What would a Delta outbreak look like with higher vaccination rates?

By Stuff reporter

The Whole Truth: Covid-19 Vaccine | By Stuff reporter Nikki Macdonald.


We now know how quickly Delta can take hold when only 20 per cent of the population is fully vaccinated. And it’s bad news, sending the entire country into lockdown. 

But modelling shows higher vaccination levels would slash the numbers of cases, hospitalisations and deaths in any future outbreak. And the impact increases rapidly as more of the population gets jabbed. 

Unvaccinated people who catch Delta are about twice as likely to be hospitalised as those infected with the original Covid strain. However, the variant has not significantly dented the Pfizer vaccine’s effectiveness against hospitalisation and death from Covid-19. That means more than nine out of 10 fully vaccinated people will be protected against getting seriously ill or dying. 

So what would that look like in a Delta outbreak? 

Canterbury University mathematics professor Michael Plank and research assistant Nicholas Steyn modelled what an uncontrolled Delta outbreak would look like in the whole population, based on different levels of vaccination in Kiwis aged 15 and over. 

By day 10, with a completely unvaccinated population, there would be 14 Covid-struck New Zealanders needing hospital treatment. With 40 per cent of those 15 and older fully vaccinated, that would fall to 4.5, and at 80 per cent there would be just one hospitalisation. 

By day 20, the impact is much more pronounced. An unvaccinated population would produce 734 hospitalisations, compared to just four when 80 per cent were fully vaccinated. 

By day 28, in an unjabbed population, 17,355 Kiwis would be in hospital - a number that would break the system. But if 80 per cent of those over 15 had had two jabs, that would fall to a manageable 14.

The same goes for deaths. By day 28, a Delta outbreak in an unvaccinated population would lead to 438 funerals or tangi. At 40 per cent vaccination, there would still be 22 deaths. With 80 per cent of Kiwis aged 15 and over vaccinated, however, the average number of deaths would be under one. 

Case numbers are a bit more complicated. 

Delta’s arrival killed off any hope that vaccination alone could stamp out Covid-19. That’s partly because the variant is twice as infectious as the original strain. But it’s also because Delta has eroded the Pfizer vaccine’s effectiveness against infection. 

While initial studies found the vaccine was more than 90 per cent effective at stopping people catching the Covid-causing virus SARS-CoV-2, research estimates for Delta vary wildly, with some as low as 40 per cent. Plank and Steyn’s modelling is based on a 70 per cent reduction in infection. But the impact of preventing even one infection is exponential over time, as every Delta case infects an average of six other people.

So even at that reduced effectiveness, the impact on case numbers would be huge, the modelling shows. At day 10, there would be 105 new cases a day in a completely unvaccinated population. With 40 per cent of Kiwis 15 and over fully vaccinated, that would drop to 25 and at 80 per cent vaccination, there would be just five new cases a day. 

By day 28, with no vaccination and no lockdown, there would be a staggering 128,990 new cases a day. At 40 per cent vaccination, the 1pm press conference would be reporting 3780 new cases. At 80 per cent fully vaccinated, there would be 65 new cases a day. 

These numbers are averages not absolutes. As we’ve seen in the latest Delta outbreak, one super-spreading event can have a huge impact on case numbers. 

But the verdict is clear - while vaccination alone cannot eliminate Covid-19, higher vaccination levels can prevent thousands of cases, hospitalisations and deaths.

Reporting disclosure statement: Canterbury University mathematics professor Michael Plank provided expert advice for this post.