New Zealand's latest community cases have prompted calls for a wider roll-out of saliva testing, which health experts argue could be carried out daily on MIQ workers. What does it involve - and what is New Zealand already doing? Science reporter Jamie Morton explains.
What is saliva testing?
Saliva-based testing has been increasingly touted as a quick and simpler option for those who need regular checks, like border staff and healthcare workers.
While based on PCR methods currently used for analysing virus samples, saliva tests require only a small sample - or people essentially having to spit into a container.
"Simply put, it is an easier, non-invasive sample that patients can self-collect," explained Dr Nikki Freed, a senior lecturer at Massey University's School of Natural and Computational Sciences.
"This can reduce exposure for public health staff and reduce patient discomfort."
Examples include the SalivaDirect method developed by Yale University scientists and now part of an ESR and Air New Zealand trial, and the University of Illinois' Shield, being used in Auckland Airport by Rako Science.
The disadvantage, Freed said, was that it was slightly less sensitive than the standard Covid-19 test in New Zealand, which draws on nasopharyngeal swabs.
"Less sensitive means that a person will need more copies of the virus to be detected, so saliva-based tests might miss a very small fraction of people who are just starting to get sick or who are just at the end of Covid-19 illness."
How much less effective is it than swabs?
A recently-published systematic review suggests saliva tests perform only slightly worse.
It found that, when it pooled the different saliva test types together, they were about 83 per cent accurate - meaning about 17 per cent of true positives would be missed.
That wasn't far off the effectiveness of swab tests, which proved 85 per cent accurate, and would miss about 15 per cent of true positive cases.
"So, even though on average when all results are pooled together, it does appear that the saliva tests perform slightly worse than nasopharyngeal swab tests, they are not all that different," said Associate Professor Arindam Basu, of Canterbury University.
"Of the two different saliva tests being used in New Zealand, if you compare the performance, limit of detection, and convenience between SalivaDirect and Shield tests, the administration and principles are identical, and both employ innovative ways to cut down on the time of processing."
SalivaDirect was able to detect with 94 per cent accuracy true positive results returned by a comparable nasopharyngeal swab, while Shield was able to detect true positive results with 88.9 per cent accuracy.
"This indicates that the performance of Shield is roughly similar to what is expected of all saliva tests, and not much different from SalivaDirect."
Basu stressed that while the systematic review and SalivaDirect research was assessed on the basis of peer-reviewed publication, the research on Shield was not peer-reviewed and therefore open to bias.
How is it being used here already?
Last month, Air New Zealand and ESR began a three-month trial in which staff members provide saliva samples alongside their regular nasopharyngeal swabs, so the two can be compared.
ESR's chief scientist, Dr Brett Cowan, said there was hope saliva testing would be more comfortable than current methods, and, if successful, could give New Zealand another public health tool.
In a separate three-month trial with Kiwi business Rako Science, Auckland Airport's international terminal has been set up to provide the saliva testing.
Airport staff taking part in the saliva tests were doing so on a voluntary basis, along with the mandatory nasal-swab testing required by the Government's border policies.
Elsewhere, staff at Ryman Healthcare are now being offered voluntary saliva tests.
Ryman, which has retirement villages and rest homes across the country, has contracted Hill Laboratories to carry out the testing.
What are experts calling for?
In a recent blog post, Otago University public health experts said the Government should mandate daily PCR-based testing of saliva for MIQ workers.
"This option could also be explored for travellers in MIQ, in addition to the current testing regimen to allow for comparative assessments," they wrote, adding this testing was already being used in parts of Australia and other countries.
Otago epidemiologist Professor Nick Wilson told RNZ that New Zealand should have moved from weekly testing long ago, and should be saliva testing border staff daily.
"The data I've seen suggests it may be, just may be, as reliable [as PCR tests via nasal swab], but even if it is less reliable, because it's done every day, it's probably far superior to just weekly testing ... and it's much less onerous on staff.
"Other countries have been using this for months now."
Otago University epidemiologist Professor David Skegg said even if there were shortcomings to the saliva test, doing it daily would help compensate.
"If you're doing it every day it'll be better ... also it'll give us an earlier warning that we need to act."
What does the Government say?
Covid-19 Response Minister Chris Hipkins said he wouldn't rule out expanding the use of saliva testing.
"But it would be as an addition to our existing testing regime, not as an alternative to it."
Hipkins pointed out that saliva testing still required the same resourcing to process test results as current PCR-based swab tests.
"But the overall confidence in each individual test result is lower than with PCR," he said, adding the current method was still regarded around the world as the "gold standard" test.
The Government had to consider the impact on workers being more regularly tested, how to provide the capacity to do the extra testing, and whether it would all give an extra layer of reassurance, given the higher chance of getting false negatives.
Director general of health Dr Ashley Bloomfield said the Government wasn't reluctant about using saliva tests, which had also been deployed in some quarantine facilities.
But he added that not all saliva tests were the same, with some better than others.
Data had been presented to the Government from one US company he didn't name.
"We would need to validate that in the New Zealand setting, just as we did with PCR testing at the outset. And that's what we want to do."