Before vaccines, nearly every child in the United States caught the virus. Today, however, public health experts emphasize that while natural immunity does occur after infection, the risks of measles are far too high when compared to the protection offered by the measles, mumps and rubella (MMR) vaccine.

Early 2025 data show rising cases in the U.S., a reminder that measles still spreads quickly wherever vaccination rates dip.

Natural immunity and the role of vaccines

“Natural immunity is wonderful. It's part of the evolution of the human body that has taken all our exposures and creates a memory for them, so that we can protect ourselves in the future. Vaccines augment (improve) that. There are some infections that an individual doesn't have to go through, or are so severe to go through, that receiving a vaccine is more valuable,” says Doug Kasper, MD, infectious disease specialist with OSF HealthCare.

He notes that measles is one of those infections.

“Measles is highly infective and has potential complications in our child population. The key with measles is that the vaccine is so effective at preventing infection. It's unanimously beneficial to receive and has been that way for a long time in the United States.”

For older generations who remember having measles as kids, it can be easy to assume it wasn’t serious. But modern evidence paints a different picture: measles can lead to pneumonia, hospitalization, brain inflammation, long-term complications and death – even in healthy children.

Why measles vaccination remains essential for kids

“Fortunately, measles vaccination remains part of the normal pediatric vaccine schedule. It's something that would be discussed with the parent regarding their child at a standard visit,” Dr. Kasper says. “We strongly recommend all children receive all their childhood vaccinations, which includes the measles, mumps and rubella (MMR) vaccine. The timing of that vaccine begins at one year of age, but it can sometimes be given earlier if there is a local outbreak of measles in the community.”

MMR vaccination rates need to remain extremely high, typically above 95%, to prevent outbreaks. However, declining vaccination rates across parts of the Americas have caused concern. The Pan American Health Organization recently warned that the region may lose its measles elimination status without urgent action.

Vaccines build a “library” of protection

The goal of vaccination isn’t to replace the body’s defenses – it’s to strengthen them.

“In the hospital, we tend to see people that get sicker, so that's what we focus our efforts on. But a lot of medicine is done outside of the hospital, and that is preventive medicine,” Dr. Kasper says. “So, the strategy with vaccine is to try not to get sick or try to get less sick than you would've. Use the utility of the vaccine, along with natural immunity, to build up your individual reservoir as much as possible to protect you in the future.”

This “reservoir” or “library” of immunity is especially important with a virus as contagious as measles, which can linger in the air for hours and infect up to 90% of unprotected people who are exposed.

Localized outbreaks are driving current cases

While measles is not widespread across the U.S., localized outbreaks continue to occur in communities where vaccination rates have dropped. Public health departments monitor these cases closely and respond with tools such as:

• Rapid case investigations
• Community education
• Mobile vaccination units
• Early MMR vaccination for children who qualify

Without high vaccination coverage, these small outbreaks can grow quickly. That’s why maintaining strong MMR rates is essential – to protect children, prevent community spread and preserve decades of progress toward measles elimination.

For more information on measles symptoms and long-term risks, visit the OSF Newsroom.

Video Interview Clips

The respiratory viral infection, although thought eliminated in the U.S. in 2000, has reemerged, especially among unvaccinated individuals. As of early April 2025, the Centers for Disease Control and Prevention (CDC) has reported more than 600 confirmed cases of measles.

Measles is extremely contagious

"The key to measles is how highly infectious it is. Each person that has measles will infect between 12 and 18 people in an unvaccinated situation," says Doug Kasper, MD, an infectious disease specialist with OSF HealthCare. "The key for controlling measles is widespread vaccine use."

For reference, each influenza patient, on average, will infect one to two others.

The measles vaccine is normally offered once a child has reached one year of age. It's a two-shot series, which came on the market in 1963. Dr. Kasper says this gives medical experts decades of data that show a correlation between people receiving the vaccine, and close to zero circulation of the virus in the United States.

