Amid growing Texas outbreak, how contagious is measles?

Measles has been spreading across the U.S. for the last several weeks, sickening people in at least nine states amid a growing outbreak in western Texas.

Several people have been hospitalized and at least one unvaccinated school-aged child in Texas has died, marking the first U.S. measles fatality in a decade.

Some may believe that measles is a harmless childhood illness that causes a fever and a rash, clearing after a few days. However, it can also lead to serious health complications, especially in children younger than 5 years old.

Measles is highly contagious in a totally unprotected group. One infected patient would be able to spread the illness to an average of 18 people.

How contagious is measles?

Measles is one of the most contagious viruses known to humans. Just one infected patient can spread measles to up to?nine out of 10 susceptible close contacts, according to the Centers for Disease Control and Disease (CDC).

"So, if you have a bunch of unvaccinated kids in the school or in a daycare, nine out of 10 would likely be infected," Dr. Peter Hotez, professor of pediatrics and molecular virology at Baylor College of Medicine in Houston, told ABC News. "That's why you often see measles as your first breakthrough epidemic … once vaccination rates go down below 90% because this virus is so highly infectious and can transmit so easily."

Another way to describe how contagious measles is its basic reproduction number (R0), a theoretical number that suggests how many people an infected patient may infect in a totally susceptible or unvaccinated population.

The R0 for measles ranges from 12 to 18, meaning if no one had any immunity, an infected person could transmit the virus up to an average of 18 people.

"Contrast that with the seasonal flu, and we're going through a pretty severe flu season," Dr. Nicholas Cozzi, EMS and disaster medicine medical director at Rush University Medical Center in Chicago, told ABC News. "One person with the flu can infect up to three people. And so, measles, one of the most infective viruses that we have, can infect almost four times as many people as the flu."

Why is measles so contagious?

Hotez said that it takes a very small amount of virus, or a few measles virus particles, to infect somebody.

It can be transmitted through direct contact with infectious droplets or through the air when an infected person coughs, sneezes or breathes, according to the CDC.

Measles virus can linger in the air and live on surfaces for up to two hours after an infected person has left a room.

Hotez said this is why there was a recent concern when a person infected with measles visited two colleges campuses in Texas and visited establishments to have lunch and dinner.

"So, each place that individual went, you had to worry that he or she left a trail of virus in the atmosphere," he said. "So even after that individual left, another unvaccinated individual walking into the restaurant or walking into the same classroom space where this visitor was at could become infected."

Measles complications

Some people who contract measles may suffer severe complications as a result of infection. Those most at risk include children younger than age 5, pregnant people and those with weakened immune systems.

About in one in five unvaccinated people who contract measles are hospitalized and about one in 20 children with measles develop pneumonia, which is the most common cause of death in young children who get infected.

About one in 10 children infected with measles develop ear infections as well, which can lead to hearing loss, according to Hotez.

Additionally, about one out of every 1,000 children with measles will develop encephalitis -- which is the swelling of the brain and can lead to brain damage -- and one to three out of every 1,000 children with measles will die from respiratory and neurologic complications, the CDC says.

Vaccine availability drives down cases

In the decade before the measles vaccine became available, nearly all children contracted measles by age 15, according to the CDC.

The federal health agency estimates that 3 to 4 million in the U.S. were sickened by measles every year, about 48,000 were hospitalized and about 400 to 500 people died. About 1,000 people suffered encephalitis.

In 1963, the first measles vaccine became available, followed by an improved vaccine in 1968. CDC data show that cases fell from 385,165 confirmed cases in 1963 to 26,686 cases in 1973.

In 1971, the measles, mumps, rubella (MMR) vaccine became available and, in 1989, the Advisory Committee on Immunization Practices, the American Academy of Pediatrics and the American Academy of Family Physicians recommended children receive a second MMR dose.

By 2000, annual recorded cases had fallen to just 85 and measles was declared eliminated, meaning it was without continuous spread for at least 12 months.

MMR vaccine is like a shield

The CDC currently recommends that children receive two vaccine doses, the first at 12 to 15 months and the second between 4 and 6 years old. One dose is 93% effective, and two doses are 97% effective. Most vaccinated adults don't need a booster.

Cozzi, from Rush University Medical Center, described the MMR vaccine as a shield and measles as an enemy trying to pierce the shield with arrows.

"So, if you're protected and you have that vaccine, you have a shield in front of you," he said. "And if something were to get past that shield, it may infect you, but it's less severe."

He noted that people who have the measles vaccine can still be infected.

"However, just like the flu, their symptoms and their duration of illness is much less," he said.

Cozzi said this is evident in Texas, where an outbreak is occurring. As of Friday, just five of the 146 cases were vaccinated – and with just one dose of the MMR vaccine. The remaining cases are among those who are unvaccinated or with unknown vaccination status.

Rise in vaccine hesitancy, exemptions

Despite the protectiveness of the MMR vaccine, CDC data show vaccination rates have been lagging in recent years and the percentage of exemptions have risen.

An October 2024 CDC report found that vaccination coverage among kindergartners decreased from 95.2% during the 2019–2020 school year to 92.7% in the 2023–2024 school year.

While medical exemptions have held steady for the past decade between 0.2% and 0.3%, non-medical vaccine exemptions rose to 3.1% during the 2023-24 school year, which is the highest figure recorded in at least 13 years.

Cozzi said there has been an increase in vaccine hesitancy after the COVID-19 pandemic, which has seen some kindergartners not receiving the MMR vaccine. The same has been true of the polio and DTaP vaccines, with the latter protecting against diphtheria, tetanus and pertussis.

"The onus is really on us as physicians and health care clinicians to have good, honest conversations with our families and our parents, discussing all of the positive things with vaccines -- hearing them, understanding their concern -- but still providing that proactive nature to prevent all of the severe infections like measles, especially the very devastating consequences that it can have, specifically for those unvaccinated individuals," he said.