Measles explained: how it spreads, what complications it causes and how well two vaccine doses protect

On Monday, October 5, 2026, Pennsylvania’s health department said 1,004 cases of measles had been confirmed in the state this year. That makes it the largest outbreak in any US state in more than three decades. The department also reported 198 hospitalizations, and five people in the state have died.

Pennsylvania is the center of a wider resurgence. By October 1 the Centers for Disease Control and Prevention (CDC) had counted 3,887 confirmed cases this year across the country, and the US is enduring its worst year for measles since 1991. On October 5 New York also declared a state of emergency over its own cases.

Measles had become rare enough in the United States that many people have never seen a case. This explainer sets out what the disease is, how it spreads, how often it causes harm and what is known about the vaccine against it.

What measles is and how it spreads

Measles is caused by a virus that infects the airways and then travels through the body. It passes from person to person through the air when someone who is infected coughs, sneezes or simply breathes.

Two features make it unusually hard to contain. The first is persistence. A person can catch measles in a room where an infected person has been, and this can happen even up to 2 hours after that person has left. The second is timing. The World Health Organization notes that it can be transmitted by an infected person from four days prior to the onset of the rash to four days after the rash erupts. People spread it before they know they have it.

The result is that measles reaches almost everyone exposed. If one person has measles, up to 9 out of 10 people who come into close contact with that person will become infected if they are not protected.

Epidemiologists express contagiousness with a number called R0, the average number of people one case infects in a population with no immunity. For measles the textbook figure is that each person with measles would, on average, infect 12-18 other people in a totally susceptible population. A systematic review led by Public Health Ontario found that real-world estimates vary more than the often cited range of 12-18, depending on the setting.

A high R0 has a practical consequence. The more contagious a disease, the larger the share of people who must be immune to stop it circulating, a level known as the herd immunity threshold. For measles the working figure is 95%. The Guardian reported that US vaccination rates have fallen in recent years to below the 95% coverage needed to prevent outbreaks.

What measles symptoms look like, day by day

The illness unfolds on a predictable schedule. Symptoms usually begin 7 to 14 days after infection, according to the CDC. The first signs resemble a bad cold: a high fever, cough, a runny nose and red, watery eyes.

Next comes a sign found in almost no other disease. Tiny white spots (Koplik spots) may appear inside the mouth two to three days after symptoms begin.

The rash follows, 3 to 5 days after the first symptoms. It usually begins as flat red spots that appear on the face at the hairline. They then spread downward to the neck, trunk, arms, legs, and feet. Fever often peaks as the rash emerges.

People who have been vaccinated and still catch measles tend to have a milder illness. No drug attacks the virus itself. “There is no specific treatment for measles,” the WHO fact sheet says, and care consists of fluids, comfort and treating complications as they arise.

How often measles causes complications

Most people recover fully. A substantial minority do not recover cleanly, and the CDC publishes rates for the main complications.

Complication	How often, per CDC
Ear infection	About 1 in 10 children
Hospitalization	About 1 in 5 unvaccinated people in the US
Pneumonia	As many as 1 in 20 children
Encephalitis (swelling of the brain)	About 1 in 1,000 children
Death	1 to 3 in 1,000 children

Ear infections occur in about 1 out of every 10 children with measles. About 1 in 5 unvaccinated people in the U.S. who get measles is hospitalized. As many as 1 out of every 20 children with measles gets pneumonia, the most common cause of death from measles in young children.

The brain is affected less often and more seriously. About 1 child out of every 1,000 who get measles will develop encephalitis, which can lead to convulsions and leave a child deaf or with intellectual disability. Between 1 and 3 of every 1,000 children who become infected with measles will die from respiratory and neurologic complications.

