Coronavirus: Delta's R number close to seeing COVID outbreak 'spiralling out of control' - expert

Auckland's move from level 4 to level 3 has pushed the Delta outbreak's R number above one, meaning it's now probably spreading exponentially, an expert has said.

And he's warning a strict two-week level 4 lockdown might soon be needed to "prevent the outbreak from spiralling out of control".

The rapid plunge into level 4 after a single case was found in August undoubtedly prevented a much bigger outbreak, daily case numbers peaking in the 80s about 12 days into the lockdown before falling down to as low as eight by late September. By then Auckland had moved to level 3. 

But it takes about a week or so for the effects of a change in restrictions to start appearing in the statistics, because infected people generally don't have symptoms until five or six days later. Figures show the seven-day rolling average of daily case numbers in New Zealand was slowly trending downwards until a week after Auckland moved into level 3.

Since then, the seven-day average has doubled to 40. The three-day average has tripled, from 14 on September 28 - five days after Auckland went to level 3 - to 48. 

"It's clear now cases are trending upwards and it looks like the shift from level 4 to level 3 has contributed to that," said Michael Plank, disease modeller and researcher at Te Pūnaha Matatini and the University of Canterbury.

The Government on Monday said Auckland would spend another week at its current alert level 3, step 1 settings, which unlike other areas in level 3, allow for limited contact between households. Dr Plank said Cabinet had no choice. 

"Any significant easing of restrictions now could really accelerate that case growth - we could see spiral out of control and put large numbers of people in hospital." 

Without restrictions, Delta has an estimated R number of six - that means each infected person on average would infect six others. The number of infected quickly adds up - those six would infect 36 more, who'd pass the virus onto 216 others, and so on. It only takes a few more generations to see millions infected. On a graph, it looks like a ramp that gets steeper and steeper. 

Dr Plank said at present, modelling suggests the R number in our Delta outbreak is "around about 1.2 to 1.3". 

"Although that might sound like it's only slightly greater than one, because of the way that compounds over time it can still lead to a significant number of cases... The real problem is if that R number increases just a little bit more to 1.4 or 1.5, then cases could start increasing very rapidly." 

A little bit of maths can illustrate just what he means - think of it like compound interest.  Starting with a single case, an R number of 1.2 after 10 generations would result in 6 infections; after 20, 38 infections; and after 30, 237 infections.

If the R number was 1.5, that single case after 10 generations results in 57 infections - nearly 10 times more than at an R of 1.2; after 20, it's at 3325, and after 30 more than 191,000 - a more than 800-fold increase. 

Michael Plank.
Michael Plank. Photo credit: Newshub.

It's not known how many people were already infected with Delta before the level 4 restrictions were put in place. The restrictions are designed to push the R number below one - if each person on average infects fewer than one other person, the outbreak eventually dies out. It can be difficult to estimate the R number in outbreaks like New Zealand's however, when the daily case numbers are still low enough that they can fluctuate a lot from day-to-day. 

Whether Auckland's step 1 loosening of restrictions, allowing households to mingle outdoors and reopening daycare centres, will push that R number up remains to be seen.

"It could go one of two ways - we may be able to keep case numbers under control until we get enough people vaccinated... but the trouble is there's quite a narrow margin for error now," said Dr Plank. "If we start to see cases growing much more steeply  than they are at the moment, then our healthcare system will be under strain and it may be that we need to tighten restrictions or have something like a two-week strict lockdown to prevent the outbreak from spiralling out of control." 

With the Government signalling it's unlikely to do that, Dr Plank said tighter geographic border controls might be needed, with rapid tests and proof of vaccination required for anyone crossing.

"None of these things will keep COVID contained forever but they could delay it, and that could buy us some valuable time to get people vaccinated."