Why vaccinated people may still be able to spread Covid-19 – BBC News

Herd immunity is the indirect protection from an infectious disease that populations acquire when enough people are immune. The threshold needed to achieve it depends on many different factors, such as the reproduction number of the virus, or “R” – the number of further people 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 when scientists do know more, there will be no set threshold for herd immunity that works everywhere – but it’s possible to estimate roughly what it might be.

For example, one calculation suggests that for a vaccine that totally eliminates transmission, 60-72% of the population would need to have it, it order to accomplish complete herd immunity. But if the vaccine’s effectiveness were 80%, between 75 and 90% of people need to have it.

This is potentially higher than the vaccination ambitions of many countries. The UK is aiming to immunise every adult by September, which equates to around 51m out 67.5m people – 75% of the population in total. That’s assuming that every adult in the country is willing to be vaccinated, and healthy enough to be eligible.

However, most scientists aren’t expecting to eliminate the virus entirely. For the moment, the goal is to reduce its transmission as much as possible. “Even if you vaccinate, you’ve still got a fairly large number of susceptible people there,” says Head. “So, we will still see outbreaks happen. I think they would be fairly localised, 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 may prove crucial for those who are unable to be vaccinated, for example because they are pregnant, too young, or too unwell.

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 though we haven’t been vaccinated, even if we have. 

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