Kiwi researchers are exploring how effective the Pfizer vaccine was against Omicron, in what was one of the world's few infection-naive countries. Photo / Bevan Conley
Kiwi researchers are exploring how effective the Pfizer vaccine was against Omicron, in what was one of the world's few infection-naive countries. Photo / Bevan Conley
New Zealand confronted Omicron as one of the most vaccinated populations on the planet – so what difference did that make in blunting the worst impacts?
That's what researchers plan to find out in a new study exploring the effectiveness of multiple doses of the Pfizer vaccine against the variant, in what was also one of the world's few "infection-naïve" populations.
Study leaders Dr Anna Howe and Dr Matt Hobbs also aim to answer another critical question: what protection the vaccine gave Māori, Pasifika and other high-risk groups.
By the time the Omicron outbreak forced the whole of New Zealand into the red traffic light setting on January 23, about 93 per cent of our eligible adult population – that's 3,910,251 people – had already received at least two doses of Pfizer's Comirnaty vaccine.
On top of that, 974,784 of us had been boosted and more than 36,000 Kiwis had received their third primary doses, giving the country one of the highest levels of coverage anywhere.
By contrast, New Zealand had only confirmed 15,175 cases of Covid-19 – most of those from the Auckland-centred Delta outbreak that began five months before.
Some 1.7 million confirmed cases later – modellers estimate the true proportion of infection could amount to up to two thirds of the population – it's clear that high vaccination rate wasn't enough to hold back the quick-spreading Omicron's tide.
But, as vaccinologists have pointed out to the Herald, there's much to suggest vaccination played a big part in giving Aotearoa one of the lowest case fatality rates in the world, by helping slash the risk of severe sickness, hospitalisation and death.
As of Friday, the Ministry of Health had recorded 1245 deaths where Covid-19 had been formally coded as the underlying cause, along with 13,443 hospitalisations – 484 of which required ICU care.
While experts say most Kiwis should be relatively well shielded against the very worst outcomes of Omicron for now - thanks to vaccination but also "hybrid" immunity gained from natural exposure to it – knowing the vaccine's impact was nonetheless important.
"In the high-coverage post-Omicron environment, some data sources showing only numbers of cases and hospitalisations in vaccinated and unvaccinated people can allow misleading interpretations such as vaccination increasing disease risk," said Howe, of the University of Auckland.
"Internationally, there have been challenges in vaccine effectiveness analyses and uncertain applicability to the infection-naive New Zealand population, particularly Māori and Pasifika."
Although there had now been many effectiveness studies of the vaccine, few had been carried out during Omicron's dominance, she said – and there had been none conducted in a population with such a unique immunity profile as ours.
"This information is vital for equitable immunisation policy decisions going forward for New Zealand."
The team would be drawing on health data to measure what the overall number of vaccine doses, as well as the timing of those doses, generally meant for vaccine effectiveness.
Naturally, that wasn't a straight-forward undertaking in an ongoing pandemic – and similar overseas research efforts have repeatedly run up against tricky methodological issues.
But recently, a World Health Organisation advisory group set out steps on how to get a clearer measure of effectiveness, which the New Zealand team would align their study with.
"This project will use a layered approach, starting with validated data on severe disease - hospitalisation, ICU admission and death - and vaccine coverage in descriptive analyses," said Hobbs, of the University of Canterbury.
"We'll then use these foundations to generate high-quality, reliable vaccine effectiveness estimates with appropriate statistical adjustment."
"Our findings will provide accurate data that accounts for local context, as trustworthy local data presented in a relatable way is vital for community trust and confidence."
The study would also put special focus on key subgroups, including Māori, Pacific and other ethnicities by age group and presence of medical risk factors.
"This is a priority concern," Howe said.
"If we look at this in the context of historical evidence of inequity in pandemic management and response as shown in the 1918 flu pandemic, Māori had a mortality more than eight times higher than the general population."
That inequity has again been borne out in this pandemic, with Māori and Pacific people accounting for a disproportionate number of hospitalisations – while also trailing other groups in vaccination coverage.
Gaining a deeper insight into vaccine effectiveness in Māori and Pacific people, Howe said, would also be "essential" in informing strong, culturally appropriate programmes in future.
Encouragingly, studies focused on earlier variants have indicated Māori and Pasifika were no less protected by the vaccine than other groups.
The programme is among 23 new research projects just awarded funding through a special Ministry of Health pool.
By the time Omicron arrived in New Zealand, nearly four million people had received at least two doses of the Pfizer vaccine. Photo / Andrew Warner
In another project, ESR virologist Dr Sue Huang will investigate the vaccine's effectiveness among several thousand adults and children routinely surveyed through the WellKiwis and Shivers studies.
Huang told the Herald she also wanted to assess how the vaccine might have reduced spread between household members.
Separately, the study's large cohort offered one of the few current bases to measure Covid-19 reinfection in New Zealand – and would also enable her team to gather detailed information on factors like symptoms, severity, and how long infections had lasted.
And just as importantly, it would yield more valuable data on how the virus had touched population groups differently.
"The results will guide improved surveillance methods at community level particularly for those at-risk vulnerable sub-populations, such as Māori and Pacific groups," Huang said.
By the numbers
• 11,742,497 – Number of all Covid-19 doses given to date.
• 91.5% - Proportion of Kiwis aged over 12 at least partially vaccinated.
• 13,443 – Hospitalisations with Covid-19 to date.
• 1,762,125 – Total Covid-19 cases confirmed in New Zealand to date.
• 49% - Booster effectiveness against symptomatic Omicron infection relative to two doses, according to one large Qatar-based study that also indicated 76% effectiveness against hospitalisation and death.
