Herd immunity is the indirect protection against an infectious disease that populations acquire when there are enough people. The threshold required to reach it depends on many different factors, such as the number of reproductions of the virus or “R” – the number of people still infected by each carrier – which itself varies greatly. Some factors that affect the latter include where you live in the world, the variant involved and the conditions on the ground, such as blockages.
This means that even when scientists know more, there will be no set threshold for the immunity of the herd that works everywhere – but it is possible to estimate approximately what it could be.
For example, a calculation suggests that for a vaccine that completely eliminates transmission, 60-72% of the population should have it, in order to achieve complete herd immunity. But if the vaccine was 80% effective, between 75 and 90% of people should have it.
This is potentially greater than the ambitions of vaccination in many countries. The UK aims to immunize every adult by September, which means about 51 million out of 67.5 million people – 75% of the total population. This assumes that every adult in the country is willing to be vaccinated and healthy enough to be eligible.
However, most scientists do not expect to completely eliminate the virus. For now, the goal is to reduce its transmission as much as possible. “Even if you get vaccinated, you still have a fairly large number of susceptible people there,” says Head. “It simply came to our notice then. I think they would be quite localized, but it would still cause concern and cause a burden of disease. ”
Some scientists claim that the focus on preventing transmission is a red herring, because once enough people have been vaccinated, it doesn’t matter if they are still able to spread the virus – everyone will have immunity.
However, it can be crucial for those who cannot be vaccinated, for example, because they are pregnant, too young or too ill.
Until we have an answer, maybe we should all have in mind the story of the 11-year-old boy with mumps – and behave as if we hadn’t been vaccinated, even if we had.
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