Covid-19 Omicron outbreak: NZ's fatality risk lower than Oz, Taiwan

High vaccination coverage and public health measures have likely pushed Covid-19's case fatality risk in New Zealand to less than 0.1 per cent. Photo / Auckland DHB

High vaccination coverage and public health measures have likely pushed Covid-19's case fatality risk in New Zealand to less than 0.1 per cent – a rate lower than even that of Australia, Singapore and Taiwan.

But public health experts behind a new analysis warn that risk could change with future waves and variants – and are urging the Government to beef up its surveillance of Covid-19.

In a just-published commentary, Otago University's Dr Jennifer Summers and professors Michael Baker and Nick Wilson assessed two key indicators to measure virus-related deaths: our cumulative case fatality risk (CFR) and infection fatality risk (IFR).

CFR estimated the proportion of deaths among confirmed cases, while IFR estimated that among all infected people – the true number of which remained unknown in New Zealand.

According to their calculations, our cumulative CFR for the whole pandemic could be sitting at just 0.08 per cent.

Our IFR, assuming around half our population has already been infected, was lower still, at 0.03 per cent.

The Ministry of Health has reported just over 900 Covid-related deaths – 28 of them confirmed on Wednesday, and the vast bulk of them linked to the Omicron wave.

But that toll – translating to a crude, cumulative mortality rate of around 16 to 17 deaths per 100,000 of population - remained strikingly low compared to other countries that never achieved elimination.

Latest Public Health Blog: Covid-19 Case-Fatality Risk & Infection-Fatality Risk
Professors Michael Baker, Nick Wilson & I explore two key mortality indicators in NZ & internationally#COVID19 @publichealthUOW @Otago https://t.co/5qpkVM3JNO pic.twitter.com/zEw47jP0c5

— Dr Jennifer Summers (@Dr_Jenn_Summers) May 11, 2022

While most other nations recorded major increases in excess mortality over the first two years of the pandemic, New Zealand's zero-tolerance approach to the virus brought a net decline - and also a bump in life expectancy.

In their analysis, the researchers looked at how New Zealand's cumulative CFR stacked up against other countries that pursued elimination.

As at this week, they found our rate of 0.08 per cent to be lower than Australia's (0.12 per cent), Singapore's (0.11 per cent), Taiwan's (0.75 per cent) and Hong Kong's (0.77 per cent).

Summers told the Herald that Taiwan and Hong Kong's higher rates might be explained by factors like variants, differing demographic characteristics, testing capabilities – and vaccination levels.

In New Zealand, about half of Covid-19 deaths had occurred among those who hadn't been boosted – with deaths proportionately higher among the unvaccinated.

Ideally, Summers said, we'd also be able to accurately calculate the IFR. But our hidden picture of infections, which modellers have suggested could even stretch to around three million, made that tricky.

Still, if we assumed that about 50 per cent of the population had been infected, as the Centers for Disease Control and Prevention (CDC) has in the US, then the IFR came to 0.03 per cent.

"This estimated Covid-19 IFR would then be lower than the seasonal influenza IFR in New Zealand, of around 0.039 per cent," she said.

"However, to provide a robust comparison to seasonal influenza, New Zealand would need to conduct a population-based antibody survey, and would also need to adjust for factors such as for age, ethnicity, and vaccination status."

Baker suspected that, if Omicron had been allowed to run rampant over a New Zealand population that wasn't highly vaccinated, the sheer number of people it infected might have amounted to an IFR similar to that of the original strain of the virus.

He added that, although New Zealand reported deaths in line with World Health Organisation protocol, our numbers still didn't give an accurate picture of how many people were really dying from the virus.

While mortality statistics included anyone who'd died within 28 days of being infected – which could put pre-infected car crash or heart attack victims among the total – they could also well represent an undercount.

For instance, he said, the number of flu deaths recorded in New Zealand likely only accounted for one in 20 of those who actually died from it.

"That might not be the case with Covid, given there's so much attention on it with testing and screening – but it's still a struggle to measure this directly."

The researchers noted the huge shift that Omicron – this week pushing our cumulative confirmed case count over the million-mark – made to CFR, which was around 0.34 per cent before the variant began spreading here.

To get a better picture of the true risk of the virus, they said the Government needed to provide better data, and suggested a population-based antibody survey like that run by the CDC.

Keeping a close eye on new variants also meant that genome sequencing needed to be maintained – such as with random sampling of international arrivals and hospitalised cases.

"There may even be a case for routinely sampling wastewater from incoming international flights – as successfully used in Australia."

As well, they called for targeted interventions to protect the most vulnerable – including fourth shots before winter arrived – along with better efforts to improve ventilation and maintain mask use indoors.

Although New Zealand's death rate remained small, experts have pointed to the hidden threat of Long Covid – a myriad of lingering, post-infection symptoms that could come with 10 per cent of infections, and pose enormous problems for the country's health and healthcare system.