Coronavirus Covid-19: How fast could virus spread if unleashed?

The chances of a fresh Covid-19 outbreak after the Government's border blunder is likely to be small - but experts warn the virus can spread fast once unleashed in the community.

The Government has come under mounting criticism over allowing two women out of isolation early to attend a funeral, before discovering they were infected with Covid-19, along with a series of other lapses that have come to light this week.

With a new and unrelated case reported today, the number of active cases in the country stands at three. But there's no suggestion of the Government moving New Zealand back to level 2 yet.

Officials have now traced and tested all but 27 of 364 potential contacts of the two women - although the bulk of these weren't "close contacts" but those who'd been on the same flight and staying in the same hotel.

How quickly could the virus spread if the pair had been able to pass it on?

In models, epidemiologists use a measure called a basic reproductive number – or an R0 value – to calculate how quickly a virus can spread from person to person.

Studies from overseas have suggested the R0 value for the SARS-CoV-2 virus that causes Covid-19 sat between two and three, meaning that one infected person could pass it on to two to three people, a higher rate than influenza.

Director General of Health Dr Ashley Bloomfield told media today he wasn't concerned about any hidden Covid-19 clusters in New Zealand. Photo / Mark Mitchell

This value however wasn't set, and could change depending on what prevention measures were put in place to stop it, and how little the virus was circulating in the community.

Before New Zealand went in lockdown, for instance, researchers estimated the R0 value was 1.8 – and had been quashed to just 0.35 as a result of it, leading to its effective elimination in the country.

Top infectious diseases modeller Professor Mick Roberts, of Massey University, said that, with New Zealand at level 1, the R0 value was now well above one.

"That means that any community transmission could result in the onset of a large epidemic, and a return to lockdown."

Another modeller, Professor Michael Plank of the University of Canterbury, said the fact that the two women travelled outside their quarantine facility was "significant", as there was a chance they might have come into contact with others.

"This reinforces the importance of our contact tracing system and, crucially, everyone keeping a record of where they've been and who they've been in contact with."

Otago University epidemiologist Dr Amanda Kvalsvig said New Zealand was now in a situation where hundreds of people had travelled for several hours in an enclosed space with someone who was infectious.

"There is an enormous amount of mopping up to do to make sure that all potential transmission chains have been extinguished," she said.

"It's also notable that one of the people who tested positive had no symptoms and might never have known they were infectious if the person with them hadn't become unwell."

Kvalsvig said a situation like this week's first two announced cases could easily trigger a major outbreak and take New Zealand back where it was in March.

"Inevitably, some travellers will have Covid-19 infection and inevitably, some of those will be making the long trip back because they need to be with family at a sad time. There will be more cases like this one in the future."

Vehicles queue at a Covid-19 testing centre near Destiny Church in Wiri, Auckland, today. Photo / Michael Craig

Associate Professor Patricia Priest, of Otago University's Department of Preventive and Social Medicine, didn't see R0 values as relevant to New Zealand, given it didn't appear to be spreading in the community.

"But you just need to look at what happens in places where there were no interventions early on in the pandemic," she said.

"I was watching cases in Italy at the time, and [daily case numbers] would be three, four, five, four, three, and then it was 20, 100, 500, to 2000… once it's going, it goes really fast."

Still, she saw the current risk to the community from the latest cases as low.

If they had been able to infect any others, contact tracers had the opportunity to find them and isolate them before a new chain of transmission began, she said.

The Ministry of Health was currently able to trace around 5000 contacts a day - well enough capacity to deal with a limited number of cases.

Given the two women would've been infectious during their stay in the quarantine facility, their closest contact would only have been with a "very, very small" number of people, Priest said.

As for any pre-existing threat in the community, director general of Health Dr Ashley Bloomfield said today he wasn't concerned about any hidden community clusters.

"The two cases that we confirmed on Tuesday and the one today have clearly come in across the border," he said.

"I can't think of any explanation of why we would have cases undetected out in the community when we haven't actually found any for many weeks now."

Asked whether New Zealand might have to move back to level 2 as a result of the new cases, Bloomfield said that cases at the border were to be expected, and more would come.

Bloomfield pointed out that Victoria in Australia, 11 of 18 new cases reported were picked up at the border.

"Remembering that the actual number of cases worldwide continues to grow, and also we've got New Zealanders still wanting to return from all parts of the globe… so we can expect to see more cases at the border."

Priest said officials would opt to move out of level 1 if there appeared to be new cases of transmission in the community, that couldn't be linked to a known one.

She noted that New Zealand hadn't actually recorded a first wave of infection, as it had managed to get on top of the virus first.

"As long as we continue to allow people to fly to New Zealand from countries where there is community spread of the virus, we will see the occasional case picked up in quarantine if our systems are working," she said.

"However, if they are detected in quarantine and managed appropriately from there, they won't cause spread into the community."