As many as 20 per cent of unvaccinated people with Covid could be missed by "leaky" rapid antigen testing, says leading epidemiologist Michael Baker.
Free rapid antigen testing will soon be available for unvaccinated people in participating pharmacies around the country.
As the Auckland border lifts this week, unvaccinated travellers from the city – as well as domestic travellers with some transport companies – will be required to show proof of a supervised negative rapid antigen test.
Epidemiologist Professor Michael Baker said the system of testing unvaccinated travellers with the rapid antigen model would allow some infected Aucklanders to slip through.
"Potentially for every 10 infected unvaccinated people leaving Auckland, you're going to miss 20 per cent of them – two out of 10 people with rapid antigen tests. It is leaky."
GeoHealth Laboratory Co-director and University of Canterbury Senior Public Health Lecturer Dr Matthew Hobbs said PCR tests had been used during the elimination phase as "the cost of missing a case was deemed to be too high".
He said the rapid antigen test was less sensitive, and required a higher quantity of the virus to be present in the sample.
As people are unable to travel with a positive test result, health authorities are advising travellers to allow sufficient time to be tested, and re-tested, if necessary.
The Ministry of Health advised rapid antigen tests can sometimes give false-positive results, and said unvaccinated people who receive a positive test result will be "strongly advised to seek a confirmatory PCR test".
But Baker said due to the reduced sensitivity of the rapid antigen test, a false negative would be more common than a false positive.
"You will get false negatives. Particularly in the early phase of infection or the later stages where there's much less virus present you may not pick up enough to activate the test."
"False positives should be very rare, but false negatives are the concern. People talk about them being 80 per cent sensitive."
A more robust testing regime would see the PCR test used for unvaccinated travellers, and perhaps a rapid antigen test for those who were vaccinated, Baker said.
"The fact that we're not testing vaccinated people either, and we're not testing the largest unvaccinated group which is children … basically the regime we've got is full of holes for people coming out of Auckland."
"If we were really working hard to limit the virus out of Auckland we would be doing testing and vaccination."
"We would say if you are fully vaccinated as an adult you would need the rapid antigen test as well … and we might say that entirely unvaccinated people have a PCR test."
While this regime would be more labour intensive and expensive, Baker said it would be more effective at reducing the virus coming out of Auckland.
Rapid antigen tests will be available at 483 pharmacies around New Zealand, for unvaccinated asymptomatic people who are aged above 12 years and three months.
The tests will be free from December 15 to the end of January, to help meet domestic travel requirements over the holiday period.
There are a total of 345 pharmacies in the North Island – including 138 in Auckland – offering free antigen testing. There are 138 in the South Island.
Participating pharmacies can be found at www.healthpoint.co.nz under 'Supervised rapid antigen testing for travel' under Covid-19 Testing. Bookings are recommended, but some pharmacies will accept walk-ins.
As the rapid antigen tests are for asymptomatic people only, those who are unwell are asked to stay at home and seek a PCR test.