It sounds simple. We roll the Covid-19 vaccine out to the public and enough of us become immune to stop the coronavirus causing any more big outbreaks. But, as University of Auckland vaccine experts Dr Helen Petousis-Harris and Dr Janine Paynter tell Jamie Morton, herd immunity isn't always straightforward.
We've been hearing much about "herd immunity" and how and when we might reach it with the vaccination roll-out. What actually is herd immunity?
Dr Janine Paynter (JP): That's when there are enough people in a population who are not susceptible to a disease - or within whom a disease organism can't replicate enough - that people who aren't immunised have no or very low risk.
Dr Helen Petousis-Harris (HPH): It's really "community immunity", where there are enough people protected to stop the chain of transmission.
The number of people that need to be immune to create this community immunity differs depending on the disease - like how infectious it is, and how it spreads.
So what viruses have we actually achieved herd immunity against?
JP: There's smallpox, rubella, mumps, human papillomavirus (HPV) and rotavirus.
The HPV vaccination has been a lovely example of protection through herd immunity because, initially, campaigns only involved vaccinating girls.
However, we saw reductions in genital warts in boys – who were unvaccinated, when HPV coverage in girls was greater than 50 per cent. The vaccinated girls were actually protecting the boys.
What diseases haven't we quite got there with? The 2019 outbreak showed we're certainly still vulnerable to measles.
HPH: Another example of a disease that remains elusive to community immunity at the moment is whooping cough.
Not even experiencing that disease can protect you from getting it again for life.
There are also diseases, like tetanus, which aren't spread from human-to-human, but remain lurking in the environment, like in the soil.
Those we will always need to vaccinate against.
What are some of the big misconceptions about the concept, generally? And what are the variables that make the herd immunity threshold lower or higher, such as the much-discussed "R" value, or basic reproduction number?
JP: One is that we can achieve herd immunity through natural infection.
If the virus has infected everyone, and they've become immune but some people have ended up getting sick or dying in the process, then this isn't "herd immunity" because the unimmunised weren't protected.
It's just survival of the fittest.
Another one is there's a "fixed target" and we can stop or slow the vaccination effort when we've reached it.
It's all predominantly based on the "R value" - which is the average number of other cases that one infected person will create.
HPH: Indeed, if herd immunity through natural infection was actually achievable, then we wouldn't have the measles.
In the absence of vaccines, pretty much everyone would get measles and those who survived would be generally immune for life.
Yet measles epidemics keep occurring without vaccines, so community immunity against measles is only achievable through using them.
Last week, an interesting University of Sydney study suggested that Australia could reach herd immunity with a 70 per cent vaccination rate, using solely the Pfizer-BioNTech vaccine the New Zealand public will receive this year. It also found that Australia's current "hybrid" plan - using the Pfizer vaccine, but also AstraZeneca's shot, which has a lower efficacy - wouldn't achieve herd immunity even if the vaccination rate was as high as 90 per cent. What can we make of such studies? And do they mean anything for New Zealand?
JP: It will have its biggest implications when Australia and New Zealand's borders start to relax.
Until then perhaps it's a useful guide to when might be a good time to start relaxing border controls - and how we go about that.
If we're struggling to achieve good coverage - less than 70 per cent, for instance - and if there's overwhelming pressure to open the borders, then we still have the other tools to stop outbreaks.
We can ensure there's still good investment in contact tracing, and perhaps maintain a requirement to wear masks on public transport, or still limit gatherings in response to outbreaks.
Some commentators are already arguing that global herd immunity to Covid-19 is likely impossible, for a combination of reasons such as uneven vaccine roll-out, mixed vaccine efficacy, new variants that evade immunity, and the possibility immunity might not last forever. Is it indeed fanciful that the world could ever become truly "immune" to Covid-19?
JP: I think it's too early to say this – we don't know how well the vaccine will perform long term.
However, it shouldn't scare us to inaction or hopelessness.
It'll join the list of all the other infectious diseases and sit on a spectrum with them.
Some of them - like flu, Ebola and Marburg virus, and to a lesser extent measles and pertussis - still perturb us.
Then there are others that've been relegated to animated discussions about the horrors of the past. Think polio and smallpox.
HPH: I think experience has taught us that vaccines can often exceed expectation in the real world, when it comes to the overall impact.
We can only run estimates and make a range of assumptions - but sometimes we underestimate the effectiveness.
That said, we're also capable of over-estimating, so we need to wait and see.
If there's one thing for certain, it's that we need to aim high - and not let up.
JP: I'd add that we still need to be mindful that we have a health system that is more easily overwhelmed than in other countries.
We already have populations that bear a ludicrously, shamefully high burden of infectious disease, even without Covid-19 in the picture.
The measures we took to reduce Covid-19 also had dramatic impacts on other respiratory diseases like flu, respiratory syncytial virus (RSV) and pertussis.
I hope that our glimpse of what is possible inspires us and policy makers to sustain action to keep these diseases at a lower level, across the whole population.