Herd immunity is the indirect protection against an infectious disease that populations can grow when a sufficient number of people are immune. The threshold required to reach it depends on many different factors, such as the number of reproductive numbers of the virus, or “R” – the number of additional humans infected by each carrier – which itself varies widely. Some factors that affect the latter include where in the world you live, the variant involved, and the conditions on the ground, such as lockdowns.
This means that, even as scientists know more, there won’t be a set threshold to herd immunity working everywhere – but it’s possible to roughly estimate what it could be.
For example, one calculation suggests that for a vaccine that completely eliminates transmission, 60-72% of the population would need it to achieve complete immunity to the herd. But if the vaccine’s effectiveness is 80%, 75 to 90% of people will need it.
This is potentially higher than many countries’ vaccination ambitions. The UK is aiming to vaccinate every adult by September, which amounts to about 51 million of the 67.5 million people – 75% of the total population. That assumes that every adult in the country is willing to get vaccinated and is healthy enough to qualify.
However, most scientists do not expect to eliminate the virus completely. At the moment, the goal is to reduce its transmission as much as possible. “Even if you vaccinate, you still have a pretty high number of sensitive people out there,” says Head. “So we will still see outbreaks happening. I think they would be fairly local, but they would still be of concern and cause a disease burden.”
Some scientists argue that the emphasis on preventing transmission is red herring, because once enough people are vaccinated, it doesn’t matter if they can still 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 sick.
Until we have an answer, maybe we should all keep the story of the 11-year-old boy with mumps in mind – and pretend we’re not vaccinated, even though we did.
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