what the vaccine's done in this instance.
"This person was only detected because of routine testing. So it may be consistent with the idea that this vaccine was just as effective as we know it is."
In other words, the vaccine is designed to stop people getting sick if they're exposed to the virus; this person was exposed but had no idea until a routine test picked it up.
"The vaccine is designed to stop you getting sick - symptomatic illness," said Dr Baker. "That doesn't seem to be what this person has. That's consistent with the vaccine doing its job."
There is growing evidence that by reducing symptomatic illness, the vaccines also reduce transmission - while it's not known for sure just yet if that's happening, vaccines for other diseases work that way.
"It's likely that the vaccine will have decreased their levels of infectivity to others,' said Dr Baker. "We don't know yet whether the vaccine entirely stops transmission, but certainly someone in this situation - who's fully vaccinated and has no symptoms - is far less likely to infect other people than someone who has symptomatic illness."
While some experts - notably Dr Baker's University of Otago colleague epidemiologist Nick Wilson - have labelled the latest case a border failure, he doesn't see it that way.
"This in many ways shows the system is working well - it's picking up infected people early."
While the airport cleaner's infection has been linked to an imported case, it remains to be discovered just how the virus was transmitted.
"That's the big mystery with this case... did they have contacts with other border workers who did have front-facing roles? Because at the moment, there's really no evidence of people getting the infection from cleaning aircraft or surfaces that might have been touched by a case that arrived in New Zealand."
There was one high-profile previous suspected case of transmission via surfaces - a rubbish bin in a managed isolation facility. A later study found that was highly unlikely, the infection more likely to have been spread via the unventilated air in the hotel's hallway.