Covid 19 Delta outbreak: Contact tracers to 'prioritise' new cases based on risk

Public health experts say contract tracers will have to prioritise which new Covid cases they fully investigate after yesterday's record 160 cases reached the threshold at which the system becomes "overwhelmed".

Yesterday's 160 community cases - 151 in Auckland - was the highest registered in a single day since the beginning of the pandemic.

On October 15, Ministry of Health (MoH) director of public health Dr Caroline McElnay said contact tracing services would be overwhelmed at around 160 or 170 daily cases.

University of Auckland data expert Dr Andrew Chen has been analysing the Government's Covid-19 tracer app and the public health contact tracing protocols since the beginning of the pandemic.

Chen says the 160 threshold met yesterday will mean contact tracers will no longer be interviewing every new case multiple times to get precise details of exposure sites.

"It's going to be quite challenging and they're going to have to prioritise," Chen said.

"So they may not be able to trace every single possible case that needs to be traced as we may have expected in the past.

"They'll still try to do the first interviews with the [new daily] cases but from there they may have to identify high priority locations of interest, for example, and places where interactions may be more fleeting, or where the risk is lower.

"It won't be zero risk, but if it's lower risk then perhaps it's less important."

Auckland University researcher Andrew Chen. Photo/Supplied.

For new Covid cases who visited sites of lower risk for further spread, contact tracers may not revisit them with further interviews.

Up until now, people who test positive for Covid will be notified and a contact tracer will call to make sure they are safe and well and have a suitable place to isolate.

They will then call back and interview the case to determine where they have been during their infectious period.

Once the contact tracers have an initial list of locations of interest they will analyse the risk of those sites.

Subsequent interviews with the case will get specific details on the length of their exposure to different sites.

"Previously they might have said 'Okay, well we need to know how long did you actually spend at this place, was it 5 minutes or 20 minutes?' Those sort of details they might [now] only ask for higher risk locations," Chen said.

In mid-October, director of health Dr Ashley Bloomfield said eventually they plan to just send message alerts to cases when the system becomes too overwhelmed with new daily cases for even first interviews.

"Rather than them all being personally contacted we will be sending alerts out through the app," Bloomfield said.

"Of course our great hope is that as many of those contacts as possible are fully vaccinated, then their period of isolation is likely to be shorter and of course they will just have mild, if at all, any symptoms and be able to be looked after at home."

Director general of health Dr Ashley Bloomfield says eventually contact tracers may not ring new Covid cases once high daily infection numbers become the norm. Photo / Mark Mitchell

Bloomfield said businesses or organisations may follow up with new cases and make sure they are staying home and isolating.

University of Auckland microbiologist Associate Professor Siouxsie Wiles said the breadth of yesterday's cases across Auckland, Waikato, Northland and Canterbury will pose an increased challenge for contact tracers.

"The reality is that more daily cases mean that systems are being put under increasing pressure, and so we will likely see lags in data getting into the system," Wiles said.

"The range of cases reported [yesterday] just show the different scenarios encountered by the public health teams doing contact tracing and case investigations. For example, one is a person in Christchurch who has recently been through MIQ. Genomic sequencing will help to understand if they were infected overseas and have had an unusually long incubation period."

Chen said despite the dramatic increase in case numbers over the past week, New Zealanders can still help the MoH contact tracers.

"There's also a lot that we could all still do as individuals by keeping good records of where we've been and where," Chen said.

"If you can do it through NZ Covid tracer, if you do become a case and you need to provide your data to contact tracers, it's a lot faster."

The NZ Covid-19 contact tracer app. Photo / George Novak

If a new case's Covid tracer app has been thoroughly updated with location visits then a code can be sent to the person to enter it into the app and give covid tracers a direct head start, rather than the person trying to remember where they may have been and verbally giving that information to a contact tracer.

"So that can save time as well," Chen said.

"Time is where the capacity limits are. If we can reduce the time spent per case then that will make a significant difference."

However, Chen said he did not believe contact tracing will be abandoned when New Zealand reaches a 90 per cent double vaccinated rate across all its DHBs.

At that point lockdown's will be abandoned in favour of a traffic light system, but daily case numbers are expected to remain at a level New Zealand has not experienced before.

"This is my guess, but we'd have to be talking about many thousands of cases a day before we were to abandon contact tracing and say it's not useful and to redeploy those resources elsewhere," Chen said.

"Even if it's having some impact that's better than no impact. Contact tracing is still being run in many jurisdictions where there are hundreds or thousands of cases a day. It is one of our last lines of defence because the virus is already in the community at that point. You can't control that risk at the border any more.

"I think we will still need contact tracing, but the way that it works is going to change. To my understanding I think the protocols are in the process of being rewritten for a little while now so we'll see what that means in terms of actual impact."

Chen said unfortunately New Zealand's contact tracing capacity can not simply be solved by advertising for more roles, because those already employed in the roles would need to take time off to train them.

Ideally, contact tracers would also have some medical background and a capacity to calmly deal with potentially distressed members of the public who have just realised they have Covid.