Causes and Risk Factors of Measles

Measles is an extremely contagious viral infection caused by exposure to a person infected with the virus. Sneezing, coughing, and talking can spread it, but the virus can even live on surfaces and the air for a limited time—long enough to infect someone new. Before routine measles immunization began in the United States in 1963, there were about 3 to 4 million cases of measles each year. Though vaccination has virtually made measles a worry of the past in the United States, it is still a concern in other countries. Outbreaks all over the world still occur and those who are not immune to the virus can put themselves and others at risk.

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Common Causes

Measles is caused by an extremely contagious virus called paramyxovirus that replicates in your throat and nose. It's spread through respiratory droplets when an infected individual sneezes, coughs, or even talks. The virus can live in the air and on surfaces for up to two hours after a person with measles symptoms has left the area. It invades your respiratory system, causing fever and flu-like symptoms, and then spreads throughout your body. As your antibodies attack the virus, damage to the walls of tiny blood vessels occurs, leading to the measles rash.

An infected person is contagious for around eight days—four days before through four days after the measles rash appears. Measles is so contagious that one infected person who is exposed to 10 people who aren't immune to measles will infect 9 of the 10.

Approximately 20% of measles cases require hospitalization and even more go to the doctor or the emergency room because of the high fever. This can put other people in those settings, particularly those with immune system problems, at risk if they are not carefully separated. Unfortunately, when parents take their kids with measles for medical attention, they rarely suspect that they have measles and expose many people to their children when they are most contagious.

Being vaccinated for measles doesn't just protect you from getting sick, it also stops you from spreading measles to others. Unvaccinated people continue to travel to other countries where measles is common and bring it back here, spreading it to others. Despite widespread concerns about the link between vaccination and autism, numerous studies have shown that there doesn't appear to be any association between the two.

Measles After Vaccination

There is another more severe form of measles called atypical measles. This occurs in people who were immunized with the first measles vaccine between 1963 and 1967, which contained killed or inactive virus. Because they didn't develop full immunity, these people can still contract the virus when exposed to someone with measles. The symptoms are more severe and usually begin with a high fever and headache. The rash usually starts on the wrists or ankles instead of the face and head, and it may never get to the trunk at all. This form of measles seems to not be contagious and is pretty rare now.

Most people who have had both measles, mumps, and rubella (MMR) vaccines are immune to the measles, though around 3 out of 100 people who've had both doses may still get measles if they're exposed to it. Health experts aren't sure why this is, but it could be because some people's immune systems just don't respond to the vaccine well. However, if you have had your vaccinations and you still get measles, known in these cases as modified measles, the illness will most likely not be as severe. It's less contagious, too.


An outbreak occurs when more cases of a disease occur in a community, geographical area, or season than are normally expected. Several factors help to limit outbreaks of measles in the United States, though we've seen more of them in the past decade. Most important is the fact that despite the talk of personal belief vaccine exemptions and anti-vaccine parents not getting their kids vaccinated, we still have high population immunity.

In the United States, 91.9% of children get at least one dose of the MMR vaccine by the time they are 35 months old and 90.7% of teens have had two doses. While not perfect, that is still far higher than the many other immunization rates around the world.

Instead of overall low immunization rates, like many countries have, the United States has clusters of intentionally under-vaccinated children. It is in these clusters and communities that outbreaks typically occur.

In 2019, the United States saw a major outbreak of measles, with 1,250 confirmed cases between January and October. While incidents were reported across 31 states, 75% of them occurred in New York, largely within unvaccinated communities.

Prior to that, the biggest outbreak was in 2014 with a documented 667 cases of measles reported across 27 states. The biggest regional outbreak, affecting 383 of these 667 people, occurred in primarily unvaccinated Amish communities in Ohio. Many of these cases turned out to be linked to the Philippines, where there was also a large outbreak of measles.

Very few of the measles cases in these outbreaks are in people who are completely vaccinated. For example, in the outbreaks in Europe in 2011—when 30,000 people got measles, causing 8 deaths, 27 cases of measles encephalitis, and 1,482 cases of pneumonia—most cases were in unvaccinated (82%) or incompletely vaccinated (13%) people.

In addition to many developing countries where measles is still endemic, international measles outbreaks have been reported in Japan, the United Kingdom, the Philippines, and other countries, which makes it important to make sure you are fully vaccinated before traveling out of the United States.

Anatomy of an Outbreak

A closer look at a measles outbreak in San Diego, California, in 2008 can help you understand even better what happens during one of these outbreaks and how many people one infected person can expose.

