Auckland's raging epidemic has surprised experts and could flag a different progression for the Omicron outbreak than expected, Marc Daalder reports
A top disease modeller thinks the outbreak in Auckland may peak sooner than previously expected, as the city continues to smash daily case records. More than 11,000 new cases were reported across Auckland's three District Health Boards on Tuesday and it makes up nearly two thirds of each day's cases despite having just a third of the national population.
"The fact that we've had just such high case numbers coming through recently, and that they were preceded by this period that had some strangely low case numbers, suggests that actually there was more transmission going on, more infections going on than we were measuring," Covid-19 Modelling Aotearoa's Dion O'Neale told Newsroom.
"It would possibly make sense to shift forward that curve by maybe a week, at least for the case of Auckland. Auckland you could shift by a week or maybe even more forward - and it will peak quite soon. Some other parts of the country seem to have had quite a delayed spread. It's almost surprising that we didn't see spread happen sooner into other parts of the country or take off sooner."
This is one of the difficulties of tracking and projecting what University of Otago epidemiologist Michael Baker has called New Zealand's "multi-speed" outbreak. Auckland's outbreak is more much advanced than those in other parts of the country, making it harder to estimate when we might hit a national peak but also serving as a model for what the rest of the country might expect when Omicron takes off.
"It also gives the opportunity for the rest of the country to learn from the Auckland experience. So we're arranging seminars and webinars so that Auckland clinicians can update their colleagues around the country, in different specialties, around just what they've learnt and how they can prepare and also manage when they do get that wave," Director-General of Health Ashley Bloomfield told Newsroom on Tuesday.
Bloomfield conceded that, nationwide, we still have no real sense of how many Covid-19 infections are caught through testing and reported to the Ministry of Health. The daily figures could be half or a third of the real number, he said.
While O'Neale has called for a prevalence survey to test random New Zealanders in an effort to discover the true extent of the spread of the virus, Bloomfield said the ministry would only look into doing that after the peak of the outbreak.
In the interim, O'Neale said, we're left with no real early warning system to prevent a large crush of hospitalisations. By the time case numbers spike - they're now 10 times above the daily level from 10 days ago - it's too late to stop some of those cases from ending up in hospital. By the time the hospital system starts to overflow, it's too late to stop some of those patients from needing ICU care or dying. Besides that, the messy case data is hard to rely on as a proxy for the real trend or number of infections.
"The most reliable data we've got at the moment is hospital beds occupied. That's a really dangerous thing to be using as your most reliable indicator of infections because that's late along your disease progression pipeline," he said.
"If you wait until hospitalisations look bad before you act, the only things that come after hospitalisations are ICU admissions and deaths. Those are difficult to influence. You can't go back and do a public health measure to stop people from dying once they're already in hospital. It's too late."
Despite all this, O'Neale says international experience shows us that our outbreak could involve two patterns that might not be expected. The first is that we might not have one peak but rather two or more, as regional outbreaks like the one in Auckland wane before others truly pick up.
This happened in New South Wales, where Omicron started off in Sydney night clubs, peaked, and then spread to the rest of the state.
"With those big Australian cities, you see the multi-peak spread through the different suburbs and different communities," O'Neale said. "That's looking more and more like it could be on the cards as things accelerate lots in Auckland. They're quite far ahead of other regions. We might have a more bumpy peak than what we otherwise would have had."
Bloomfield told Newsroom this was a possibility as well.
The other pattern was that we might see a "multi-mode" outbreak which temporarily breaks the correlation between case and hospitalisation numbers, O'Neale said.
"Young people interact so much more and are more likely to be in work like hospitality. They get infected really quickly, lots of them are asymptomatic which means a little bit more spread amongst them. You go through them quite quickly, with a relatively low hospitalisation rate," he said.
"Then, over time, because you still have transmission in the community, you get into older demographics. With cases still trickling along, they're going to eventually get exposed. You'll see the hospitalisations stay higher for a longer period of time than your confirmed cases. Cases come down faster than hospitalisations and I think Australia's seen a really strong signal of that."
While New Zealand has seen a sharp rise in cases recently, O'Neale said this "multi-mode" factor meant the corresponding rise in hospitalisations might not be as sharp in the coming days.
"The rapid spread is happening through younger people. So that's going to slow the hospitalisation rise a little bit."
