Herd immunity is the indirect protection against an infectious disease that populations acquire when there are enough immune people. The threshold required to achieve this depends on many different factors, such as the number of reproductions of the virus or “R” (the number of people infected by each carrier), which in itself varies greatly. Some factors that affect the latter include where you live, the variant involved, and terrain conditions, such as blockages.
This means that even when scientists know more about it, there won’t be a set threshold for herd immunity to work everywhere, but it’s possible to estimate roughly 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 immunity from the herd. But if the vaccine was 80% effective, between 75 and 90% of people need to have it.
This is potentially higher than the vaccination ambitions of many countries. The UK aims to vaccinate all adults by September, which equates to approximately 51 million people out of 67.5 million people, 75% of the total population. This means that all adults in the country are willing to be vaccinated and are healthy enough to be eligible.
However, most scientists do not expect to completely eliminate the virus. For now, the goal is to minimize its transmission. “Even if you get vaccinated, there are still a fairly large number of susceptible people,” Head says. “So we’ll still see outbreaks happen. I think they would be pretty localized, but they would 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 doesn’t matter if they are still able to spread the virus: everyone will have immunity.
However, it can be crucial for those people who cannot get vaccinated, for example, because they are pregnant, too young, or too ill.
Until we have an answer, maybe we should keep in mind the story of the eleven-year-old boy with mumps and act like we haven’t been vaccinated, even if we did.
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