8 Sep 2021

Elimination possible in coming weeks - Covid-19 modeller Shaun Hendy

9:27 pm on 8 September 2021

Te Pūnaha Matatini Covid-19 data modelling expert Shaun Hendy says alert level 4 in Auckland is working nearly as well against Delta as it did last year.

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Covid-19 modeller Shaun Hendy Photo: Shaun Hendy

He was explaining some of the different models that were used to estimate things like population immunity thresholds and virus spread to the Health Select Committee this morning, and said the estimated effective reproduction, or R, number for the current outbreak was now about 0.4.

"That's our median estimate at the moment, obviously there's a range of uncertainty about that, 0.4 is actually extremely good. The March-April outbreak was about 0.35 so alert level 4 in Auckland is working almost as effectively as it did last year."

Prof Hendy said getting to a stage of elimination - where the virus was no longer circulating in the community - would soon be possible if restrictions in Auckland continued.

"We're still forecasting around about 1000 cases in total, and we're starting to see the chance that we eliminate the virus in the coming weeks, assuming that we stay at alert level 4.

"You may remember me in the media saying I'm a bit anxious about level 4 and we might need to strengthen it because of Delta ... what we've learnt I think in this outbreak is actually our alert level 4 settings are effective and although we're sitting at the outer edges of this we are seeing elimination."

He said it was hard to estimate how New Zealand would have fared without the lockdown, but it seemed likely healthcare would have been at absolutely capacity by this week.

Modelling estimated the time to elimination could be roughly halved by increasing vaccination levels from 30 to 50 percent of the population.

"As you push up vaccination rates the time to elimination starts to decrease."

He said it was important not to consider current vaccination coverage independently of the effectiveness of the current lockdown on the spread of the virus.

"When we've observed how well alert level 4 has worked, we've done so in the presence of the vaccine coverage that we've had, so those two things I wouldn't want to look at them independently."

"However, in general, the higher our vaccination coverage then the more effective our alert level system becomes."

It was nearly impossible to achieve herd immunity solely through vaccinations alone, however.

"In theory it's possible if we hit very very high rates of vaccination in some optimistic scenarios but if you switch to the pessimistic side of the evidence and the data then it says that even at 100 percent [vaccinated] you'd still have substantial outbreaks because of the vaccine effectiveness.

"There is still considerable uncertainty around that and vaccine development is ongoing, but I think the weight of evidence at the moment suggests that it's not possible with our current suite of vaccines that are available ... to achieve herd immunity."

Including other measures as well however could achieve it.

"You do need some other controls in place, and with those other controls you can achieve herd immunity - so with widespread mask use, with some forms of social distancing, with rapid testing - you could talk about herd immunity but not solely from the vaccine.

"Then I think it's also important to point out that some of those things require compliance as well."

He said Delta was very difficult to deal with - particularly for workplaces and education settings.

The understanding of the risks had improved a lot, but some changes could be made based on the science of the risk of aerosol transmission, compared to airborne and surface-based transmission, he said.

"I do have continuing concerns and I think we do need to continue to look at the science as well as looking at what's reasonably practicable to be done in indoor spaces.

"We haven't designed our indoor spaces to be ... respiratory virus resistant and there's a lot more we could do there. I do think we have to take better advantage of mask use - we tend to encourage mask use into the low-risk environments and we're not requiring mask use necessarily in those high-risk environments which are those workplaces and education settings.

"I would like to see more rapid testing, I mean I think that's a tool that we haven't taken advantage of and I think we're going to have to ... that could make a significant difference."

He said regardless, the more people were getting tested, the more confidence it would give authorities that the virus was being stamped out.

"It's hard for us to give a sort of a hard number. I guess I would like to see on the order of about 10,000 tests a day over the next couple of weeks."

Hendy said the rapid antigen tests could become extremely useful next year as the country began to move towards higher vaccination coverage and a strategy that may accept a greater level of risk of infected people coming into the country.

"The rapid antigen tests aren't as sensitive as the PCR tests ... they don't do as well picking up cases late in the piece but the fact is because you can get those turnaround times much more quickly they can be really useful."

He said the speed also meant that for example they may be more effective for pre-departure testing.

"Also because you can test more frequently you can more than make up for in some of the deficiencies in sensitivity by testing on a daily basis."

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