As Kiwis count down the hours to the slightly looser life of alert level 2, scientists say New Zealand still faces a big unknown: just how much is the virus still circulating?

Today, director of general of health Dr Ashley Bloomfield announced another day of no new cases, keeping New Zealand's tally at 1497, of which 94 per cent had recovered.

There are now only 74 active cases – and just two people in hospital – while nearly 6000 tests were carried out yesterday.

Bloomfield said while the numbers showed again "we're on the right path", he stressed New Zealand couldn't afford to give its gains away – a point also emphasised by Prime Minister Jacinda Ardern.


All the while, scientists say it's unclear how transmission of the virus has changed as New Zealand dropped down from level 4 to level 3 on April 27 – and how that might change as restrictions again ease after tomorrow when level 2 begins.

Associate Professor Siouxsie Wiles said she would have preferred level 3 continued longer to ensure the lockdown gains weren't squandered.

But the University of Auckland microbiologist was nonetheless grateful that level 2 would involve a staggered approach where sizes of gatherings would be initially limited.

Otago University epidemiologist Dr Amanda Kvalsvig explained the number of contacts people had was one of the main drivers of Covid-19 transmission.

A key baseline was the virus' basic reproductive number – or "R0" – or the average number of people directly infected by a single infectious individual.

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Modelling has suggested that, if that number could be kept below one, the virus could be eliminated.

Kvalsvig noted that level 3 saw a flurry of Kiwis expanding their bubbles and moving back into public spaces.

"During level 2, the number of contacts will increase enormously as people return to work, shops, and public transport," she said.


"Covid-19 is highly infectious, so with each step down in level any virus still hanging around will have many more opportunities to spread."

It was tough to assess how widely the virus was still circulating, given the fact people could be infectious but show no symptoms – and we still didn't have an accurate handle of the proportion of hidden cases.

That was partly due to what scientists call the "lag" or "long tail" of Covid-19.

While the virus' average incubation period – or the time from infectious contact to symptoms appearing - was estimated to be five days, the range was as wide as between two and 14 days.

That meant a person becoming infected today may only show symptoms in two weeks' time.

However, it was common for people to be infectious before they developed symptoms, so on Day 12, that person could unknowingly pass the infection on to someone else who might only become symptomatic two weeks after that.


"It can also take some time to discover chains of infection between people who don't show strong symptoms," said Professor Shaun Hendy, director of Te Pūnaha Matatini, which has been modelling Covid-19 in New Zealand.

"Until the disease gets passed to someone who develops symptoms and goes to get a test, these chains of transmission can persist for some time.

"Hopefully, these invisible chains have died out during level 4, but it may be that they re-emerge once we leave our bubbles and expand our contact network."

"Until the disease gets passed to someone who develops symptoms and goes to get a test, these chains of transmission can persist for some time," Professor Shaun Hendy says. Photo / Greg Bowker

Was it possible that rates might have quietly picked up again after the switch to level 3?
Kvalsvig said that, because of so much variability in transmission cycles, it was unlikely there'd be an overnight step-up in cases as the time lag kicked in.

"But a slow build-up of cases over the next two weeks would be worrying because it would suggest that a new epidemic curve is developing as a result of the change in physical distancing."

Modelling by Te Pūnaha Matatini suggested that if level 3 had been ineffective, such an increase in cases would emerge over coming weeks.


It was also possible that the small length of time New Zealand spent in level 3 - and the relatively small numbers of cases the country has recorded - may mean scientists never get a full picture of how well level 3 worked compared to level 4.

What was important, Kvalsvig said, was that measures to control Covid-19 stayed in place.

Under level 4, physical distancing was applying most of that control – but now it would shift to other interventions.

"That includes strict quarantine at the borders and active case finding, including looking for asymptomatic cases in high-risk populations such as care homes," she said.

"It would also be good to see further testing of workers who are in contact with large numbers of people, whether that's through paid work or volunteering. The final barrier is to stop virus from being passed along.

"That means meticulous attention to handwashing and cough hygiene. Together with colleagues at the University of Otago I support the use of face coverings on public transport and other crowded, closed settings to reduce the risk of virus particles being dispersed into the air by coughing, laughing, or just breathing."


Hendy also singled out the vital role of contact tracing – an area which the Government is beefing up after a review by Otago epidemiologist Dr Ayesha Verrall set out various shortcomings.

"While contact tracing apps can make a contribution, it is ultimately our human contact tracers who need to really be on their game," Hendy said.

"We can help them by keeping good records of the places we have been and the people we have met.

"Keep a diary, ask for peoples' cellphone numbers where appropriate, and avoid large gatherings that will make contact tracing very difficult.

"With good contact tracing and moderate physical distancing, Te Pūnaha Matatini's model suggests that we can keep R0 below 1 while case numbers remain low."

New modelling is expected to be released later this week.

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