Covid-19: Why do fully vaxxed people show up in daily cases?

A Herald analysis has revealed the wide gulf in case rates between unvaccinated people and those who have received both jabs of the Pfizer shot.

Ministry of Health figures show that, since Auckland's Delta outbreak began in August, 3832 cases of Covid-19 have been recorded among eligible but unvaccinated people – 317 of whom have been hospitalised.

That compared with 1465 cases among people who were fully vaccinated – and just 36 hospitalisations.

When those numbers were set in context, and considered at per 100,000 people, the unvaccinated were shown to be at far greater risk.

The Herald looked at the rolling average of case rates over the past two weeks: among the eligible but unvaccinated, there were nearly 111 Covid-19 infections for every 100,000 people.

For every 100,000 people who had received the vaccine, meanwhile, there were just 8.3 cases.

Because the ministry doesn't publish age data alongside case rates by vaccination status, the Herald wasn't able to produce an accurate, age-adjusted analysis to show such differences in hospitalisation rates.

In the US, the hospitalisation rate among unvaccinated people aged over 18 was about eight times higher than those fully vaccinated.

In New Zealand, experts have repeatedly pointed to the simple reason why hundreds of fully vaccinated people have been infected: there are far more of them.

As at today, nearly in nine in 10 eligible Kiwis have received both doses – accounting for 3,719,864 vaccinations - while 94 per cent have received one.

Just 6.4 per cent of the eligible population – or 269,304 people – hadn't been vaccinated.

The vaccine has been shown to give 95 per cent protection against the symptoms of Covid-19 at clinical trials - and to also provide a strong buffer against infection.

A recent UK report found that contacts of those who were fully vaccinated with the Pfizer shot were 65 percent less likely to test positive with the virus, compared with the contacts of those who were unvaccinated.

Covid-19 modeller Professor Michael Plank said there were few surprises in the current case data.

"It's quite clear that vaccines work: the risk of catching Covid-19 is far less if you're vaccinated," he said.

"And, as the number of fully vaccinated people increases, you expect the proportion of cases that are fully vaccinated to increase in step with that."

With vaccination rates now so high, he said the proportion of vaccinated Covid-19 cases might now start tapering off.

Two factors could however shift that.

"Because we know that immunity does wane over time, if people don't get their booster shots, this could contribute to increasing numbers of fully vaccinated cases."

More than 132,000 booster doses have been administered since they became available on November 29.

"The other thing to consider is the populations that the virus is spreading in," Plank said.

"So far, we've still seen relatively localised spread of the virus, partly in populations where vaccination rates have been relatively low.

"As the virus becomes more evenly distributed across the country, that might have an increase in the proportion of cases that are fully vaccinated."

Plank emphasised the importance of age as a risk factor in severe illness with Covid-19 – something that could complicate analyses of hospitalisation data by vaccination status.

"Most of our unvaccinated people are kids, who are at low risk getting severely ill or needing to go to hospital," he said.

"That actually means that we get more hospital cases among the fully vaccinated than we might expect if we didn't account for that.

"When you compare data for vaccinated people and unvaccinated people, you are effectively comparing an older group to a younger group.

"That's not a fair comparison because the older group fundamentally has a higher level of risk.

"So if you don't control for age the data on hospitalisation or deaths can mislead you into thinking that the vaccine is less effective than it really is."

Meanwhile, the Herald's vaccination tracker now projects that all but three DHBs – Northland, Tairawhiti and Whanganui - will have a full vaccination rate of 90 per cent of eligible people or higher by January 1.

"It's great to see New Zealanders continue to roll up their sleeves to get vaccinated, even with over 88 per cent of Kiwis already fully vaccinated," the ministry's covid vaccine and immunisation programme group operations manager Astrid Koornneef said.

"Over the last two months, the number of fully vaccinated Māori has more than doubled."

Koornneef said it was also reasonable to surmise that the shift to the new traffic light system will have encouraged some who may have been hesitant to get vaccinated.

"In addition, it is highly likely that people, through their own careful decision-making, have decided to be vaccinated," she said.

"It is never too late to get your first dose if you haven't yet, and get your second dose even if your first was some time ago."

How long till each DHB reaches 90%?

As more people become vaccinated, there will be fewer and fewer people who are eligible for vaccinations. Reaching and convincing the remaining people will become harder, slowing down the rate of vaccinations the further we go. An estimate that doesn't account for this slow-down will constantly underestimate how far there is to go.

Using vaccination data from the Ministry of Health, we've created a simple statistical model for vaccination rates in the next four weeks.

This model is our best effort to estimate the vaccination trends, but it is only an estimate and should be used with caution. Forecasts will be updated daily, and will change to incorporate the latest figures.

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We estimate that the DHB -.

This is - the forecast from one week ago. Click onTap the line below to see more details.

This is a logistic growth model with weekly seasonality using fbprophet based on daily vaccination data as at -. Changes are measured in percentage points (e.g. A change from 50% to 51% is a "1%" change).