The states with measles outbreaks are in unvaccinated people, and no breakthrough cases (in those who have received the vaccine) have been reported, Dr. Kasper adds.

The long-term risks for unvaccinated measles patients 

"There is emerging information that vaccination not only protects an individual from not becoming ill or having less severity of illness with a viral exposure, whether that's measles, chicken pox, influenza or COVID-19, but that there are long-term risks from viral infections that we do not appreciate in the moment," Dr. Kasper says. "Some of these have been associated with memory loss or dementia as somebody ages. Some of these are associated with organ disease like hepatitis or kidney disease."

Subacute Sclerosing Panencephalitis (SSPE)

SSPE is a rare, deadly degenerative disease of the central nervous system that can happen seven to 10 years after a measles infection. Young children, pregnant women and immunocompromised people are the most at-risk for SSPE. One to three out of every 1,000 children who become infected with measles will die from respiratory and neurologic complications, the CDC says.

SSPE symptoms happen over four stages.

-          Stage 1: Personality changes, mood swings or depression. There may also be fever and headaches, this stage can last up to six months

-          Stage 2: Muscle spasms and uncontrolled movement problems. Loss of vision, dementia and seizures can occur

-          Stage 3: Twisting movements and rigidity. Sometimes death

-          Stage 4: Serious brain damage, including areas of the brain that control breathing, heart rate and blood pressure, leading to coma and death

Immune amnesia

Another long-term impact of measles is known as immune amnesia. The American Society for Microbiology (ASM) calls immune amnesia “one of the most unique and most dangerous features of measles pathogenesis.” The extremely rare condition causes people’s immune systems to “forget” how to fight off infections and makes the person more susceptible. The ASM determined it normally takes two to three years after a measles infection for protective immunity to return.

A measles outbreak was declared in Northeast Illinois in early 2024 and declared “over” by the Illinois Department of Public Health (IDPH) in June. The expectation of statewide health experts is that measles will return to Illinois in the coming months. This is mainly due to heavy travel through Chicago airports and summer festivals. County health departments have now been tasked with keeping an eye on their measles vaccine rates and identifying areas where vaccine rates are low. 

Peoria County's measles vaccine rate is quite high, around 95%. But outlying communities in central Illinois have lower vaccine rates, normally more rural counties. Health experts at Michigan Medicine call the Great Lake State “ripe for a measles outbreak,” as only 66% of toddlers receive the recommended childhood immunizations.

Measles symptoms 

"Measles typically presents with a high fever, typically much higher than what we'd see with seasonal colds or influenza. Fevers can be 104 or 105 degrees Fahrenheit," Dr. Kasper says. "That's followed by a runny nose and a characteristic rash. This typically develops on the head and then spreads down the rest of the body."

“We don’t want to go back”

"Measles at its peak, in the 1950s before the vaccine, led to a significant number of hospitalizations in young children. Around 50,000 hospitalizations were estimated per year and about 500 deaths," Dr. Kasper says. "We don't want to go back to that scenario in any capacity, where we have a large number of unvaccinated people. The outcomes could be too severe." 

Immunocompromised people like those who have received an organ transplant, cancer patients or those on long-term therapies that could impact their immune system, are at high risk for measles. 

Why vaccines are so important right now

Summer travel along with large crowds at places like airports and theme parks, will likely bring more measles cases to the United States. 

"This time, as good as any time, is to make sure somebody is updated on their vaccinations. If somebody isn't sure if they were vaccinated as a child, they should talk to their primary care provider about testing or receiving the vaccine. If someone is immunocompromised and they might be at a higher risk for measles through travel in areas that have exposure, they should talk to their primary care provider about the vaccine," Dr. Kasper says. "If someone has young children or is at an age where vaccine is recommended, we wholly recommend for them to discuss with their provider to remain up to date on their vaccine series."

Vaccine hesitancy continues to be a topic of discussion, one that Dr. Kasper recommends patients openly discuss with their medical provider. 

Natural immunity is not a viable option with measles. The highly infectious nature of the virus, along with the severity of symptoms and potential for long-term complications, make it an illness people need to try and avoid entirely. 

