Contact tracing can cut the spread of Covid-19 by as much as 60 per cent - but the most critical factor in curbing an outbreak remains how well and quickly people isolate themselves.
That's according to new modelling that comes at a time experts warn New Zealand is just as vulnerable as ever to another big flare-up.
In their study, University of Canterbury mathematician Associate Professor Alex James and her colleagues at Te Punaha Matatini developed a smart model that quantified, for the first time, just how much of a difference contact tracing can make against an outbreak.
That included modelling how many contacts could be tracked down, the length of time it took to reach them and tell them self-isolate, and how effective that self-isolation ultimately proved.
While data supplied by New Zealand's contact tracing system helped in their estimates, the researchers were still able to test several other values to find out which variables were the most important.
Their analysis suggested that, amid bringing the country's national outbreak under control during April last year, the test-and-trace system cut the rate of spread by between 35 and 45 per cent.
And because the system had improved since then, they found contact tracing brought down transmission by more than that during the response to the Auckland August cluster.
In all, the modelling indicated that a high-quality, rapid contact tracing system, combined with strong support for people in quarantine or isolation, could slash the rate of spread by up to 60 per cent.
But if cases weren't isolated and contacts weren't quarantined so smoothly, or some contacts weren't traced, or traced more slowly, that impact dropped to around 40 per cent.
Interestingly, they also found that the best performance measure of a contact tracing system wasn't the proportion of cases tracked down before they became symptomatic, but the time between quarantining an infected person and quarantining their contacts.
If at least 80 per cent of cases were quarantined or isolated within four days of quarantining or isolating the source or "index" case, for instance, then the rate of spread could be brought down by at least 40 per cent.
The researchers noted how epidemiologist Dr Ayesha Verrall singled that out as a key criteria in her urgent and influential audit of New Zealand's system last year, before later becoming Associate Minister of Health.
"We have shown that if the effectiveness of quarantine and isolation can be guaranteed, this criterion can be useful to benchmark a system," the researchers said.
They also reported the speed and capacity of contact tracing systems was likely to be more cost-effective than enforcing social distancing measures, even though these might also be needed.
"The crucial importance of effective quarantine and isolation makes it essential that there is universal provision of social security such as paid leave entitlements for pre-symptomatic or asymptomatic individuals in quarantine, and adequate job security and unemployment benefits," they concluded.
"Likewise, the need to rapidly trace the majority of contacts mean that investment in skilled professionals and workers trained in public health work is essential.
"Experience from the New Zealand contact tracing effort shows that the development of trusted relationships by public health officials is critical to an effective system."
Study co-author Professor Michael Plank added that, with the national vaccine roll-out still in its early stages and highly infectious variants dominant globally, New Zealand remained as vulnerable to Covid-19 as ever.
"So it's really important to keep going with using the app and for anyone who is even slightly sick to stay at home and get tested."
A Ministry of Health spokesperson said New Zealand had been using the performance indicators suggested in the study since August last year.
The study comes as experts have been calling for improvements to the country's system since a review of the August outbreak found the number of cases which could be traced fell well short of the promised 1000 per day.
Before the outbreak hit, it was believed contacts of 1000 cases could be traced a day, with Auckland Regional Public Health ARPH expected to trace 120 of that number.
But the cluster was far more complex than the first outbreak in April because it largely affected the Pacific community who closely interacted with their community.
While the two experts who led a review into the response - Sir Brian Roche and Professor Philip Hill – found the testing and tracing system ultimately delivered a timely and informed effort, they nonetheless suggested its capacity and financial stability needed to be "urgently addressed" within two to three years.
Since then, however, the ministry spokesperson said the system's capacity had been "significantly increased".
During the January and February outbreaks, the ministry's National Investigation and Tracing Centre (NITC) and public health units were able to manage cases and contacts without drawing on the wider "surge" workforce, which numbered several thousand staff.
Along with having improved technology and operating procedures, the NITC was also now able to track more than 40,000 travellers, as part of follow up from quarantine-free travel zones.