What is herd immunity?
Quick Answer
Herd immunity occurs when enough people in a population are immune to a disease, through vaccination or past infection, that its spread is greatly slowed, protecting those who cannot be vaccinated.
The Full Story
When a large percentage of a community is immune, the pathogen has few susceptible hosts to jump to, and chains of transmission break. The threshold varies by disease: measles, one of the most contagious diseases, requires about 93 to 95 percent immunity, while polio needs about 80 to 85 percent. Herd immunity shields vulnerable people who cannot be vaccinated, such as newborns, the elderly and immunocompromised individuals. It can be achieved through vaccination programmes or widespread natural infection, though the latter comes at the cost of many illnesses and deaths. Herd immunity does not work for all diseases, especially those that do not confer lasting immunity.
Key Facts
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Suggested video title for this topic:
"Safety in numbers — how herd immunity works"