What is “Herd Immunity”?


The term herd immunity (or “herd effect”) is often used to describe a reduction in incidences and outbreaks of a particular infectious disease in a population, after a sufficient percentage of that population has been immunized against that disease. As more people are immunized, the number of people who can become infected – and thereby pass on that infection to other people – is reduced.


The herd effect provides indirect protection to those who have not yet been immunized (like, for some diseases, infants under 2 months), those who cannot be immunized (like those with severe allergies to components of some vaccines), and those who have a compromised immune system (such as those undergoing chemo- or radiation therapy, or have a disorder like alymphocytosis).

Does it work?

We know that when a sufficient number of people are immunized against a disease, there will be fewer incidences of that disease, and the occurrence of outbreaks will also be limited. The greater percentage of a population immunized, the better. We also know that immunizing a sufficient percentage of a population (between 75 – 95% depending on the disease) will result in the eradication of that disease.

But, how do we know this? Fortunately, the history of vaccines is relatively recent and has provided us with observable, measurable evidence. The introduction of the conjugate vaccines against pneumococcal and haemophilus infections provide impressive evidence. In those examples, the rates of infection in people who were too old to receive the vaccine equaled at least one third of the total reduction of infection. Another great example is smallpox. So far, smallpox the only disease to have been eradicated from the entire planet. Yet, by the time smallpox was eradicated in the 1970s, not every person on the planet had been vaccinated against it yet. The herd effect has protected the rest, thereby rendering the smallpox virus extinct.

Can I just rely on herd immunity, instead of risking injury by vaccination?

The lawyer’s favorite answer is always, “it depends.” In this case, the more individuals who believe they can rely on herd immunity, the less likely it will be that they can rely on herd immunity. Also, different diseases have different percentages of a population who are required to be immunized (the “threshold”) to consider the disease eradicated. Measles, for example, has a relatively high threshold of 93-95%.

Because of widespread vaccination, endemic measles was considered to have been eradicated in the United States by 2000. However, because measles was not eradicated globally and rates of immunization in the United States drop periodically, several outbreaks have occurred since then. These outbreaks have often started in populations who chose not to immunize, and then spread to those who cannot be immunized, such as those too young to receive the MMR vaccine.

The relative risks of vaccination should also be considered. In an average group of one million people, 139 will die in a motor vehicle accident, but less than 1 will suffer serious allergic reaction to the ingredients in an MMR vaccine. However rare, though, injuries do still happen. If you have experienced an injury which you suspect is caused by a vaccine, you should contact an attorney with experience in the Vaccine Injury Compensation Program to help you determine if you can be compensated.


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