Herd Immunity is a term that takes into consideration an entire community, and when a large percentage of the population is vaccinated against a disease, its spread is limited.
According to website for The College of Physicians of Philadelphia, this indirectly protects unimmunized individuals, including those who can't be vaccinated and those for whom vaccination was not successful. This is the principle of herd immunity.
"If that disease starts to hit the population, it doesn't have anywhere to go," said Karen Yeargain, communicable disease coordinator for the Crook County Health Department. "So many people are immune that with exposure, they don't get infected, so they don't pass it on."
She added that this dynamic benefits communities because there are people who aren't able to get immunized for a number of reasons.
"We need to protect them by not having them exposed to disease," she added.
A vaccine is a product that stimulates a person's immune system to produce immunity to a specific disease. Vaccines are usually administered through needle injections but can also be administered by mouth or sprayed into the nose.
According to the Centers for Disease Control website, Japan had a successful pertussis, or whooping cough, vaccination program in 1974, with nearly 80% of Japanese children vaccinated. That year only 393 cases of pertussis were reported in the entire country, and there were no deaths from pertussis. But then rumors began to spread that pertussis vaccination was no longer needed and that the vaccine was not safe, and by 1976 only 10% of infants were getting vaccinated. In 1979, Japan suffered a major pertussis epidemic, with more than 13,000 cases of whooping cough and 41 deaths. In 1981, the government began vaccinating with a cellular pertussis vaccine, and the number of pertussis cases dropped again.
Yeargain explained that there are two ways to get immunity — natural immunity and vaccinations. She added that "natural" immunity consists of having the disease — which she said is not so natural, given the consequences of having severe diseases.
"That's why I don't like the term 'natural immunity,' which means you had the disease," she said. "When you get the disease, you have a risk of having it severe, rather than mild, and you have a risk of having big complications or even dying. That's not a good balance in how to get immunity. It's much safer and better to plan ahead by getting vaccinated for the diseases we can."
Common vaccines include pertussis, diphtheria, measles, meningitis, tetanus, polio and mumps.
"Those should be terms that most people are familiar with, and those are the common vaccines that we give to our young kids, so we are planning ahead. We are providing them immunity from a weakened version of that disease so they can build an immune memory to it," said Yeargain.
She added that by having that memory already in place, it protects people from coming down with a disease and exposing others who can't fight it off.
"By being vaccinated, it is kind of like putting a wall between disease, you and your community," she said.
Herd immunity helps to minimize the spread of disease in the community and protect those who could have severe outcomes if they contract diseases.
The College of Physicians of Philadelphia website went on to say that in diseases spread from person to person, it is less common to maintain a chain of infection when much of the population is vaccinated. As the number of those vaccinated increases, the protective effect of herd immunity increases. Depending on the contagiousness of the disease, vaccination rates need to be as high as 80% to 95%. This percentage is called the herd immunity threshold.
When only a small percentage of a community's population is vaccinated, the risk of disease outbreak is greater than if a higher percentage is vaccinated. Of additional importance, unvaccinated people are not indirectly protected, so each community member has a higher risk of becoming infected.
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