Just two years after a deadly outbreak of polio raged across the country, the children lined up in the basement gymnasium of a Pittsburgh elementary school for what became the largest medical experiment in America.

One by one, they were led to tables draped in white cloth and covered with vials, waiting for their turn to be injected with a vaccine newly created to combat the catastrophic disease.

One giggled. Others fidgeted. Some of the youngest shrieked as needles with the red liquid were inserted into their arms.

In less than two hours, it was over. Dr. Jonas Salk, the creator of the vaccine, injected 137 children before they were led to an area shrouded by curtains to rest.

Under the glare of news cameras, the first public trial of the vaccine at Arsenal Elementary School in 1954 was a turning point in the battle against an epidemic that had left thousands of people dead and even more sickened and paralyzed.

The shots would nearly eliminate the polio cases spreading across the country and help build a broad acceptance of childhood immunizations for decades to follow.

But the Pittsburgh school that helped launch one of medicine’s most towering achievements is now at high risk of another dangerous childhood disease: measles.

At a time when the disease is surging across the country, the vaccination rates reported by the historic school have plunged to levels not seen in years.

Just 4 miles away, at Faison Elementary, an institution named after the city’s first Black superintendent, it’s the same alarming trend.

At Pittsburgh Allegheny on the North Side and Concord Elementary near the city’s southern border, the number of kindergartners who are immunized against measles and other diseases has dropped for the last six years, leaving hundreds of students without the critical protections that have been in place for generations.

Statewide, thousands of children are unvaccinated against the measles, putting communities below the critical threshold of herd immunity — the level that experts say is needed to stem the spread of the disease.

And it happened under the watch of principals and superintendents who are entrusted with protecting the health of students.

A six-month Pittsburgh Post-Gazette investigation found that school leaders have allowed thousands of children into schools without all the required vaccines even though they had no special exemptions that let them forgo the shots on religious or philosophical grounds — a violation of state vaccine laws.

Last year alone, school officials let nearly 16,000 into classrooms, despite warnings from public health officials that students face severe risks when rates fall to unsafe levels.

Combined with the students who have the legal exemptions — about 23,000 — the total number who are attending classes in Pennsylvania without the shots has soared to at least 40,000, records show.

Pennsylvania’s state health department says all students must meet vaccine requirements “or risk exclusion,” and district leaders are responsible for enforcing those mandates.

But the Post-Gazette found that, in the last two years, hundreds of superintendents and principals have failed to ensure that the requirements were met.

CDC/NIP/Barbara Rice, Public domain, via Wikimedia Commons

The first symptoms of measles typically appear one to two weeks after infection and frequently include the hallmark rash and a fever.

“How are you not enforcing this?” said Dr. Amesh Adalja, an infectious disease physician and a scholar at the Johns Hopkins University Center for Health Security. “What are you doing? You’re creating an unsafe environment.”

Though the state Department of Health sets the rules for which vaccines are required and collects the immunization data from schools, state officials say they have no power to force district leaders to follow the law.

Yet on its own website, the state health agency’s requirement is clear: “All students regardless of classroom setting must meet the immunization requirements or risk exclusion.”

If a student doesn’t abide by the requirements, the health agency directs school leaders to “undertake exclusion measures” until they have all their shots.

In an email to the Post-Gazette, a health department spokesperson acknowledged the state collects the data from the schools and monitors the numbers of parents opting out of the shots for their children.

But when it comes to enforcing the state’s requirements — standards set by the health department — the agency says the responsibility falls on “school directors, superintendents, principals, or other persons in charge of the schools.”

The increase in the number of unvaccinated students comes amid the nation’s largest surge of measles cases in decades — including an outbreak in Texas last year that left two children dead and dozens of others hospitalized.

A burst of cases in South Carolina that spread to nearly 900 children has led to at least 19 hospitalizations and forced hundreds of others into quarantine.

Pennsylvania officials argue that because the statewide immunization rate is just below herd immunity, the disease is unlikely to impact as many people.

But rates in hundreds of classrooms have plunged far below that number, wiping out herd immunity in those schools and exposing entire communities to the spread of the disease.

