The Ministry of Health and Statistics New Zealand have different views on how many people are in the country, which means we may think we’re at 90 percent vaccinated but actually fall short of it, Marc Daalder reports

Analysis: More than 49,000 Asian men in their 30s in Auckland have had their first dose of the Covid vaccine, which is pretty impressive when you consider there are only 47,000 Asian men in their 30s in Auckland.

In fact, when you break the vaccination statistics down by ethnicity, age group, gender and region, nearly a third of all groups report a first dose rate above 100 percent. Even if you exclude those that consist of a just a few dozen people and count only those with a population of at least 1000, 64 groups have a vaccination rate above 100 percent.

Consider white Cantabrians aged between 80 and 84. There are about 10,670 of them, according to Ministry of Health data, but nearly 11,000 first doses have been distributed to that group.

On the one hand, these data glitches make for amusing anecdotes. But experts say they also pose a risk to our ability to loosen restrictions in an evidence-based manner. If the Government reopens because it thinks 90 percent of the population has been fully vaccinated and it turns out that only 87 percent have had two doses, the consequences could be severe.

That throws a wrench in the Government’s new traffic light system, which will only be introduced once 90 percent of the eligible population of each DHB is double dosed.

The core of the issue is that there are differing estimates for how many people actually live in New Zealand. Statistics New Zealand says that, as of the end of June, there are 5,122,600 people in the country. That’s based on an estimate of population growth since the 2018 Census.

Our World In Data, which provides a data explorer with statistics on Covid-19 and which is often used by the Prime Minister to compare New Zealand’s vaccination rollout to that of other countries, thinks we have just 4.8 million people.

The Ministry of Health provides yet another estimate, saying there are just a hair over 5 million people in the country. This estimate, which is used to inform official vaccination rates and Newsroom’s own vaccination charts page, is based on the Health Service User system, which records when people interact with the health system.

Brian Cox, an associate professor in the University of Otago’s Department of Preventative and Social Health and a vaccinator, says he regularly sees people coming in to be vaccinated whose data is out of date.

“A lot of people, probably about a quarter of people who come in for vaccination, the system has an old address. That’s because the address that’s recorded is their address when they last contacted the health service,” he said.

That means the Ministry of Health’s population data could be inaccurate across regions, as people have since moved. It also doesn’t count people who haven’t interacted with the health system, such as recent migrants.

Compared to the Stats NZ data, the Ministry of Health says there are 122,131 fewer people in the country.

That means that if the population over the age of 20 were to be 90 percent vaccinated according to the Ministry of Health, the actual rate might be just 86.9 percent.

“Vaccination rates are very critical to what we can expect as we release constraints on the population. From about 70 percent up to 90-plus percent, it’s not a linear relationship. It only takes a small change in those vaccination rates to make quite a big difference in the control of the epidemic,” Cox said.

“If you’re making decisions based on vaccination rates that are 3 or 4 percent out, that’s a real danger.”

Michael Plank, a mathematics professor at the University of Canterbury and a disease modeller with Te Pūnaha Matatini, said that just one or two percentage points might not make a huge difference in terms of the spread of Covid-19. But it would be worrying if particular communities or age groups were found to be significantly less vaccinated than suspected.

As it turns out, that may be the case. Based on the HSU data, 96.7 percent of people aged 80 and up had received their first dose of vaccine as of October 19. But if you use the Stats NZ figures, that falls to just 90.8 percent.

"There is a big difference between 90 and 96 percent vaccinated in those older age groups. You can turn that around and it's the difference between 4 percent and 10 percent that aren't vaccinated," Plank said.

That may not sound like such a significant gap, but here's another way to think about it: The Government thinks there are only 6130 unvaccinated, unprotected people over the age of 80, but there might be nearly three times as many - some 17,837.

So which one should we be using? There are arguments on both sides, but Plank said we should use the larger population estimate just to be safe.

"I think there's an argument for looking at the larger denominators from a precautionary point of view for opening up, because it does make a big difference in the number of people who are unprotected."

A spokesperson for Covid-19 Response Minister Chris Hipkins said the Government was still planning to use the HSU data to decide when to transfer to the new traffic light system.

"The HSU ensures the same source of demographic information is used in the numerator and the denominator. This combats numerator denominator bias. There are some drawbacks to this, but the HSU will provide the most accurate analysis of vaccine coverage by ethnicity and location, allowing more granular analysis," the spokesperson said.

"We don’t use Stats NZ data, as this is a projection from 2018 estimates and it is known that the older population had a low response rate in the 2018 Census. Whereas, the HSU population is generally believed to be more accurate in older age bands as the use of health services is generally more prevalent amongst these groups."

The Stats NZ and HSU figures also disagree on the populations of each DHB. Stats NZ estimates there are more people in every DHB area other than Tairāwhiti, compared to HSU.

That means the Government could think each DHB is at 75 percent vaccinated (which corresponds to about 90 percent of the eligible population being vaccinated) when the real rates are off by as much as 3.5 percent, as in the case of the Southern DHB area.

Marc Daalder is a senior political reporter based in Wellington who covers climate change, health, energy and violent extremism. Twitter/Bluesky: @marcdaalder

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