Measles and Herd Immunity: What It Is and How It Protects Our Community
March 25, 2025
Robin Oliverio, Ph.D.
This is the third story in a series of articles opens in a new window related to the recent measles outbreaks in the U.S.
The recent measles outbreaks 
in the United States serve as an important reminder of the power of vaccinations and the role of community protection in the fight against infectious diseases.
Herd immunity occurs when enough people are immune to a virus, which helps to significantly limit its spread. Think of each potentially infectious person as a stepping-stone for the virus. The more immunized people there are, the fewer stepping-stones, and the virus is unable to spread further. The percentage of individuals required for herd immunity varies by virus and depends on how contagious it is.
Because measles is considered highly contagious , experts recommend that at least 95% of the population be vaccinated to maintain herd immunity. But what happens when vaccination rates fall below that threshold? What are the consequences of weakened protection across an entire community?
How the Body Reacts to Viruses
When your body encounters a virus, your immune system collects identifying information about it, a unique structure called an antigen . Once your immune cells recognize an antigen, they produce specialized proteins known as antibodies , which helps your body eliminate the virus.
While getting sick with measles allows your body to naturally develop antibodies to fight off future infection, the risks associated with the virus are severe and can lead to pneumonia and inflammation of the brain . In fact, one in four individuals who contract measles require hospitalization.
Vaccines offer a safer alternative. They work by exposing the body to a piece of the virus –the antigen – so the immune system can develop antibodies without the risk of illness. The measles vaccine, typically administered as part of the measles, mumps, and rubella (MMR) vaccine and given between 12-15 months old and again between the ages of 4–6 years, contains an attenuated (weakened) form of the virus.
This weakened form of the virus does not cause measles symptoms but instead trains the immune system to recognize and fight the virus if it’s encountered in the future. An easy analogy to consider is to think of it as showing your cells a “Wanted” poster of the virus so they can recognize and eliminate it before it causes harm.
Achieving Immunity as a Community
The MMR vaccine is highly effective – 97% of people who receive both doses are protected against measles for life. Vaccinated individuals who do contract measles tend to experience much milder symptoms than those who are unvaccinated . Click here for more information on the importance and effectiveness of vaccination.
However, not everyone can get vaccinated. Some individuals – such as cancer patients undergoing chemotherapy – have weakened immune systems and cannot safely receive live vaccines. Others may lose immunity over time due to medical conditions. For these individuals, herd immunity is crucial. When a high percentage of the population is vaccinated, it helps to prevent the virus from spreading and therefore protects those who are most vulnerable.
The risks of low vaccination rates are real. In 2015 , a person with a compromised immune system died after being exposed to measles during an outbreak of just five cases in Washington.
Where Are We Today?
So far in 2025 , 42% of reported measles cases in the United States have occurred in individuals between the ages of 5 and 19, while 35% have been in children under 5. Early childhood is a critical period for measles protection, as young children are particularly vulnerable to severe complications.
Vaccination rates have declined in Allegheny County (click here for more information ). The most recent data show that 94.7% of kindergarteners receive the MMR vaccine – just below the 95% threshold needed for herd immunity.
As vaccination rates decline both nationally and locally, the number of measles cases increase. This year, a child died from measles . In Pennsylvania , at least two cases of measles have been reported. With two cases in the state since the start of 2025, it’s a good reminder about the importance of herd immunity and a reason to improve and maintain vaccination rates.
Vaccination is the safest and most effective way to stop measles from spreading and to ensure our community remains protected.
About Robin Oliverio, Ph.D.
Robin holds a Ph.D. in neuroscience and is passionate about bridging the gap between science and policy. She focuses on using research to inform evidence-based policies that drive positive societal change. She is strongly committed to projects that make a meaningful difference in people’s lives, and she thrives in roles where science meets advocacy, communication, and impact.
If you have any questions or would like a particular topic to be featured, please reach out to Dr. Oliverio, science communications & media specialist, at OliverioR@CarnegieScienceCenter.org