Science
What is Herd immunity?
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.
See it, don’t just read it.
Watch a 2-minute lesson with voice + animation that explains herd immunity.
Key things to understand
- 1Once immune people are common, the germ runs out of new hosts and outbreaks fizzle.
- 2It shields the vulnerable who can't be vaccinated, such as infants or immunocompromised people.
- 3The threshold depends on how contagious the disease is — measles needs about 95% immunity.
- 4Reaching it through vaccines avoids the deaths and complications of mass infection.
Frequently asked questions
- 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.