To choose a primary care provider, visit the OSF HealthCare website.

Video Interview Clips

Measles can spread through respiratory droplets (like when you cough and sneeze), and it only takes a small amount of the virus to cause infection in others. 

"It can remain in an area for an hour or two after the infected person has left the area. The difference: in a setting that you have many people in a convalescent center, social center or shelter, you'd be concerned about exposure to the unvaccinated people," says Doug Kasper, MD, an infectious disease specialist with OSF HealthCare. 

The World Health Organization (WHO) calls measles a "serious airborne disease,” reporting 107,500 measles deaths globally in 2023. Most deaths were in children under the age of five. The WHO partially blames the COVID-19 pandemic for the reemergence of measles globally, saying immunization rates and surveillance declined because of it. In 2023, 10.3 million measles cases were reported worldwide, a 20% increase from 2022. 

In 2024, the Centers for Disease Control and Prevention (CDC) reports 280 measles cases have been reported in the United States from 32 jurisdictions, including Washington D.C.

Sixteen outbreaks have been reported this year, compared to four in 2023. Outbreaks are when there are three or more related cases. The CDC adds that 40% of the reported 280 cases ended up in the people being hospitalized.

Who is most at risk?

"The particular populations that we are most concerned about if they were to become exposed to measles are pregnant women or children under the age of five. They can have more severe clinical outcomes," Dr. Kasper says. “We are not seeing any concern for measles in Central Illinois. There has been sporadic reporting of cases by the Illinois Department of Public Health out of Chicago. But the unique details of those cases have not been shared so far. Where did the people travel from? Had they been previously vaccinated? Or were there exposures in the household? We await some guidance if there is a unifying feature among them.”

Measles outbreaks in Illinois

This fall, the Pan American Health Organization, the WHO'S branch in the Americas Region, issued an epidemiological alert for measles cases in the Americas, with the U.S. accounting for the most cases. 

Illinois and Minnesota have reported the most measles cases in the U.S. this year. 

In March, the Chicago Department of Public Health (CDPH) reported a measles outbreak in a migrant shelter in the city's Pilsen neighborhood, located in the Lower West Side. The CDPH considered the outbreak "contained" in late May, crediting a rapid vaccination response. 

Vaccine mitigation

One dose of the measles, mumps and rubella (MMR) vaccine is 93% effective, while two doses jump to 97% effective. If you've been vaccinated against measles, you're unlikely to have any symptoms at all. The measles vaccine is given alone or often combined with vaccines for mumps, rubella and/or varicella.

"Fortunately, we have widespread vaccination as a standard part of medical care in the United States. As children we receive two shots of the measles, mumps and rubella (MMR) vaccine starting at age one. The second shot will be before entering school at age four or five. The vaccine is highly effective at preventing measles. So, while measles is highly infectious, we have a nationwide response to prevent outbreaks of infection,” Dr. Kasper says.

The recommendation is to complete your childhood vaccine series, which Dr. Kasper calls "very safe, durable and long-lasting." 

Measles symptoms

Symptoms for unvaccinated people: 

• Upper respiratory infections (resulting in a cough or runny nose)
• Followed by fever or rash – fever can be very high (103-104 degrees) 
• Blue spots in the mouth or along the gumline (seen a week into infection)
• Red and watery eyes
• Small white spots inside the cheeks  
• Illness can lead to meningitis or encephalitis

Because of the potential lead to meningitis or encephalitis, Dr. Kasper says getting vaccine protection is the definitive recommended option instead of developing natural immunity. He adds neurological effects down the road are also a concern. 

If families with children under one year of age plan to travel outside the country or to a large metropolitan area in the U.S. with low or no measles vaccination rates, parents should speak to their child's pediatrician. The child's doctor may recommend starting the measles vaccinations as early as 6 months of age. Currently, Dr. Kasper says there is no concern for widespread measles activity in schools, senior living communities or the general public. 

Video Interview Clips