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What is herd immunity?

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Quick answer

Herd immunity is when enough people in a community are immune to a contagious disease — through vaccination or past infection — that it can no longer spread easily. This indirectly protects those who aren't immune, such as newborns and people who can't be vaccinated.

Key things to understand

  1. Once immune people are common, the germ runs out of new hosts and outbreaks fizzle.
  2. It shields the vulnerable who can't be vaccinated, such as infants or immunocompromised people.
  3. The threshold depends on how contagious the disease is — measles needs about 95% immunity.
  4. Reaching it through vaccines avoids the deaths and complications of mass infection.

People also ask

What percentage is needed for herd immunity?
It varies with contagiousness: highly contagious measles needs about 95% immune, while less contagious diseases need a lower fraction.
Why does herd immunity protect people who aren't immune?
If almost everyone around an unvaccinated person is immune, the disease has no chain of hosts to travel through to reach them.
Can herd immunity be reached without vaccines?
Through mass infection, yes — but that path causes the very illness, hospitalizations, and deaths that vaccines prevent, so it's far more dangerous.