30 Jun 2021

Covid-19 vaccination alone won't achieve herd immunity - study

From Checkpoint, 5:38 pm on 30 June 2021

A new study from Te Puna Matatini suggests it is virtually impossible for New Zealand to achieve herd immunity only with vaccinations, against the Covid-19 Delta variant.

In a worst-case scenario, modelling shows thousands could die if we do not maintain safeguards like masks and a closed border with isolation measures, while bumping up immunisation numbers.

"For the Delta variant, that's probably somewhere in the 90 percent mark, maybe up towards 100 percent," Covid-19 modelling expert and the director of the research centre Professor Shaun Hendy told Checkpoint.

"To combat the Delta variant, which is the variant that we're seeing emerge as the dominant variant around the world, we're really going to need very high coverage."

Professor Hendy said that figure covers the whole population, children included.

"It's not something that we can just achieve by vaccinating the adult population. The good news is we can use a combination of measures, so we don't need to just entirely rely on the vaccine.

"We could continue to use masks, we can continue to use our QR codes to scan in, and that could actually put us in that population immunity zone as well, provided we're prepared to mix in these other measures with the vaccine.

"This is going to be a global problem with the Delta variant. We're saying Delta cases pick up in the UK which is one of the countries that has, at the moment, one of the highest vaccination rates in the world.

"The vaccine's not going to be a silver bullet that takes us back in time to 2019."

The study modelled what could happen if New Zealand borders opened with relatively high vaccination rates but no other measures to curb infections.

"There could still be a significant outbreak," Professor Hendy said. 

"Up until we had that population immunity threshold or at least get very close, if we completely open the border that will open us up to the possibility of a really significant outbreak.

"I don't think anyone would consider that. We would want to use other measures to control it, but it would be significant. It is important to remember the vaccine has a couple of effects. It's known to be very effective at reducing severe disease.

"It doesn't necessarily prevent everybody who's had from getting the disease, although it will prevent some people entirely from getting the disease. So that still means there's the possibility for spread, and it does mean there'll be a small number of people therefore, who potentially have severe health consequences."

Professor Hendy said in a worst-case scenario, if New Zealand's borders opened without reaching a level of population immunity, and without other health measures, then there would be the possibility of thousands of deaths. 

"It wouldn't be as bad as if we hadn't vaccinated. Vaccinations certainly will save lives, but we'd still have a significant health impact here in New Zealand.

"The vaccine is very good, in fact it's one of the best vaccines that we've developed globally. But there will still be some people who are vaccinated [and] the vaccine doesn't produce a sufficiently strong immune response.

"They would still remain vulnerable. Vaccines always have that mixed effect, they don't work on everyone. And that's why the population immunity threshold is usually so important. It's because once we hit that, actually we're protecting everybody else at that point. So the fact that it doesn't work on some people, or some people don't choose to use it, has less of an impact."

The data modelling is important to help with considering the consequences of policy choices, Professor Hendy said. 

"The government is going to be facing pressure to open the borders as we go through the vaccination rollout programme. We can see that playing out in other countries who are further ahead of us in the vaccine rollout.

"And the modelling figures really just reinforce that actually some of those decisions that maybe looked plausible would have severe repercussions."

He said there was no particular ideal level of vaccination for borders to open. 

"Those choices become potentially more manageable the closer you get to the population immunity threshold.

"There's no magic number that we can tell the government 'here is the point where you can do this' but it does suggest those choices will become easier to make the closer we get to that threshold."

He said once the vaccination rate got above 70 percent, the possibility of relaxing MIQ restrictions to low-risk countries became more viable provided mask wearing and contact tracing efforts were maintained. 

"Then something like normal life might start to come back. But globally this is going to continue to be a problem for some time."

Given Covid-19 will continue to be a health issue, there is the question of at what point does it become something that is generally accepted, like the road toll. 

"These are ethical questions that I think we do need to have a conversation about as a country," Professor Hendy said. 

"Speaking for myself personally, I actually think we've developed some fantastic new tools now for fighting infectious disease. Previously we've been tolerating 500 deaths a year from seasonal influenza, I think we could now look at that with some of the tools we've developed and say 'well, actually, that's no longer tolerable'.

"I think, personally, these infectious disease impacts tend to hit people from lower socio-economic backgrounds. They impact more on Māori and Pacific Islanders because of racism in the healthcare system.

"So it is an important ethical question as to whether we're prepared to allow those impacts to fall on certain parts of our community, while the rest of us perhaps, go back to enjoying a normal life."