New modelling has highlighted how vaccinating as much of the population as possible – and as quickly as possible – remains the best hope to stop new Covid-19 variants popping up and circulating permanently.
In a paper just published online, Te Pūnaha Matatini researchers modelled factors that could mean the difference between the virus either being quashed, or being allowed to evolve to the point where there were too many variants of it to be eliminated.
Their results indicated that speed was the key – and also suggested that it could be better to prioritise giving updated variant-resistant shots to those who hadn't yet been vaccinated, over high-risk people who'd already received two doses of vaccine.
"When researchers have modelled Covid-19 in the past, we've tended to assume that only one variant will be active at one time," study author Elliott Hughes said.
"However, we need to understand how we should respond to outbreaks of different variants which have different levels of resistance to vaccines.
"For example, if a variant emerged that was highly resistant to the Pfizer-BioNTech vaccine we use in New Zealand, how much risk would that pose?"
"And if we later developed a vaccine that was effective against this variant, should we start by revaccinating high-risk individuals, or should we focus on people who have not received any vaccine yet?"
In the paper, which is yet to be peer-reviewed, the researchers used a model that randomly generated different possible Covid-19 outbreaks to explore how the virus could spread and create new variants.
"While most of these variants die off almost immediately, some may be more infectious or more resistant to vaccines, and these are more likely to start spreading in the population."
In one scenario, which assumed no vaccine was available, the modelling showed that either the disease proved to be self-limiting and variants died out after the population reached herd immunity, or it evolved too fast to be stopped, hitting the point where it began circulating like influenza viruses.
In the latter case, the researchers observed how in the second of two waves, peaking around day 450 of their modelled outbreak, a large number of new variants sprouted, quickly rendering natural elimination through herd immunity impossible.
A second scenario they explored assumed that regularly-updated vaccines were available, and that they had 100 per cent effectiveness and take-up among the population.
Further, they assumed that each of the new vaccines were aimed at the variant that was most prevalent on the day the vaccines began to be rolled out, and which hadn't been targeted before.
While this modelling found that faster vaccine updates – happening as rapidly as every 50 days – were more effective in achieving elimination, this only had a marginal effect if the vaccination rate was slow.
"We show that if we don't vaccinate people quickly enough, there is a risk that the virus could continue to circulate indefinitely," Hughes said.
"If a vaccine-resistant variant does emerge, we found that the faster we could pivot to vaccinating against this new variant, the higher the probability of elimination.
"We also found that vaccination strategies that maximised coverage seem to have a higher chance of eliminating the virus than those that revaccinated high-risk individuals against variants - but it's important to note that we did not model the impact of different strategies on hospitalisations or deaths."
Because of that, he said, he'd caution against using the results as a basis for policymaking without considering the other impacts of different variants.
Ultimately, vaccinating as many people as possible as rapidly as possible - here and overseas – minimised the risk of more variants emerging, study author Dr Rachelle Binny said.
"In other countries, like the UK and Brazil, we're seeing different variants driving waves of infection today compared to early on in the pandemic and current vaccines may be less effective against some of these newer variants," she said.
"While the number of people who say they're willing to take the vaccine is growing globally, we're still not seeing high enough uptake rates to eliminate the virus without the need for additional public health measures.
"Our modelling shows that the faster we can roll out vaccines to as many people as possible, the better our chances of avoiding future large outbreaks where new, more dangerous, variants might arise."
As at midnight last night, around 1,149,608 doses of vaccine had been administered across New Zealand, and more than 444,000 Kiwis had received their second dose.
While coverage was running about 7 per cent above target, just 10.6 per cent of the country's adult population has been fully vaccinated so far – a rate only slightly better than Australia's 7.2 per cent.
"We also improve our chances by keeping transmission rates low and detecting cases early," Binny added.
"We can all do our part, by wearing masks in crowded places, keeping track of where we've been, and getting tested if we start experiencing symptoms."
The new research comes as Te Pūnaha Matatini modellers this week warned New Zealand may never reach herd immunity against Covid variants like the deadly Delta strain - or be able to completely abandon public health measures.
Another paper, published today, indicated that 83 per cent of Kiwis would have to be vaccinated against less-transmissable virus strains for measures like lockdowns and 14-day quarantine to be no longer needed.
However, the modelling suggested 97 per cent of Kiwis would need both Pfizer jabs to abandon such measures if the country was hit by a wave of a strain as transmissible as the Delta variant.
Study co-author Professor Shaun Hendy said it was "pretty unlikely" New Zealand would reach that level of vaccine coverage, which he said would require previously unseen uptake rates.
"Time will tell and it may not be achievable. Life's not going to go back to 2019 any time soon."