The full scope of the problem in Pennsylvania remains hidden from public view because state health officials will not release the immunization rates for a third of all schools — roughly 1,400.

The reason: the classrooms are so small — fewer than 20 children — the students could be identified, say officials.

But for the vast majority of schools, the data remains striking. One in every four elementary schools in Pennsylvania — 654 in all — have fallen below herd immunity for measles, the investigation found.

In Altoona, nearly every public school in the city has dropped below the threshold of safety.

In the city of York, five of the eight public schools are in that category.

And in the Cheltenham School District just outside Philadelphia, every school has dropped below the threshold of protection.

A highly contagious and sometimes deadly disease, the virus can run rampant through a school and the nearby community unless at least 95% of students are vaccinated.

At Arsenal, the rate is 85%, and at Faison and Concord in Pittsburgh, just 74% — levels that public health experts say are so unsafe that even a single case could ignite an outbreak.

Since January, at least a dozen cases have been reported in Pennsylvania, starting in Lancaster County and spreading to other counties.

“It’s scary to see these numbers falling,” said Dr. Peter Salk, an infectious disease physician and son of the man who developed the polio vaccine. “The protection that has been so solidly built over these last many years” is waning.

The decline in rates has left thousands of children — especially those with compromised immune systems — at risk of diseases that were eliminated decades ago.

“This could be fatal,” said Annette Hall, whose 9-year-old daughter, Maddie, undergoes blood infusions every week for a genetic disorder that’s left her immune system badly weakened and open to infection.

The third grader who suffers from primary immunodeficiency disorder attends Woolslair Elementary, a Pittsburgh public school that was safely vaccinated in 2019, but has since fallen below the threshold.

“What do you think is going to happen?” said Ms. Hall, who injects her daughter with antibodies once a week to boost her immunity. “She’s going to get sick more. She’s going to have more issues.”

‘It’s only going to get worse’

State officials were warned about the breakdown in enforcement a decade ago, when a government commission found that school districts, along with the departments of education and health, all “share oversight” of making sure that children are vaccinated.

The panel urged regulators to create a clear enforcement system — one that would ensure school leaders carry out the state’s vaccine mandate.

But that didn’t happen.

With no single government agency in charge of enforcing the law, the state has been left with a patchwork system of enforcement, with each district policing itself and no accountability at the state level.

In an emailed response from the health department, the agency said neither the departments of health or education were empowered to change the enforcement rules.

That authority belongs to state lawmakers, said Neil Ruhland, the health department’s press secretary.

Despite the commission’s recommendation for the changes, “the legislature did not implement any additional enforcement mechanisms,” said Mr. Ruhland.

However, the report by the government commission directly instructed the health and education departments to collaborate with the legislature to create a more clearly defined system to enforce the law.

The lack of action comes amid a historic shift in public attitudes toward vaccines as parents increasingly reject key scientific research on the safety of the shots.

Across Pennsylvania, parents have looked for ways to bypass the vaccines for their children through the exemptions — special provisions allowed by most states — by claiming religious or philosophical objections to the shots.

In fact, in the past six years, the number of families seeking such waivers has nearly doubled — one of the steepest increases of any state in the country, federal data shows.

Those numbers alone are enough to put hundreds of classrooms at risk, but the decisions by school leaders to allow children to attend class without any of the legal requirements is a key factor in putting even more schools in jeopardy.

In at least 128 schools, the number of students attending classes without the legal requirements is driving the rate for measles vaccinations to dangerously low levels.

At one Pittsburgh school, Concord Elementary, more than a third of the children didn’t have all the required shots or the exemptions, and at Banksville Elementary, it was one in every four.

As the numbers grow, the potential for outbreaks increase because the disease spreads so quickly, say medical experts.

One sneeze, and the tiny droplets that linger in the air can infect a dozen or more people, even up to two hours later.

