Herd immunity is the indirect protection against a contagious disease that populations contract when enough people are immune. The threshold required to achieve this depends on many different factors, such as the reproduction number of the virus, or “R” – the number of further people infected by each carrier – which itself varies greatly. Some factors that influence the latter include where you live in the world, the variant in question and the conditions on the ground, such as connections.
This means that, even when scientists know more, there will be no set threshold for herd immunity working anywhere – but it is almost possible to estimate approximately.
For example, one calculation suggests that 60-72% of the population for a vaccine that totally eliminates the transmission, it needs to bring about the herd immunity. But if the effectiveness of the vaccine was 80%, between 75 and 90% of people should have it.
This is possibly higher than the vaccination ambitions of many countries. The UK aims to immunize every adult by September, amounting to about 51 million out of 67.5 million people – 75% of the population in total. This is the assumption that every adult in the country is willing to be vaccinated and healthy enough to be eligible.
However, most scientists do not expect to eliminate the virus completely. At present, the aim is to reduce its transmission as much as possible. “Even if you’ve been vaccinated, there are still a fair number of people who are susceptible there,” says Head. “We will still see outbreaks happen. I think they will be fairly localized, but they will still cause concern and cause a burden of disease.
Some scientists argue that the emphasis on preventing transmission is a red herring, because once enough people have been vaccinated, it does not matter if they can still spread the virus – everyone will have immunity.
However, it can be very important for those who cannot be vaccinated, for example because they are pregnant, too young or too ill.
Until we have an answer, perhaps we should all keep in mind the story of the 11-year-old boy with pumpkins – and act as if we have not been vaccinated yet, even if we did.
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