New calculations from the country's expert disease modellers indicate that achieving herd immunity is no guarantee, Marc Daalder reports
Even if vaccines remain highly effective against the more transmissible Delta variant, as much as 81 percent of the entire population would need to be vaccinated to reach herd immunity.
That's according to the latest modelling from Te Pūnaha Matatini, which released the first estimate of New Zealand's likely herd immunity threshold on Wednesday morning. The research has not yet been peer reviewed.
"The vaccine rollout is good news but life is not going back to normal for some time," the research centre's director Shaun Hendy said.
Researchers lead by University of Canterbury research assistant Nic Steyn and Canterbury mathematics professor Michael Plank developed models showing the likely size of an outbreak at different stages of the vaccine rollout, the level of measures needed to suppress outbreaks as the rollout proceeds and the likely herd immunity threshold.
They found that, as the rollout went on, the hospitalisations and fatalities resulting from an outbreak dropped precipitously. This was based on the fact that those most likely to be vaccinated early are those most likely to suffer severe outcomes from Covid-19. By the time all of those over the age of 65 are vaccinated, the modellers found, the number of likely deaths from an outbreak of a more transmissible variant, like Delta, would have fallen by nearly two-thirds and the number of hospitalisations by almost two-fifths.
The number of total cases in an outbreak would also fall, but less dramatically. There would be 20 percent fewer cases in such an outbreak than without any vaccinations. This is partly because younger people are more mobile and more likely to spread the virus, even if they don't suffer the same severe outcomes.
The further into the vaccine rollout we are, the lower the reproduction number of the virus. That's the number of people infected by each case, on average. If the reproduction number falls below one, that means an outbreak will die out of its own accord. So the herd immunity threshold is the percentage of the population that needs to be vaccinated to drive the reproduction number below one.
That said, even a reproduction number above one can be lowered through non-lockdown public health measures. For example, effective contact tracing that leads to 70 percent of symptomatic cases being isolated within two days of symptom onset lowers the reproduction number by about 11 percent. Therefore, the combination of effective contact tracing and high vaccination rates could control the virus even if we haven't quite hit the herd immunity threshold.
"Increasing levels of vaccination will make maintaining an elimination strategy easier and allow the country to eventually move from relying on population-wide interventions like lockdowns to more targeted controls like contact tracing in the later stages of the rollout," Steyn said.
However, the likelihood that all restrictions will be able to be lifted in the near-term has been thrown into doubt by the findings of the research. The modellers analysed a number of different scenarios, including for a less transmissible version of the virus (like the original virus), one of medium transmissibility (like the Alpha variant) and a highly transmissible variant like Delta. They cautioned that these scenarios don't exactly map onto each different variant of the virus, particularly because Delta is still the subject of some uncertainty and a lot of ongoing research.
With that caveat, if the dominant variant of the virus worldwide becomes a highly transmissible one, with a reproduction number of six in a population with no restrictions, that could challenge New Zealand's ability to reach herd immunity. It's worth noting that the Delta variant increasingly makes up a majority of the cases sequenced in a number of countries, including the United Kingdom, Israel and Russia.
Depending on the effectiveness of the vaccine, between 81 and 97 percent of the entire population could need to be immunised to reach herd immunity. If vaccines are 90 percent effective at preventing infection, then the lower threshold would be called for, while vaccines that are only 70 percent effective would necessitate the higher one. At this stage, the Pfizer/BioNTech vaccine that New Zealand is using is thought to be in the 90 percent range or above, but the evolution of new variants of the virus could challenge that efficacy.
Because it is unlikely, at this stage, that anyone below the age of 12 will be vaccinated, a 97 percent threshold would mean herd immunity is out of reach. Such a scenario - or one where enough eligible people decline the vaccine such that even the 81 percent threshold isn't met - would mean New Zealand would have to maintain some additional measures like masking or contact tracing in order to keep the reproduction number below one.
"Mask wearing, testing and tracing will be necessary for some time yet," Massey University mathematical biology professor Mick Roberts said, after examining the necessary herd immunity thresholds.
This is also something Director-General of Health Ashley Bloomfield has hinted at. In an exclusive interview with Newsroom in May, Bloomfield said, "Some of these things are with us for at least a good period. People say three to five years. No one can see into the future.
"The question will be, in addition to vaccination - and this will be something for us in New Zealand to be thinking about particularly over the next few years - what are the other measures we will need to have in place to help protect New Zealanders and help protect our healthcare system? That's where I've talked about ... the idea that there will need to be a baseline level of measures," he said.
"I do think that, even wide vaccination as we open our borders will need to be complemented with a level of baseline measures around gatherings, scanning and these sorts of things, and ongoing contact tracing and isolation and testing, to help reduce the burden of Covid-19 across the community. So we're not getting the unnecessary hospitalisations and deaths that might otherwise happen."
Even if New Zealand reaches herd immunity, it would still see some cases, hospitalisations and even deaths from Covid-19. The Te Pūnaha Matatini researchers simulated the effects of five new Covid-19 cases entering the country each day for two years. If 90 percent of the population were vaccinated with a 90 percent effective vaccine, even a highly transmissible outbreak would see just 77 deaths and 710 hospitalisations.
Compare that to the consequences of not quite reaching herd immunity: If the vaccine was only 70 percent effective, then 1700 people would die in the first two years, 14,000 would be hospitalised and 770,000 would be infected.
"We’re not going to one day magically hit a population immunity threshold where we can open the borders and everything goes completely back to normal. It will be more of a gradual relaxation of border measures alongside continued testing and contact tracing measures," Plank said.
"If we relax border restrictions, we will see Covid-19 cases and it’s quite likely that we’ll see outbreaks. The way to protect against those outbreaks is to get vaccinated."
