Herd Immunity

You may have already learned that immunity means

protection from disease. If you are immune to a disease,

you are protected from getting it. But, what is “herd

immunity”? It means the same protection from disease,

but now it applies to a group of people, not an individual.

Image: Young girl receives vaccine. Content Provider: CDC/Amanda Mills. http://phil.cdc.gov/phil/details.asp#modalIdString_CDCImage_0 ID# 13525

The term “herd immunity” does not sound like it should apply to humans. We usually talk about herds when we talk about other animals such as cattle or sheep. But, in this case, herd immunity is a good term for the immunity that applies to a group of people.

Herd immunity protects people from disease who may not be able to receive vaccines. But, how does that work? How can you be protected against disease if you can’t get a vaccine? Herd immunity prevents the disease from taking hold in a community.

Think about it using your school as an example. Let’s say that most people in your school get a certain vaccine. The vaccine protects them from getting the disease. Now, let’s say you are not able to get the vaccine. Maybe you are allergic to the vaccine or maybe you have a health condition that doesn’t allow you to receive the vaccine. Because most people in your school got the vaccine and do not get the disease, you are less likely to get the disease because you are not around a lot of people who have it. You have been protected by everyone else—the herd.

A virus needs a certain number of people in the

community who can get the disease in order for it to

spread and stay active. When enough people in the

community are vaccinated and protected against the

disease, the disease cannot easily be passed on. The

disease dies out.

Image: Small child who had to have her arms and legs partially amputated due to meningococcal disease. Mass vaccinations and herd immunity can prevent children who are too young to be vaccinated. Image: https://en.wikipedia.org/wiki/Herd_immunity

This means that people who cannot get immunized, are too young to be immunized, or are too sick to be immunized are still protected because other people are protected. The pathogen (disease-causing organism) cannot take hold in the community. The unvaccinated are protected by the herd.

There are many examples of herd immunity. Adults are immunized against pertussis (whooping cough) and this protects infants who are too young to be vaccinated. Immunizing people against the seasonal flu protects our elderly population who are so susceptible to severe lung disease.

The number of people in a population who need to be immunized for herd immunity to be effective is different for each disease. If the required number of people do not get immunized, herd immunity will not develop.

A Case for Herd Immunity

As an example, consider herd immunity for measles

in England. Measles cases were low in England until

people stopped getting their children vaccinated due to

lack of outbreaks. Herd immunity dropped and measles

outbreaks began to appear. When cases in a long

outbreak reached the highest levels in 18 years, health

experts began to encourage parents to vaccinate their

children. Vaccination rates went back up, but herd

immunity takes time to build. Eventually, herd immunity

percentages were reached and measles cases dropped.

Image: Child with measles rash. Content Provider: CDC/NIP/Barbara Rice http://phil.cdc.gov/phil/quicksearch.asp ID#132

People who may have to depend on herd immunity include:

elderly people with a compromised immune system
people on chemotherapy treatment
HIV patients
children too young to be immunized
people too ill to be immunized

Young girl sitting in a chair getting a vaccine from a doctor.

Small child missing parts of her arms and legs sitting on a floor.

Small child with a rash over face, arms, chest and stomach.