Pennsylvania’s hospital figures are in line with those rates. Among the pediatric measles cases, 18% have needed hospitalization, NBC News reported. Its five deaths among 1,004 cases, which include adults and a newborn, work out to a higher rate than the CDC’s range for children. Doctors there described what admission looks like. “One of the more common things that the measles virus can do is cause hepatitis or infection of the liver,” said Evan Shirey, a pediatric infectious disease physician at Penn State Health Golisano Children’s Hospital. Dehydration in babies and brain inflammation in some children were among the other reasons for admission.

Risk is not evenly spread. The CDC lists children under 5, adults over 20, pregnant women and people with weakened immune systems as the groups most likely to have complications. Measles during pregnancy can lead to premature birth or a baby with low birth weight.

How measles weakens the immune system afterwards

Measles has an effect that outlasts the illness. The virus infects immune cells, and for some weeks afterwards a child is more open to other infections. Research over the past decade has shown the damage runs deeper than that.

In 2019 a team in Boston led by Michael Mina and Stephen Elledge examined blood from 77 unvaccinated children before and 2 months after natural measles virus infection. They used a test that reads which germs a person’s antibodies recognize. Measles caused elimination of 11 to 73% of the antibody repertoire across individuals. The children had lost part of their immune memory of other infections they had already fought off.

The phenomenon has been nicknamed immune amnesia. The losses were not permanent, since recovery of antibodies was detected after natural reexposure to pathogens, which means the children had to catch or meet those germs again. For the debate about vaccination, one detail stands out: these immune system effects were not observed in infants vaccinated against MMR, the combined measles, mumps and rubella vaccine.

Population records suggest the effect costs lives. An earlier analysis of national data from high-income countries found that measles has a more prolonged effect on host resistance, extending over 2 to 3 years, with deaths from other infections tracking the level of measles two to three years before. Host resistance means the body’s defenses against infection. The CDC’s summary is more guarded, saying this damage to the immune system might last months or even up to a few years.

The late brain complication of measles

One consequence appears long after recovery. Subacute sclerosing panencephalitis, or SSPE, is a degenerative disease of the brain caused by measles virus that has persisted in the nervous system. It is always fatal.

SSPE generally develops 7 to 10 years after a person has measles, even though the person seems to have fully recovered. It is rare. Among people who caught measles during the US resurgence of 1989 to 1991, 7 to 11 out of every 100,000 were estimated to be at risk for developing SSPE.

The risk is much higher for babies. California health officials reviewed the state’s records from 1998 to 2015, and seventeen SSPE cases were identified. Among children who had measles before the age of five, the rate was about 1 in 1,367, and among those infected before their first birthday about 1 in 609. The disease surfaced after a latency period of 9.5 years, with a range from 2.5 to 34. The authors concluded that SSPE cases in California occurred at a high rate among unvaccinated children, particularly those infected during infancy.

Infants are too young for routine vaccination in the first months of life, so their protection depends on the people around them being immune.

How well the measles vaccine works

The most thorough assessment of the vaccine is a Cochrane review, a systematic review prepared to a published protocol, last updated in 2021. Its authors included 138 studies (23,480,668 participants) covering effectiveness and harms of the combined vaccines.

On effectiveness, the review reported that effectiveness in preventing measles was 95% after one dose and 96% after two doses. The first figure came from seven cohort studies, which follow vaccinated and unvaccinated children over time, covering 12,039 children. The second came from five such studies of 21,604 children, and the reviewers rated the evidence as moderate certainty. The CDC’s figures are close: two doses of MMR vaccine are about 97% effective at preventing measles; one dose is about 93% effective.

The vaccine also has some effect when given after contact with a case. In two cohort studies of 283 children, effectiveness in preventing measles after exposure was 74%, though the reviewers rated that evidence as low certainty.

The same review counted harms. The clearest is febrile seizure, a convulsion brought on by fever in a young child. Febrile seizures normally occur in 2% to 4% of healthy children at least once before the age of 5. The additional risk from vaccination is estimated to be from 1 per 1700 to 1 per 1150 administered doses.

