Under what is called “herd immunity,” when most people in a community are vaccinated against a disease, the virus lacks a host and will eventually go away because there are so few susceptible people left to infect, a CHOC Children’s infectious disease specialist says.
An instance when herd immunity broke down and a disease spread rapidly and broadly was in 2010, when a statewide whooping cough epidemic killed 10 infants and sickened 9,120 people in California. Earlier this year, the California Department of Public Health declared another statewide whopping cough epidemic.
“With the last epidemic, it was shown that there were higher rates of disease in geographic pockets where there were lower rates of vaccination,” says Dr. Jasjit Singh, associate director of pediatric infectious diseases and medical director of infection prevention and epidemiology at CHOC.
The herd immunity principle is especially important for protecting people who cannot be vaccinated, including children who are too young to be vaccinated and people with immune system problems. Of the 10 infants who died in the 2010 whooping cough outbreak, nine were too young to be vaccinated.
Unless society eliminates a disease, it’s important to keep immunizing people. If the protection given by a vaccination is removed, more and more people will become infected and spread diseases to others. In time, diseases that today are almost unknown and rare in the United States, such as polio, could return.
And though vaccinations benefit everyone, they do not offer a full guarantee that a vaccinated person won’t get a disease if exposed to an infected person, Dr. Singh says.
“Even those who have been vaccinated are at increased risk from those not vaccinated,” she says. “That’s been shown clearly with measles and whooping cough. By being in proximity or near an individual who has not been vaccinated, you are at a higher risk.”