News & Views: Herd Immunity and Vaccine Duration
Published on in Vaccine Update for Healthcare Providers
Published on in Vaccine Update for Healthcare Providers
Some believe that the lack of vaccine boosters given to adults provides evidence that herd immunity is a myth. So let’s look at this notion more closely.
Herd immunity is the concept that when most people in a community are protected against a disease, everyone in the community is better protected. A highly vaccinated community means that fewer people are available to spread viruses and bacteria, which results in lower levels of disease. Herd immunity does not apply for diseases in which person-to-person spread is not a means of transmission, such as tetanus.
While the general concept of herd immunity is the same for all transmissible diseases, the specifics of herd immunity vary depending upon the disease and vaccine used to prevent it:
When we put vaccine and disease factors together, each disease then has its own potential for the community to benefit from herd immunity. If you use the example of a ticket system, each vaccine-preventable disease, except tetanus, would be stopped in a community with only a certain limited number of “free-ride” tickets. Because some people in a community will be unable to get vaccinated for reasons such as age or health status, they will use these tickets. Likewise, people who choose not to immunize and those whose immunity is not protective will also be ticket holders. The more free-ride tickets in the community, the more likely the disease will enter the community. The diseases that can afford the fewest number of free-ride tickets before outbreaks occur are measles and pertussis. Because more and more people are asking for free-ride tickets, herd immunity has eroded and measles and pertussis outbreaks are occurring. Some authors refer to this problem as “the tragedy of the commons.”
Adults do not require as many immunizations as children because they are often immune to the diseases of childhood. For some, it is because they are old enough to have been exposed to the disease. For others, immunity is the result of vaccinations received earlier in life. However, because children often receive booster doses, people sometimes wonder why adults do not as well. The lack of need for booster doses in adult can be for one of several reasons.
Factors affecting the need for booster doses can be divided into those related to the disease and those related to the vaccine.
In summary, various factors make the potential for herd immunity different for each pathogen. In addition, whether or not booster doses are necessary depends upon both disease- and vaccine-specific characteristics. Therefore, the fact that booster doses are not typically necessary in adults cannot be used to prove or disprove the concept of herd immunity. A good rule of thumb when evaluating statements for accuracy is that broad, general statements often overlook nuances important in understanding a particular issue. So, while it might seem to make sense at face value that the lack of adult booster doses means herd immunity is a myth, taking time to explore the different aspects of the statement is important in sorting out whether the statement may be true.
Contributed by: Charlotte A. Moser, MS, Paul A. Offit, MD
Categories: News and Views About Vaccines
Materials in this section are updated as new information and vaccines become available. The Vaccine Education Center staff regularly reviews materials for accuracy.
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