COVID-19 experts are calling for all frontline border workers to undergo weekly tests for the virus, saying it would reduce the chance of having to go into another lockdown.
At present most border workers are required to undergo testing at least once every 14 days. Workers at quarantine facilities, people who transport them there and aircrew members need to get tested every seven days, and from Thursday they'll be joined by workers in managed isolation facilities and a few other groups of frontline workers.
But modelling by researchers at Te Pūnaha Matatini and the universities of Canterbury and Auckland suggests weekly tests for all frontline workers would be a "substantial improvement" on the status quo.
They modelled how big an outbreak could potentially be once a frontline worker tested positive for the virus, given different testing regimes - none, weekly or fortnightly.
"In all parameter explorations, there was a substantial increase in the expected number of exposures if the first detected case was not in the frontline worker," their study, published in the Journal of the Royal Society Interface on Wednesday, said.
If they weren't being tested at all, by the time a frontline worker was found to have the virus there would likely be 10 others in the community, potentially spreading COVID-19. That assumes 50 percent of people in the community who develop symptoms bother to get tested.
Under a weekly regime, that drops to just four - a huge difference in a place like New Zealand, where almost none of us have been exposed to the virus previously, few are immunised and "where restrictions on community activities and gatherings have been largely relaxed".
"The risk of a frontline worker inadvertently seeding a major outbreak can be significantly reduced by scheduled weekly testing combined with symptom checks by trained professionals," the study said.
From Thursday, border employers will be able to use saliva tests instead of the invasive nasopharyngeal swabs, which it's hoped will stop workers from avoiding the tests - as one reportedly did for five months.
"Saliva testing gives an alternative way of achieving that, that isn't so impactive or unpleasant," Te Pūnaha Matatini's Michael Plank told Newshub.
"The evidence that I've seen says that's pretty much as reliable as a nasal swab."
The saliva swabs used at the border use the same polymerase chain reaction, or PCR, technology as the nasal swabs. They differ from the rapid antigen saliva tests which give results in minutes, requiring a lab to analyse the sample, but are remarkably more accurate.
If the first case detected is a contact of the frontline worker, we have a much bigger problem - assuming 50 percent of people get tested when symptoms appear, "the expected outbreak size is likely to be between 19 and 26" people. If everyone got tested, it will still be likely between 11 and 14 people already.
New Zealand has successfully used short, sharp lockdowns to prevent the spread of COVID-19 when contact tracers are unable to work out where an infection came from.
The most recent case detected at Auckland Airport was fully vaccinated and detected through routine testing while asymptomatic, reducing the likelihood of a wider outbreak. All their close contacts so far have tested negative.
"We can make the system as tight as we possibly can, but there will on occasion be something that will happen," COVID-19 Response Minister Chris Hipkins told The AM Show on Wednesday.
"The vaccine is designed to stop you getting sick - symptomatic illness," University of Otago professor of public health Michael Baker told Newshub. "That doesn't seem to be what this person has. That's consistent with the vaccine doing its job."
Another finding in the research was that a high number of infections being picked up in the second week of a stay in managed isolation could be a sign there's a leak somewhere in the hotel.
At present, arrivals from anywhere in the world except Australia, Niue and the Cook Islands are expected to go into MIQ and have three tests - one on day zero, the second one day three and the last on day 12.
The modelling showed if we only had a test on day zero and no MIQ, around half of all infected people arriving in New Zealand would end up being released into the community whilst still infectious - that's because in the early part of an infection, even sensitive tests like nasopharyngeal swabs can fail to pick it up.
If there was no transmission in MIQ, around 17 percent of infections would be picked up in the second week - whether at the scheduled 12-day test or when a person becomes symptomatic. The rest are picked up on day three, the remainder giving a false-negative before being picked up on day 12.
"If this fraction increases substantially, then procedures for infection prevention in quarantine should be reviewed," the study said, saying any figure above 20 percent was cause for concern.
At the moment, guests in managed isolation are "still offered opportunities for outdoor exercise and/or smoking under supervision to ensure PPE and 2-metre physical distancing is maintained", the study says - which experts say has to end.
"The greatest reduction in risk associated with quarantined international arrivals can be obtained by minimizing mixing among guests in the facilities," said Dr Plank. "This can be achieved by eliminating or carefully managing shared spaces such as smoking, exercise areas and elevators."
The Grand Millennium and Grand Mercure facilities were recently closed due to fears of virus transmission through ventilation systems.
Neither the Ministry of Health or COVID-19 Response Minister Chris Hipkins replied to requests for comment on the study's findings.