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New modelling from Te Pūnaha Matatini shows there is a short-term risk of a Covid-19 community outbreak from a border worker, as vaccinated workers might still spread the virus, even if they are not symptomatic.
The new modelling has found risk of an outbreak could jump to 9 percent if the vaccine only halves transmission.
This week an MIQ cleaner contracted Covid-19 on the job and showed zero symptoms, but the infection was picked up during in routine surveillance - showing how important that testing is.
Professor Michael Plank is one of the researchers tracking the risk of infection against the vaccine.
"There is still a risk that someone who has been vaccinated can get the virus, and can potentially pass it on to others," Professor Plank said.
"We know that vaccines are really effective at stopping people getting sick. But there's still a risk that they could pick it up and pass it on. And in the case of border workers, if the virus stops them from getting sick, that makes it harder to spot the cases, if we do get cases in border workers.
"In some cases where we've had a border worker getting infected, we've noticed because they've developed symptoms. If the virus stops them from getting symptoms, it makes it harder to detect, so we're really relying on testing to pick those infections up."
Professor Plank said there is more data coming in on how much the Covid-19 vaccines stop or slow the rate of infection.
"It's starting to look really positive for the Pfizer vaccine, that it does have a strong effect, and that it will reduce the risk of transmission quite significantly.
"But there's still a range of uncertainty there and it's still possible it won't be as effective at reducing transmission, as it is stopping people getting seriously ill."
Data modellers were looking at the possibility of a community outbreak if the vaccine only had 50 percent effectiveness at stopping transmission.
Professor Plank said under that scenario, more testing of border workers would be needed to make sure no virus sneaked through undetected in an asymptomatic person.
"Since we've done that research actually more data has come in and we're more towards the optimistic end on the vaccine effectiveness now.
"So it's looking more positive but the key message really is that we really need to keep that routine testing of our border workers going, to make sure that we don't have asymptomatic cases slipping through the net."
Continuing all strict precautions at the border is also crucial, he said.