Covid 19 coronavirus: Why longer lockdown made beating outbreak 10% more likely

An extended Auckland lockdown has bought public health officials four more days to dampen down the city's Covid-19 cluster. With Prime Minister Jacinda Ardern warning it could be weeks before we see the end of the virus' long tail, how much more likely has the new extension made beating this outbreak? About 10 per cent, models tell us. Science reporter Jamie Morton posed these questions to Te Punaha Matatini investigators, Professor Michael Plank and Dr Alex James of the University of Canterbury, Professor Shaun Hendy and Nic Steyn at the University of Auckland, and Dr Rachelle Binny and Dr Audrey Lustig at Manaaki Whenua-Landcare Research.

From what we know from modelling, what impact can we expect this to have on transmission of the virus?

The four-day extension to Auckland's alert level 3 is welcome news.

The early signs are that the reproduction number - or the average number of people infected by a single Covid-19 case - at alert level 3 is less than one. That means that new cases will decrease over time.

Alert2

But there are almost certainly still active cases in the community, which could spark new outbreaks if we relax too soon.

Extending the current restrictions reduces the risk we will need to go back to higher alert levels in the future and gives us more time to contain the existing cluster.

Contact tracing has so far proved very effective and testing rates last week were phenomenal, but level 3 has also played a big role in reducing case numbers.

The model suggests that adding an extra four days at level 3 increases the chances of eliminating this outbreak by about 10 per cent.

That being said, because we have a much greater understanding of the specific details of this cluster than those we dealt with back in March and April.

Alert1

This means that public health officials will be more confident in exercising their judgment in relaxing controls.

Assuming they don't discover other clusters or a large side-branch later in the week, their confidence should increase as our understanding of this cluster deepens.

How might we expect this cluster to wind down under these settings, as opposed to those clusters that were extinguished under New Zealand's longer-lasting lockdown?

This has proved to be a large and complex cluster with transmission taking place in a range of settings including multiple workplaces, shops, public transport, and churches, as well as within families.

The virus is also thought to spread more effectively in winter conditions.

It has affected Auckland's Pasifika population, who are more likely to suffer worse outcomes from the disease.

So some of the risks that the Government is managing with this cluster are higher than those from the first outbreak.

It is also unlikely we will have eliminated the cluster completely by the start of next week.

This means we may need to find a way to keep the reproduction number under one at alert level 2.

The restriction on gatherings of more than 10 people and mandatory mask use on public transport will help do that, but it will also require the public to take precautions individually.

What are the key risk factors for this cluster to flare up again?

The risk is that, after Auckland moves back to level 2, increased activity will spark another outbreak.

Level 2 will allow Aucklanders freedom to go to work, school, and other activities.

But this does not mean we can relax and go back to life as normal.

People should continue to avoid high-risk settings as much as possible.

This includes the "three C's": crowded places, closed spaces and close contact.

If you feel ill, don't go to work, and if you have any of the symptoms of Covid-19, seek a test.

Make sure you wear a mask when on public transport, as will be compulsory from Monday, and consider doing so in any other situation where you are indoors in close contact with other people for an extended period.

Download the app, enter your contact details, and make sure you scan those barcodes.

If you can't or don't want to use the app, keep a diary of where you have been and how you travelled there.

Can you give me an insight into the modelling itself: what data inputs are you using to calculate these scenarios, and how robust are they?

We are tracking the average number of people that someone with Covid-19 passes it on to, or reproduction number.

The data is much better on this for the current outbreak than in March and April.

We now have fast contact tracing data and in some cases genomic sequencing to link cases.

This shows that before the move to alert level 3, people were infecting between two and three other people on average.

Te Punaha Matatini director and University of Auckland physicist Professor Shaun Hendy. Photo / NZ Herald

This was higher than the outbreak in March, which means the cluster could have grown extremely quickly if we had not acted as fast as we did.

What happens once we move to alert level 2 really depends on people's behaviour.

We don't have great insight into how well level 2 will work because when we were in level 2 in May there were very few cases, which means there is not much data to go on.

Given the new regimes that have been put in place at our borders as a result of this outbreak, does the modelling indicate that level 1 will come with a lower risk profile than before?

Regular testing of staff working at the border or at managed isolation and quarantine facilities should greatly reduce the chances of an infected worker spreading the disease into the community.

However, given that we have seen the consequences of spread at level 1 we should try to stay alert once things get back to normal.

We need to assume the disease is out there, so that we are getting tested if we have symptoms, and using the app when we are out and about.

Is there any modelling to suggest what difference NZ's new policy around masks will make?

There is modelling from overseas that suggests mask use can be very effective if it is taken up at high levels, or more than 50 per cent.

The current measures are not extensive, but they do target a high risk environment where we know transmission has taken place in the current cluster.

This should make a difference to the effective reproductive number.

Essentially, what are the take-home messages you think are important for Kiwis to remember here?

We are on the right path but we still have a way to go.

What happens next depends on all of us - we need to stick to the alert level restrictions and not take unnecessary risks.

There is still a chance of the outbreak spreading outside Auckland, and this risk will rise with increased travel between regions.

So all New Zealanders need to be cautious, no matter where you live.