THE idea of herd immunity has had a bumpy ride as the coronavirus pandemic has played out. It was initially touted in some countries as a viable strategy for dealing with the spread of covid-19, before being dismissed. Today, some headlines celebrate the fact that many places might have achieved herd immunity including and pockets of . But others warn that millions will die before we get there.
The true picture is far messier, partly because scientists don鈥檛 even agree on what herd immunity is, let alone how it might be achieved. So how will we know when populations are protected against the coronavirus?
While the definition of herd immunity depends on who you ask, let鈥檚 assume that it refers to a situation in which enough of a population is immune to a pathogen that it no longer spreads throughout a community. Those who might be susceptible to it are indirectly protected thanks to the immune responses of others.
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These immune responses might have developed after a person was infected with a pathogen or after being vaccinated against it.
Our experience of other viruses show how herd immunity can develop. Seasonal viruses like the common cold often sweep through a population until enough people have encountered them and built up a protective immune response for them to stop spreading. Widespread use of the MMR vaccine led to herd immunity for measles in some countries.
But herd immunity doesn鈥檛 necessarily last. Viruses can evolve and change. And if vaccination rates drop, viruses can make a comeback.
When it comes to the coronavirus, there are even more challenges. For a start, we don鈥檛 know what proportion of a population would need to be immune to generate herd immunity. This figure is typically estimated using the basic reproduction number 鈥� or R number 鈥� of a virus, which represents how many other people a person who has the virus will go on to infect.
鈥�10-20%
Infection rate needed to reach herd immunity, according to one study鈥�
Most estimates say that between 60 and 70 per cent of a population would need to be immune to the coronavirus to stop its spread in a community. That is based on an R number of somewhere between 2.5 and 3.
But even if these levels were reached, it doesn鈥檛 mean that the virus won鈥檛 spread at all. It could still be passed from an infected person to someone who is vulnerable, given the right setting. 鈥淚t鈥檚 a population-level statistic,鈥� says Julian Tang at the University of Leicester, UK. It is difficult to know when herd immunity has been reached, but a decline in cases would be a good indicator.
The problem is that humans don鈥檛 tend to behave as a herd. The calculation for the herd immunity threshold assumes that people are equally spaced and mixing equally with each other. In other words, 鈥渘o walls, barriers, cars or glass windows in the way鈥�, says Tang. In reality, of course, people live in varied environments, and travel and interact with others to varying degrees.
Estimates of the R number vary, too, and change over time. Our behaviour can also influence the figure. Lockdowns, social distancing and other infection control measures will have lowered the R number in many regions, for example.
This line of thought led Gabriela Gomes at the University of Strathclyde, UK, and her colleagues to recalculate the herd immunity threshold. The team developed a mathematical model based on the simple herd immunity calculation, but tried to factor in variation between individuals.
Already immune
In theory, this variation takes into account the differences across a population in susceptibility to the virus, and the chances of people coming into contact with it. The more variation there is, the lower the threshold for herd immunity, says Gomes. By her team鈥檚 calculations, only around 10 to 20 per cent of a population needs to be immune to the virus to achieve herd immunity (medRxiv, ).
By that estimate, some places have already achieved herd immunity. More than 10 per cent of the population of Madrid is thought to have developed some immunity to the virus, for example. 鈥淚n Europe, I think most countries are close to having that status,鈥� says Gomes. This would suggest that the epidemic is currently at its peak in places like Madrid, she says.
Samir Bhatt at Imperial College London disagrees. 鈥淚 personally don鈥檛 believe we鈥檙e anywhere near herd immunity,鈥� he says. He points to regions with exceptionally large outbreaks, such as Bergamo in Italy, where tested between April and June had antibodies to the virus. 鈥淚f 10 per cent were sufficient, why did they get up that high?鈥� Bhatt asks.
One issue is that estimates for the threshold vary so wildly, anywhere from 10 to 70 per cent. 鈥淚t鈥檚 impossible to actually calculate herd immunity,鈥� says Bhatt. 鈥淭here鈥檚 no simple answer,鈥� says Luis Barreiro at the University of Chicago.
Given the uncertainty, most scientists believe it would be unethical to wait for enough people to become infected with the virus to establish herd immunity. Estimates for hard-hit countries like France, Spain and Brazil suggest that only around 5 to 10 per cent of the population have encountered the virus, leaving the majority vulnerable. 鈥淭he number of people that would die from the virus would be astronomic,鈥� says Barreiro.
In theory, a vaccine could provide a safer route to herd immunity. But many questions remain as to how effective a vaccine will be. We still don鈥檛 know, for example, how long a person鈥檚 immune response to the virus lasts 鈥� whether they become infected with the virus or receive a vaccine. If a vaccine only reduces a person鈥檚 risk of infection by 50 per cent, for example, 鈥渢he threshold becomes much higher, and realistically it becomes impossible to ever reach herd immunity鈥�, says Barreiro.
Even if a vaccine does provide good protection, it might not work for everyone. Some may protect young people, but not older individuals who don鈥檛 mount as strong an immune response.
The public鈥檚 perception of a vaccine is also important. A recent Gallup poll suggests that against the virus, while a UK survey found , or refuse, to take one.鈥滻f 20 to 30 per cent of the population refuses a vaccine, we will probably never be able to reach herd immunity,鈥� says Barreiro.
