Infectious disease specialists and public health officials often talk about herd immunity, which refers to the effectiveness of immunization when the vast majority of people have been vaccinated against a disease.
- One of the key ways herd immunity works is by minimizing the number of people who are at risk of catching the disease, thus diminishing its power to spread.
When someone who is not vaccinated against a particular disease come into contact with that disease, the spread becomes exponential if other people in the community are not vaccinated against it either. Every non-vaccinated person they meet – and in turn, every non-vaccinated person they meet – is vulnerable.
More important, certain children and adults with weak immune systems--whether because of chemotherapy for cancer, other medication or because they were born like that--often can’t get vaccinated. They are also at increased risk of complications from vaccine-preventable diseases and therefore need herd immunity to remain protected.
To see how this can happen, imagine a school in a community where there’s a measles outbreak. If Olivia, one of the students, was vaccinated against measles (two doses at 12 and 18 months of age), then she’s protected and can’t pass it on to her classmate Oliver. If Oliver was vaccinated, the same holds true for everyone he comes into contact with. But since there are known cases of measles in this community, any child in the class who isn’t vaccinated is at great risk of developing measles and passing it on to every other child who isn’t vaccinated.