Herd immunity is the idea that when many people are immune to (protected from) a contagious infection, it's harder for that infection to spread in a community.

How Do People Become Immune to Infections?

There are a few ways that a person can become immune:

• Vaccines teach the body to recognize and fight germs. When someone gets a vaccine, their body makes protective antibodies against it. These antibodies stay in the body and prevent the person from getting sick from those germs in the future. This is the best way to become immune.
• Getting infected with the virus or bacterium will also teach the body to make protective antibodies that can protect a person later on. This is an uncomfortable way to become immune because it often involves getting sick. It can also be dangerous, as some people die from infections.
• Getting antibodies that are already made is called “passive immunity.” It can be done in two ways:
  • Antibody shots against specific germs can be given as protection for certain groups of people. For example, babies can get an antibody shot against RSV from birth to 8 months of age just before or during the RSV season. This is not a common way to become immune as antibody shots are available for only a few infections.
  • A mother passes antibodies to her baby through the placenta before the baby is born, and through breast milk after the baby is born.
    
    Antibodies received in a passive way do not last as long as antibodies made by the body after a vaccine or an infection.

Why Is Herd Immunity Important?

Herd immunity (also called community immunity) protects people who are not immune to a disease, such as those who:

• never had the disease
• can't get the vaccine (for example, newborn babies)
• got the vaccine but their body didn’t make enough antibodies
• can't become immune because they have a weak immune system (for example, elderly people or people getting medicine that weakens the immune system)

When about 7 or 8 out of 10 people are immune to an infection in a community, their whole community is usually protected — not just those who are immune. Diseases that are very contagious, like measles, might need almost everyone to be immune.

Why Are Vaccines Important to Herd Immunity?

Achieving herd immunity without vaccines means that many people would have to get infected by a virus or bacterium to become immune. Some of them would become sick and many might even die. Vaccines have helped us get rid of many illnesses — like measles, polio, and chickenpox — without too many people getting sick.

When groups of people do not get vaccinated, these diseases sometimes reappear. For example, there have been outbreaks of measles, mumps, and pertussis (whooping cough) in recent years. These outbreaks seem to be connected to groups of people who did not get the vaccines.

Vaccines are the best way to protect ourselves and people in our communities who need us to be immune so that they can be protected.