Simulating the spread of measles

Public health researchers at the University of Pittsburgh simulated the spread of measles at schools throughout the region. At a school like Pittsburgh Faison K-5, where less than three-quarters of kindergartners are fully vaccinated, the modeling showed that a single case of measles could lead to an outbreak impacting more than 350 people in the school and the surrounding area. At the countywide rate — 97% — the virus would spread to about five people.

For some, the symptoms can be as mild as a sore throat, a slight fever or a rash.

For others, the virus can burrow deep into the lungs, course through the bloodstream and even cause the brain to swell.

Bacterial infections can set in, causing severe pneumonia and lung failure — the leading cause of death from the illness. In rare cases, the virus can linger in the brain for years and send an infected person into a coma from which they never recover.

Though the rates for all school vaccines have been dropping for years, the Post-Gazette focused on measles because the disease is one of the most contagious of all human illnesses and is spreading at levels not seen since the early 1990s.

To conduct the analysis, the newspaper examined the rates of kindergartners in more than 1,800 schools — public and private — targeting the last six years as the levels began falling at the onset of the pandemic.

Nowhere has the drop been more dramatic than in private schools, where some classrooms are at high risk of infections, records show.

At Sequoia Christian School, just 5 miles from the state Capitol, only a third of kindergartners are vaccinated against measles.

In Weavertown Mennonite School, in rural Lancaster County, it’s just one in four.

“It’s hard to watch,” said Dr. Paul Offit, a longtime pediatrician at the Children’s Hospital of Philadelphia and one of the country’s leading vaccine experts. “It’s getting bad, and it’s going to only get worse.”

Before the pandemic, Pennsylvania was among the safest in the nation for protections against the illness, with an overall rate of 96.4%.

Since then, the falling rates have exposed students to outbreaks that could spread to dozens of people in just months, health experts say.

Consider: one case of measles in a classroom with a 74% rate — like Faison Elementary in Homewood — can erupt into an outbreak that infects 359 people in the school and the nearby community, according to researchers at the University of Pittsburgh’s School of Public Health.

The researchers create simulations — which draw on school vaccine data and other factors — showing the rates in the local schools are similar to the levels in other parts of the country where outbreaks have occurred, said Dr. Mark Roberts, former head of the university’s Public Health Dynamics Lab.

“For every 1,000 or so cases, you’re going to get about one death,” said Dr. Roberts, who has carried out the studies for decades. “And you’re going to get about 15 or 20 hospitalizations.”

Critics of the vaccines say that the shots — not dangerous diseases — pose the greatest threat to children, and have organized groups to fight the mandates and promote personal freedoms.

Members of the Children’s Health Defense of Pennsylvania, a chapter of the nonprofit founded by Robert F. Kennedy Jr., have launched protests and social media campaigns that said the vaccines were linked to autism and a range of other illnesses.

Group members continue to promote what they call natural immunity as a more effective method to fight diseases, and say the past policies of the federal government have boosted the profits of big drug companies.

“We have just really trusted these institutions for too long,” said Diane Soucy, a Montgomery County resident and a former treasurer of Children’s Health Defense of Pennsylvania who blames vaccines for her son’s autism. “Things that parents had been screaming about for decades [are] suddenly on the table for everyone.”

The evidence over the years has been overwhelming that there’s no relationship between the shots and autism.

In more than 16 major, peer-reviewed studies in the past three decades involving more than a million subjects, researchers found no links to the disorder.

As debunked claims about vaccines continue to spread, the falling rates of MMR immunizations have fueled the recent wave of measles cases across the country, say medical experts.

Nearly 2,300 people were infected last year — compared to 191 a decade earlier.

For another childhood disease — whooping cough, a highly contagious respiratory illness — the cases in Pennsylvania soared to the highest level in the country in 2024 after years of falling vaccination rates.

“We’re playing with peoples’ lives,” said Dr. Todd Wolynn, a Pittsburgh pediatrician who counsels health care workers on how to talk about vaccine safety. “As the rates drop in immunization, it’s just going to get worse.”

Representatives for York city and Cheltenham schools, Sequoia Christian School, Weavertown Mennonite and Shalom Mennonite Schools did not respond to interview requests.

