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Herd immunity is the indirect protection against an infectious disease that populations acquire when enough people are immunized. The threshold needed to achieve this depends on many different factors, such as the number of virus reproductions, or “R” – the number of additional people infected by each carrier – which itself varies widely. Some factors that affect these include where you live in the world, the variant involved, and conditions on the ground, such as lockouts.
This means that, even when scientists know more, there won’t be a fixed threshold for collective immunity that works everywhere – but it’s possible to roughly estimate what it might be.
For example, one calculation suggests that for a vaccine that completely eliminates transmission, 60-72% of the population would need to have it to achieve full herd immunity. But if the vaccine’s effectiveness was 80%, between 75 and 90% of people need it.
This is potentially higher than the immunization ambitions of many countries. The UK is aiming to vaccinate every adult by September, which equates to around 51 million out of 67.5 million people, or 75% of the total population. This assumes that every adult in the country is ready to be vaccinated and healthy enough to be eligible.
However, most scientists don’t expect to eliminate the virus completely. For the moment, the objective is to reduce its transmission as much as possible. “Even if you vaccinate, you still have quite a few susceptible people there,” Head says. “So we’ll always see epidemics happening. I think they would be fairly localized, but they would still be of concern and cause a burden of disease. ”
Some scientists claim 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 be immune.
However, this 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 act like we haven’t been vaccinated, even if we do. ‘Have done.
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