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Four Pennsylvanians who were vaccinated against measles have contracted the highly contagious illness, state officials announced Friday.

Called “breakthrough cases,” such infections in vaccinated people are rare, but sometimes occur if someone’s immune system “did not respond as well” as most patients’ to the vaccine, the CDC says.

Other times, vaccinated people can become sickened by the virus if they’re exposed for a long period to the disease — like living in a house where multiple unvaccinated people have measles. Vaccinated people tend to have a milder illness when this happens, and they’re less likely to spread measles to others.

The four breakthrough cases reported in Pennsylvania occurred in three adults and one child in Centre, Jefferson, Lancaster, and Lycoming Counties.

The state said that all of them were “appropriately vaccinated for their given ages.” None required hospitalization.

Before Friday, the state had identified no cases this year in people who were fully vaccinated. The breakthrough cases reported Friday make up less than 1% of the total cases reported in the state this year.

“When community transmission rises and our overall case numbers increase, we see that a handful of people who were vaccinated will get measles. Even so, vaccination still provides the best protection,” Pennsylvania health secretary Debra Bogen said at a news briefing.

The Centers for Disease Control and Prevention estimates that about three in 100 people who received the two recommended doses of the measles, mumps, and rubella vaccine will contract the disease if exposed. About 7 in 100 people who received one dose will contract the disease.

Sarah Long, a professor of pediatrics at Drexel University College of Medicine who previously served on two federal committees to recommend and license vaccines, called cases in vaccinated people “oddities.”

“This is a fabulous vaccine and a fabulously transmissible virus,” she said, emphasizing that the vast majority of people who get measles are unvaccinated.

A growing outbreak

Pennsylvania is in the midst of the biggest measles outbreak in more than 30 years, with 676 cases reported and 124 people hospitalized as of Friday.

The state recorded 52 new cases in the last two days alone. Local and state health officials have noted that it’s likely many cases are going unreported.

The rapidly rising case count last month, especially among adults ages 25 and 49, merits attention, said Rémy Pasco, research associate at the University of Colorado who helped develop a measles outbreak simulator based on school vaccination rates. In past outbreaks nationally, cases have been more common among children who were not yet old enough to have received both MMR doses.

He said those patterns could indicate the virus reached a community where both adults and children were largely unvaccinated.

The state has reported two deaths associated with the outbreak — namely, deaths where evidence of measles is present, even when the disease may not be assessed by the coroner as the immediate cause of death.

In its latest update Friday, the CDC still had not listed the deaths on its website highlighting data on ongoing measles outbreaks around the country.

State and federal officials have clashed over whether the deaths were tied to the state’s measles outbreak.

Lancaster County’s coroner has said he investigated one death caused by measles, in an infant with a severe genetic disorder, and another in an infant who contracted measles before birth and died shortly after. The coroner has said he does not believe measles caused that child’s death, but rather a lacerated spleen, yet prominent physicians have noted measles can weaken a person’s spleen, putting it at risk of rupture.

Amid the outbreak, Pennsylvania is seeing more interest in vaccination, Bogen said Friday. Typically, healthcare providers in the Commonwealth give out 25,000 doses of the MMR vaccine a month, she said. In August, they gave out 46,000.

“That is 46,000 times Pennsylvanians have chosen to help stop this outbreak by protecting themselves and others,” Bogen said.

Breakthrough infections

People born before 1957, before the first measles vaccines became available, are generally considered protected against measles through natural immunity — because the disease was once prevalent in children.

Between 1968 and 1989, most children in the United States received one dose of the MMR vaccine, which is about 93% effective against the virus.

In 1989, measles cases among vaccinated schoolchildren prompted health officials to recommend children receive two doses of the vaccine, which together are 97% effective against the virus, a change that also prompted some older children to get a second shot.

Measles rates plummeted during the 1990s, and the disease was declared eradicated in the United States in 2000.

The MMR vaccine’s 97% effectiveness rate makes it “not perfect, but as good as our vaccines get,” said Rochelle Walensky, who served as CDC director during President Joe Biden’s administration and currently works as a professor of medicine at Harvard Medical School.

Smaller outbreaks with dozens of cases — not hundreds — may not see any breakthrough cases.

“The larger your outbreak, the more likely you’re going to see these,” Walensky said. “It’s just simple math.”

Large outbreaks, with hundreds of cases, will inevitably include breakthrough cases, she said.

Nationwide, 95% of the 3,294 measles cases logged by the CDC so far this year have been among unvaccinated people. The agency said 3% of those cases occurred in people with one dose of the MMR vaccine, and 3% in patients with two doses.

The concern also arose in last year’s major measles outbreak in Texas. A vaccinated person in Bucks County was among those who contracted a mild case of measles last year after traveling there.

Similar stories have emerged from Pennsylvania’s growing outbreak. For example, a mother in Lancaster County told The Inquirer earlier this summer that she was vaccinated and acquired the disease alongside her eight children, who were all unvaccinated; it’s unclear whether hers was one of the breakthrough cases recorded by the state.

Breakthrough illnesses can be contagious, said Paul Offit, a physician and vaccine expert at the Children’s Hospital of Philadelphia who heads the hospital’s Vaccine Education Center.

High levels of immunity in a given community protect everyone living there, Offit said. An unvaccinated person living around mostly vaccinated people is less likely to get measles because the virus has less chance to spread.

But vaccinated people who live around many unvaccinated people are at higher risk of getting a breakthrough case, due to the virus having more opportunities to circulate, he said.

Drexel’s Long recalled caring for children during a 1991 measles outbreak that sickened more than 1,000 people in Philadelphia and killed nine children.

“It’s a horrible, horrible disease. We are extremely grateful we live in the U.S. where these vaccines are developed and proved to be safe and highly effective,” she said.