Misplaced worries about all vaccines might help explain why a quarter of Kiwis are unsure about receiving the Covid-19 shot, a new study finds.
It's also suggested that knowing more facts about the virus itself won't necessarily motivate the so-called vaccine "hesitant" to get immunised against it.
In a just-published analysis, Massey University communications lecturer Dr Jagadish Thaker has taken a deeper look at what influences that hesitancy.
It comes as New Zealand has launched its largest-ever vaccination campaign, accompanied by a major awareness and education effort to boost uptake.
Director general of health Dr Ashley Bloomfield has indicated a vaccination target of 70 per cent of the population, which experts say would be at the lower end of the threshold to achieve herd immunity against the virus.
But that could prove a challenge.
A recent Ministry of Health survey showed that while 69 per cent of people were prepared to receive a "well-tested and approved" vaccine, another 24 per cent were hesitant - and 16 per cent said they'd refuse it.
In his study, drawing on more than 1000 survey responses that were collected last year, Thaker developed a 14-point scale to tease apart the motivations behind hesitancy.
He found it could be split into two main categories: confidence in the effectiveness of vaccines in general, and perceptions that they posed a risk.
The confidence-related questions included whether respondents agreed vaccines were effective, whether they were important for their own health and that of others, and whether they could trust information from vaccine programmes.
Risk-related questions asked them if they thought the Government "over-hypes" the need for vaccines, whether they were concerned about serious adverse risks, and whether corporations making them "only care for profit".
On a scale of one to five - with the higher score indicating higher hesitancy - the average respondent scored 1.97 on the confidence factor, and 2.89 for risks - highlighting that the latter was the bigger factor in hesitancy.
Thaker said the analysis also showed that hesitancy about other types of vaccines could be linked, in that lower hesitancy toward them generally correlated to a higher "intention" to get the Covid-19 vaccine.
While having a higher trust in scientists and health experts was associated with greater intention to get vaccinated, the study also showed peoples' own knowledge about the virus - such as where it came from, how it spread and common myths - wasn't.
"In other words, knowledge about a disease doesn't appear to be sufficient enough to motivate vaccination intention."
While a "small minority" of the public may be sceptical towards all vaccines, he said people were generally likely to have different concerns about different ones.
"This study suggests that some of their underlying concerns about vaccine safety and risks may persist across different vaccines."
The Pfizer/BioNTech (Comirnaty) vaccine most Kiwis will receive has proven to be 95 per cent effective against symptomatic Covid-19, seven days after receiving two doses.
It's also shown to be safe, having been authorised by MedSafe and other major bodies overseas.
While it could cause common side effects, like pain at the injection site, serious reactions were "very rare", the ministry said.
Thaker's study meanwhile indicated that more men compared to women, more older people compared to younger ones, and those with higher incomes and more education, were more likely to say they'd get the vaccine.
The same trend was seen among those without children compared to parents, those who didn't smoke or vape, and Asian and other ethnicities compared to Māori.
"The findings suggest that a health communication campaign from trusted sources, with information that addresses prevailing concerns about vaccines, is likely to help increase Covid-19 vaccine uptake."
When do we hit herd immunity?
Researchers have suggested that Australia could reach Covid-19 herd immunity if it vaccinated three-quarters of its population using only the shot most Kiwis will receive this year.
But a New Zealand modeller cites "considerable uncertainty" surrounding some of the assumptions made in the University of Sydney study, which has been published online ahead of peer review.
In the paper, modellers compared different vaccination programmes to find that delivering one high-efficacy vaccine - the BioNTech-Pfizer shot - to 70 per cent of the population could achieve herd immunity.
Herd immunity is defined as when enough of a population has immunity - either from vaccination or a past infection - to stop uncontrolled spread.
While New Zealand's public roll-out will primarily use the Pfizer vaccine, Australia's campaign will draw on both that and AstraZeneca's.
Its programme involved a mass-vaccination approach in which a targeted group of the population - comprising individuals in high-risk age groups and healthcare workers - would receive the Pfizer vaccine.
The remaining portion of the population will receive the AstraZeneca shot, which has a lower efficacy.
The study found that even if this "hybrid" scheme covered 90 per cent of Australia's population, herd immunity still wouldn't be reached.
"However, the good news is that once Australia's hybrid vaccination programme is rolled out, the need to comply with social distancing measures in the future will decrease," said Professor Mikhail Prokopenko, director of the university's Centre for Complex Systems.
"There will likely be small outbreaks, but vaccination of 90 per cent of the population will necessitate only a 30 to 40 per cent compliance with social distancing measures.
"Without these partial restrictions, however, the outbreaks could still very well affect thousands or even tens of thousands of people."
While a 70 per cent vaccination rate has been suggested in New Zealand as a bottom-range threshold, scientists have pointed out that calculating herd immunity is fraught with complexity and uncertainty.
One recent article in major science journal Nature set out five reasons why global herd immunity to the virus was likely impossible.
New Zealand Covid-19 modeller Professor Michael Plank pointed to some of those key factors in commenting on the new Sydney study.
"Firstly, the model assumes that the vaccine is very effective in preventing transmission of the virus," he said.
"Although preliminary data about the effect of the Pfizer vaccine on transmission is promising, this is yet to be confirmed."
Secondly, he said, the study didn't take into account the effect new variants could have.
"Some of the new variants are more transmissible and others are less responsive to vaccines," he said.
"Altogether this means that vaccine coverage would need to be higher than 70 per cent to reach herd immunity."
Nevertheless, as the study showed, the more people who were vaccinated, the less severe the impacts of Covid-19 would become, and the less likely lockdowns would be needed.
"Even if herd immunity can be achieved, outbreaks can still occur and immunity is likely to wane over time," Plank said.
"This means that, regardless of where the herd immunity level sits, we need as many people as possible to get vaccinated to protect our communities.
"This is just as true in New Zealand as it is in Australia."