A 7-year-old who was unvaccinated because his parents had a personal belief vaccine exemption traveled to Switzerland with his family. A week after returning home from the trip, he got sick but returned to school after a few days. He then developed a rash and saw his family physician, followed by his pediatrician, and then made a trip to the emergency room because he continued to have a high fever and rash, both classic measles symptoms.

He was eventually diagnosed with measles, but not before 11 other children were infected with measles too. This included two of his siblings, five children in his school, and four children who picked it up at his pediatrician's office.

It's not as simple as that, though. During this measles outbreak:

  • Three of the children who became infected were younger than 12 months of age, and therefore too young to have been vaccinated.
  • Eight of the nine other children who were at least 12 months old were unvaccinated because they had personal belief vaccine exemptions.
  • About 70 children were placed under voluntary quarantine for 21 days after their last exposure because they were exposed to one of the measles cases and either didn't want to be vaccinated or were too young.
  • One of the infants with measles traveled to Hawaii, raising fears that the measles outbreak could spread there too.

Altogether, 839 people were exposed to the measles virus beginning with just one infected child.

One of them was a 10-month-old infant who got infected at his well-child checkup, was too young to have gotten the MMR vaccine yet, and ended up spending three days in the hospital in life-threatening condition.

Common Risk Factors

Being a young, unvaccinated child is the biggest risk factor for contracting the measles virus and for developing complications. If you are exposed to the measles virus and you haven't been vaccinated, your chance of getting it is 90%, no matter your age.

Other common risk factors for measles include:

  • Babies who are unvaccinated because they are too young: The measles vaccine doesn't work for infants because their immune systems haven't developed enough to create the needed protective immunologic response to the vaccine. For infants who are going to be traveling outside the United States, it's recommended that they receive one dose of their MMR at 6 to 11 months of age rather than waiting until the traditional 12 to 15 months.
  • People who are unvaccinated for medical reasons: Some people are unable to get the MMR vaccine because of issues such as being immunocompromised or the fact that they are taking certain drugs, like cancer chemotherapy or high doses of steroids.
  • Being incompletely vaccinated: Those who haven't received a second booster dose of MMR do not have full immunity to measles. Most kids don't receive their booster shot until ages 4 to 6. As an example, the first dose of Merck’s M-M-R II vaccine is around 93% effective, but the second one is 97% effective.
  • Being fully vaccinated but not developing immunity: This happens in approximately 3% of vaccinated people.
  • People who are immunocompromised: This is true even if they had previously received the MMR vaccine.
  • Vitamin A deficiency: This concern makes you more likely to contract measles and for the illness to be more severe.

Lifestyle Risk Factors

International travel and choosing not to vaccinate are the two lifestyle risk factors for contracting measles, and they are significant ones. Worldwide, measles is a leading cause of death in unvaccinated children under the age of 5. Before the routine use of the measles vaccine and the measles, mumps, and rubella (MMR) vaccine (1971), measles cases—and complications from those cases—were high. In some developing countries, they remain significantly higher than in the United States still today.

The difference now is that instead of being widespread in the United States as it used to be before the vaccine, almost all cases of measles are linked to traveling outside the country, particularly to developing countries. And instead of occurring in people who don't have access to vaccines, most cases now in the United States are in people who choose not to vaccinate themselves and their kids.

Frequently Asked Questions

  • Can you be at risk for measles if you were vaccinated?

    About 3% of people who are fully immunized with two doses of the measles vaccine may still be at risk for contracting the virus. If they do get measles, their illness is usually milder than cases that occur in unvaccinated people, and it seems that they are less likely to spread the virus. 

  • Are people in the United States at risk for measles?

    There is a very slight risk. While measles is considered eliminated in the United States, it's occasionally brought into the country by international visitors or unvaccinated Americans who return home with the virus after traveling outside the country. Other people in the United States, including those who cannot be vaccinated, may be at risk from exposure in those instances.

  • Can you pass measles on before you know you're sick?

    Yes. You’re contagious with measles for about four days before the distinctive rash appears, and you may mistake other early symptoms for a minor cold. In that time, the viral infection is easily transmitted through the air or via contaminated surfaces.

14 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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Additional Reading

By Vincent Iannelli, MD
 Vincent Iannelli, MD, is a board-certified pediatrician and fellow of the American Academy of Pediatrics. Dr. Iannelli has cared for children for more than 20 years.