Herd immunity is indirect protection against an infectious disease that populations acquire when sufficient numbers of people are immune. The limit needed to reach it depends on many different factors, such as the number of viruses reproduced, or “R” – the number of other people infected by each carrier – which in itself varies widely. Some factors that affect the latter include where in the world you live, the variant involved and conditions on the ground, such as roadblocks.
This means that even when scientists know more, there will be no set limit on collective immunity that works everywhere – but it is possible to roughly estimate what it could be.
For example, a calculation suggests that for a vaccine that completely eliminates transmission, 60-72% of the population would need to have it in order to obtain complete herd immunity. But if the vaccine’s effectiveness were 80%, between 75 and 90% of people would need to get it.
This is potentially greater than the vaccination ambitions of many countries. The UK plans to immunize all adults by September, which is equivalent to about 51 million to 67.5 million people – 75% of the population in total. This is assuming 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 entirely. For now, the goal is to reduce its transmission to the maximum. “Even if you vaccinate, you will still have a fairly large number of susceptible people there,” says Head. “So, we will still see outbreaks happen. I think they would be well located, but they would still cause concern and a burden of disease ”.
Some scientists argue that the emphasis on preventing transmission is a false clue, because once enough people have been 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, very young or very sick.
Until we have an answer, perhaps we should all keep in mind the story of the 11 year old boy with mumps – and act as if we haven’t been vaccinated, even if we have.
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