A second measured harm is a fall in platelets, the blood cells that help clotting, known as idiopathic thrombocytopenic purpura or ITP. The review put it at about 1 case of ITP per 40,000 administered MMR doses, and noted that the risk of ITP after vaccination is smaller than after natural infection with these viruses.

On autism, the review states that there is no evidence of an association between MMR vaccination and autistic spectrum disorders, on the basis of two observational studies covering 1,194,764 children. Vaccinated children were diagnosed at 0.93 times the rate of unvaccinated children, with a range from 0.85 to 1.01, where 1 would mean no difference.

One question the reviewers left open is durability. More evidence is needed to assess whether the protective effect of the vaccine could wane with time since the last dose, they wrote.

Vitamin A and measles treatment

Vitamin A comes up in every outbreak, sometimes as a claimed alternative to vaccination. The evidence supports a narrower role.

A Cochrane review of randomized trials in children with measles found no overall reduction in deaths when all trials were pooled. There was no evidence that vitamin A in a single dose was associated with a reduced risk of mortality among children with measles. The benefit appeared with a specific regimen and age group: two doses were associated with a reduced risk of mortality and pneumonia-specific mortality in children under the age of two years.

On that basis the WHO includes it in the care of children with measles. Two doses of vitamin A are recommended for all suspected measles cases in children under 5 years of age, given on consecutive days. The agency’s reasoning is that this restores low levels of vitamin A that occur even in well-nourished children. The trials tested vitamin A as a treatment for children who already had measles, in set doses. None tested it as a way of preventing infection.

Why measles has returned

Before a vaccine existed, major epidemics came every two to three years. Before the introduction of measles vaccine in 1963, epidemics caused an estimated 2.6 million deaths each year, according to the WHO. Vaccination changed that: CDC and WHO researchers calculated that during 2000 to 2023, measles vaccination saved an estimated 60 million lives.

Progress on coverage has stalled. Global coverage with a first dose slipped during the Covid-19 pandemic and stood at 83% in 2023. Estimated deaths remained high, falling only from 116,800 in 2022 to 107,500 in 2023. Over the same year the number of countries affected by large or disruptive outbreaks increased from 36 to 57. The WHO’s latest estimate is 95,000 deaths in 2024, mostly among unvaccinated or under vaccinated children under the age of 5 years.

In the United States the story is one of reversal. Measles was declared eliminated from the United States in 2000, meaning it no longer spread continuously within the country. Cases since then began with travelers and spread where vaccination was low. In Pennsylvania, Lancaster County remains the hardest hit, with 391 cases. “There’s nothing special about Lancaster. The exact same thing could happen in any county in the country that has a large percentage of unvaccinated people,” John Goldman, an infectious disease specialist at the University of Pittsburgh Medical Center, told NBC.

That elimination status is now in question. In November, international health officials will meet to determine if the US and Mexico have lost their measles-free status.

What the measles figures leave uncertain

The outbreak numbers are provisional and contested. Pennsylvania counts five deaths. The CDC, in a dispute with the state over how to classify them, currently lists two measles deaths in the US this year. Case counts also lag behind events and will change.

The vaccine’s effectiveness figures come from cohort studies, which compare vaccinated and unvaccinated children as they are found, and the review’s measles estimates rest on those studies, not on randomized trials. The Cochrane reviewers graded the main estimates as moderate certainty and the post-exposure estimate as low.

The CDC’s page does not give the period or population behind each complication rate, and the SSPE figures rest on the 1989 to 1991 resurgence and on 17 cases in one state. Rates could differ in a population with different nutrition, age structure or access to hospital care.

The immune amnesia study involved 77 children, and the population analysis that links measles to later deaths is a correlation across years. Neither can say how much any one child’s risk rises.

For a particular child or adult, questions about vaccination history and timing turn on age, health and exposure, which is assessed by their doctor.

Measles reaches up to nine in ten unprotected contacts, and two doses of vaccine were about 96% effective in studies that were observational and rated moderate in certainty.