It has been confirmed the latest managed isolation and quarantine worker to test positive for Covid-19 had recently received the required second dose of the Pfizer vaccine.
The vaccine appears to be doing its job though, as the woman is not sick, immunisation expert Dr Nikki Turner told Checkpoint.
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Another person in the cleaner's household has returned a weak positive, which needs more testing, but the other three people in the household have returned negative results.
Director-General of Health Ashley Bloomfield confirmed the worker from Auckland's Grand Millennium isolation hotel had her first vaccination on 23 February and the second on 16 March, five days before testing positive.
The vaccine is expected to take at least seven days to reach full immunity.
"She has got Covid-19 in the back of her throat but she has not gotten sick from it," Immunisation Advisory Centre director Nikki Turner said.
"So we can hope and expect that the vaccine has been working," Dr Turner said.
"Secondly, it is only five days after the second dose, so she hasn't got the full immune protection yet," she said.
"What we do know is the vaccine is highly effective against getting sick. We do know that it works a bit against spreading it. But vaccinated people can still carry and spread the virus, but they are very unlikely to get sick, and this is what we've seen in this case.
"My understanding is that this woman has no symptoms at all, but she is well. They have just picked up on a swab that they take routinely that she has got the Covid-19 virus in her throat.
"We have always known that with vaccines, they are very effective against stopping you getting sick. We do not know yet how effective they are against spread. Now there's some important things here - when you are sick, you are much more likely to spread.
"My understanding is there is only one member of the household that has got a weak positive result. The others have not. When you are symptomatic, you [are more likely to] spread. So we know that being vaccinated makes you less likely to spread. But it does not fully stop it."
That is why it was important to keep strict border protection measures, PPE regulations, social distancing, and regular monitoring of border staff, she said.
"The first principles of these vaccines are protect people against getting sick. We are hoping and expecting they will also reduce spread, but it is not 100 percent and it never will be 100 percent."
The Pfizer vaccine has been proven to have 95 percent effectiveness, but it takes at least one week after the second dose of the vaccine for that to be fully developed.
"With a few more days, [the MIQ worker] would have even higher immunity. However, I'd like to point out, she did not get sick, so the vaccine is doing what it meant to do, which is really pleasing to see," Dr Turner said.
"This is an example of the vaccine working. A vaccine is not a panacea to fully stop the virus spreading, if it was, we could stop all our personal protection and border controls now, but we can't, because we do know that even fully vaccinated people at times can still carry the virus in their throats and spread it.
"It's really important to know it's not 100 percent blocking the spread of the virus. It's certainly reducing it considerably, but not fully blocking it. We cannot let go of all our other important public health protection for our people.
"First and foremost. the use of the vaccine is to stop people getting sick. That's why we're offering the vaccine to the people who are most likely to be exposed to it.
"International data is accumulating on how effective the vaccines will be at also reducing spread. They will not be 100 percent effective against reducing spread, but we're seeing more and more data that they do quite considerably reduce the spread."