Pennsylvania’s measles outbreak, by the numbers.
Pennsylvania’s measles outbreak began in late April. As of September 9th, the state had confirmed 624 cases in 37 counties, and 109 people had been hospitalized. More than 40% of cases are in Lancaster County, according to the state’s dashboard.
Pennsylvania providers gave more than 46,000 doses of the measles, mumps and rubella (MMR) vaccine in August. A typical month sees about 25,000.
Pennsylvania has also confirmed 4 measles cases in people appropriately vaccinated for their age. Those 4 make up under 1% of the state’s total. More than 99% of cases, and every hospitalized patient, were unvaccinated or under-vaccinated.
Two doses of MMR vaccine are 97% effective. When the virus reaches hundreds of people, a few of the vaccinated will get sick.
What happens if someone gets measles after MMR vaccination?
They usually get a milder illness. A CDC analysis of 4,056 U.S. measles cases from 2001 to 2022 found that 12% were breakthrough infections. Severe disease became less common with each vaccine dose. Vaccinated people also spread it less. A second CDC study estimated that each unvaccinated case infected 0.76 people on average. Each two-dose case infected 0.27.
Newborns are too young to receive the MMR vaccine due to the immaturity of their immune systems and because the mother’s antibodies interfere with vaccine effectiveness. It is those antibodies that cross from mothers to the fetus before birth that protect babies in the first couple of months of life. But because it isn’t safe to give the MMR vaccine during pregnancy, the mother’s immunity has to be in place before she conceives.
The CDC says an MMR dose within 72 hours of exposure may still prevent measles. It may also lessen the severity of disease. In a Cochrane review, a dose after exposure prevented about 74% of cases. Anyone 6 months or older without immunity can get it. It works best after brief contact, such as at school, but for household contacts. Babies under 6 months, pregnant women, and people with very weak immune systems can’t get the MMR vaccine because it’s a live virus vaccine. They can get a shot of donated antibodies, called immune globulin, within 6 days. People who miss the 72-hour window can also get immune globulin.
Does vitamin A help?
HHS Secretary Robert F. Kennedy Jr. has promoted vitamin A for measles, including cod liver oil, since the Texas outbreak in 2025.
Vitamin A is used to treat measles, but it doesn’t prevent infection. The body needs vitamin A for immune function and to maintain the lungs and other organs. Measles lowers vitamin A levels in the blood, even in children who were well nourished. During a 1989 to 1990 outbreak in Milwaukee, 72% of 114 young children with measles had low vitamin A levels, and the sickest children had the lowest. In a South African trial, children given vitamin A recovered from pneumonia in about 6 days, versus 12 days for children given a placebo. The World Health Organization recommends it for all children with measles. So do the American Academy of Pediatrics (AAP) and the Centers for Disease Control and Prevention (CDC).
The dose depends on age. According to the AAP, babies under 6 months should receive 50,000 units, babies 6 to 11 months get 100,000, and older children get 200,000. Children are given the first dose at diagnosis and a second the next day. It is not recommended to check a blood test first. A child with signs of vitamin A deficiency, such as night blindness or dry eyes, gets a third dose 2 to 6 weeks later. Trials of this regimen have reported no toxicity. Clinicians still watch for headache, nausea, and blurred vision, which are signs of too much vitamin A. In the hospital, they may also trend liver function tests.
The strongest evidence for vitamin A supplementation comes from low-income countries, where many children be deficient. In a Cochrane review of trials there, two high doses cut the risk of death by about 80% in children under 2. One dose showed no benefit.
During a national shortage in Italy, doctors in Naples couldn’t give vitamin A to every child hospitalized with measles. They gave vitamin A to 36 hospitalized children and standard care to 72 others. The treated children did no better. Both groups had fevers of about the same length, similar lengths of hospital stay, and similar rates of complications.
Too much vitamin A can be toxic to children, because the body stores the extra in the liver. In 2025, Covenant Children’s Hospital in Lubbock, Texas, treated children for measles complications and vitamin A toxicity, including abnormal liver function. Many families had been giving cod liver oil and supplements at home. Poison centers logged 86 vitamin A exposures in children from January through March 2025, up almost 40% from the previous year.
Pennsylvania hasn’t reported how many of its 109 hospitalized measles patients have received vitamin A.
What vaccines do kids need after measles?
Measles keeps causing harm after the rash fades by erasing part of the immune system’s memory of other infections. A 2019 study in Science showed measles wiped out 11% to 73% of the antibodies children already had. An earlier study from England, Wales, the U.S., and Denmark tied measles to higher death rates from other infections for two to three years afterward, particularly bacterial pneumonia and diarrheal illness.
Recovered children still need to complete the recommended childhood vaccine schedule. A child with lab-confirmed measles is immune to measles and doesn’t need another measles dose, but measles infection doesn’t protect against mumps or rubella, so an unvaccinated child still needs MMR vaccination. A child who received immune globulin must wait before MMR vaccination: 6 months after an intramuscular shot, or 8 months after an IV dose.
Pneumococcal and Hib (Haemophilus influenzae type b) vaccines target bacteria that often cause pneumonia after measles. It wouldn’t be unreasonable to repeat those vaccines after a measles infection. No U.S. health agency tells doctors to repeat vaccines a child already got, and a 2025 review in Pediatrics found too little data to support revaccination. In the setting of the current outbreak or future outbreaks, researchers could study whether restarting the childhood vaccination schedule or administering pneumococcal and Hib boosters reduces the risk of bacterial pneumonia in the 2 to 3 years after measles.
Clinicians should educate parents about the increased risk of bacterial pneumonia and diarrheal disease after a measles infection and the need to present promptly if the child develops symptoms. The Pennsylvania Department of Health has a record of every confirmed case and could coordinate with clinicians to provide active follow-up during the 2 to 3 years of higher risk. Pennsylvania has confirmed more than 600 measles cases this year. Those patients may face a higher risk of other infections into 2028.