Why Covid-19 vaccination targets are complicated by how you count people

Roxie Mohebbi leads a discussion about the Covid-19 vaccine with immunologist Dr Maia Brewerton and general practitioner Dr Api Talemaitoga as part of Stuff's Whole Truth project.

Setting Covid-19 vaccination targets is not just an exercise in public health policy-making.

The way you count the people eligible for vaccination complicates things.

Everyone aged over 12 is eligible for the Covid-19 vaccination. On Friday, Prime Minister Jacinda Ardern announced that when 90 per cent of the eligible population in each DHB region is vaccinated, the country will shift to a new ‘traffic lights’ system for managing Covid-19.

Progress towards these targets will be calculated based on the ‘eligible population’ that’s estimated for each region by the Health Survey User (HSU) database. It has also used this database to calculate the rate of vaccination for the eligible population as a whole.

The HSU, though, has its flaws. Until now, Stuff has chosen to calculate the overall vaccination rate for the eligible population based on data provided by Stats NZ. We have done this because we believe it is a more accurate reflection of the actual number of people eligible for vaccination. It’s why our figures for the rates of vaccination have been slightly lower than those published by the Ministry of Health.

A 90 per cent vaccination target for all DHBs has been set.
Kathryn George/Stuff
A 90 per cent vaccination target for all DHBs has been set.

What’s wrong with the HSU?

The HSU database counts the number of people who interacted with the health system in the previous year, and provided detail such as residence location, age, gender, and ethnicity.

Anyone who hasn’t interacted with the health system in the last year, isn’t counted. Marginalised groups can be underrepresented, which can cause overestimation of their vaccine coverage. On the flipside, some people who have interacted with the health system but are outside of New Zealand, are counted.

The HSU puts the eligible population at about 4.2m people.

The eligible population according to Stats NZ, on the other hand, is based on a combination of the 2018 Census, and births, deaths and net migration since. It gives an eligible population of 4.35m people.

This means the HSU – which is what official vaccination targets, reporting and policy are all based on – likely underestimates the 12+ population by about 3.5 per cent.

So why are we using a less accurate data set?

Reporting on vaccination rates is done in many different ways. There are figures for hundreds of different-sized geographic locations, different ethnicities, and different age groups.

Unfortunately, there is no Stats NZ population estimate of the 12+ eligible population that aligns with all of these different ways of calculating vaccination rates. There is, however, from the HSU database.

When Ardern announced on Friday the introduction of a new system for managing Covid-19 would be tied to the achievement of vaccination targets across every DHB, these became by far the most significant figures to track. Even more important than the overall vaccination rate for New Zealand.

It's very difficult to track DHB-level rates with anything other than the HSU data for the eligible population. And even if a workable alternative was available, the targets set by the government reflect the HSU data, so are ultimately what decide when life for New Zealanders changes.

To be clear, though, when the government reports that 90 per cent of a DHB’s population is fully vaccinated, this is almost certainly an overstatement of the reality.

At Stuff, we continue to believe the Stats NZ figures represent the closest to reality. We’ll still try to show figures using this data set where possible.

But the announcement of DHB-level vaccination targets, combined with the dearth of available population data, has forced our hand. For the sake of coherence across our stories and graphics, we’ll now use the HSU database as our default measure in the calculation of all vaccination rates.