What is population immunity, and will we reach it?

By Stuff reporter

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


The holy grail of vaccination programmes is to stop the disease in its tracks.

Vaccines do that by putting up roadblocks to infection. When the virus runs into a vaccinated person who is protected against infection, it has to back up and look for a different route. 

The more people who are vaccinated, the more roadblocks, and the harder it is for the virus to find a new host.

As vaccinations increase, there’s a magic number, beyond which there are too many roadblocks for the virus to find a way through.

That tipping point is called herd immunity, or population immunity. Once it’s passed, any outbreaks naturally die out. 

Where that tipping point lies depends on how infectious the virus is, and how effective the vaccine is.

For Covid-19, experts initially estimated about 60-70 per cent of a population would need to be protected against infection to reach population immunity. But then more problematic versions of the virus emerged. 

The Delta and Alpha variants are more infectious, while the Beta mutation appears less responsive to the vaccine.

The end result is the same for both scenarios - more people need to get the jab to halt the spread. It’s like the new variants have better navigation systems, so they can find a new target before running out of fuel.

Now, modellers think 80-90 per cent of New Zealanders will need to be vaccinated to reach population immunity. That could rise to 97 per cent if the Delta variant continues to dominate.

If the one in five Kiwis who aren’t sold on the vaccine don’t get it, we won’t get there.

It would also require some children to be vaccinated, as under-16s make up 20 per cent of the population.

While young people are less likely to get seriously ill from Covid-19, severe disease can occur at any age. Children also hang out with more people, so can spread the virus more quickly.

At present, New Zealand’s vaccine regulator Medsafe has only approved the Pfizer vaccine for those aged 12 and over.

Even if we reach the 80-90 per cent vaccination threshold, Covid-19 is unlikely to be eradicated forever. The virus could still get in from overseas, but it won’t be able to take hold. 

So where does that leave us? 

If New Zealand does not reach population immunity, the majority of fully vaccinated Kiwis will still be protected. But those choosing not to be vaccinated will not.

People with compromised immune systems who might not get full protection from the vaccine will also miss out on the extra security of population immunity. They are often also more vulnerable to the disease.

But every jab still counts. Because every vaccination puts up a roadblock to infection, reduces the chance of deadly disease and makes it easier to control any outbreaks.

Reporting disclosure statement: University of Canterbury mathematics professor Michael Plank provided expert advice for this article. It was also reviewed by The Whole Truth: Covid-19 Vaccination expert panel member Dr Maia Brewerton.