Altoona schools superintendent Brad Hatch said his district makes every effort to ensure children get their shots, but acknowledged that more families are seeking exemptions.

In the last six years, the number of elementary schools in the state falling below herd immunity for measles has soared — from 274 to 654 — the majority driven by children who were granted exemptions.

Few other states showed such an extraordinary decline as Pennsylvania, which was safely above herd immunity as recently as 2021, a Post-Gazette analysis shows.

Critical breakdowns

The growing lapse in state enforcement and the drop in rates was fueled by the pandemic as protesters clashed over mask mandates and social media campaigns were launched that attacked the safety of the coronavirus vaccines.

In 2022, school immunization rates inched down from 95% to a percentage point below the threshold, as battles erupted at school board meetings across the state.

Parents fought parents in bitter exchanges on social media over school shutdowns and frequently targeted district leaders who tried to impose the orders.

“We had both sides shooting spears,” said John Bell, former superintendent of Delaware Valley School District. “We were in the middle just trying to dodge it.”

In a packed auditorium, “you had one side wearing masks and sitting in every other seat, and the other side sitting on top of each other with no masks. It was really a microcosm of how crazy America was and it was all happening right in front of us.”

During one meeting that overflowed with hundreds of people, police officers were called to break up shoving matches in the parking lot, he recalled.

“It was just freaking chaos,” he said. “We were such a fractured community.”

By 2023, as the school rates kept sliding, groups like Health Freedom Pennsylvania were pushing to remove state mandates for immunizations — including the need for any school vaccines.

People had grown frustrated over public health messages that abruptly shifted – first telling them the masks were not needed, then reversing course to say that people had to wear them in public.

“We shuttered schools, we closed businesses, we restricted travel, we masked, isolated, quarantined, social-distanced and tested, tested, tested,” said Dr. Offit, then on the CDC’s vaccine policy panel. “People didn’t like that.”

The skeptics who once questioned the safety of the MMR and other shots in fringe circles were now part of the mainstream in a dispute that was roiling the country.

Revenue — mostly donations — to Mr. Kennedy’s Children’s Health Defense nonprofit ballooned from $6.8 million in 2020 to $23.5 million two years later.

No longer was the argument driven solely by critics of vaccines, but by parents who had long followed pediatric advice and had now grown confused — and fearful.

In the end, the pandemic would lead to a growing distrust of all vaccines, and expose sweeping failures in the state’s response to the falling rates.

A small number of children have to avoid the shots each year because of medical conditions — a category that has remained steady over the years.

But the surge in the other exemptions, religious and philosophical, has reached levels that have never been seen.

Failure to Enforce

As the risks spread, hundreds of school leaders were failing to enforce the state law, records and interviews show.

In Philadelphia public schools alone, at least 1,598 students were allowed into classes without the exemptions last year.

Though some states like California can hold local districts accountable for failing to enforce their laws on immunization — in some cases by withholding funds — Pennsylvania leaves that enforcement power to the school leaders.

With the state facing a potential public health crisis, hundreds of schools continue to carry out their own enforcement with little or no consequences if they fail, records and interviews show.

Mr. Bell, the former school superintendent in Pike County, said that after years of loud protests and political battles over government mandates, district leaders who had endured the turmoil during the pandemic were no longer willing to push parents on their health decisions.

“People don’t want to deal with it,” said Mr. Bell, who left his position for a similar job in New York in 2022. “They are throwing their hands up. They are taking the path of least resistance.”

At the start of the pandemic, district leaders and principals were strict in keeping classrooms safe by making sure children received all their vaccinations before allowing them back into school.

In 2020, school leaders kept more than 1,700 students out of classrooms until they could get their shots, compared to just 310 last year, a data analysis shows.

In Pittsburgh public schools, the district excluded just four students in the past five years, and allowed hundreds of children to continue their education without getting the required shots or the exemptions.

Wayne Walters, the district’s superintendent, said there were several reasons for students not being vaccinated, including newly arrived children in schools who lack access to healthcare.

He also said the district is carrying out a plan that includes bringing in mobile vaccination units twice a month to provide the shots to the children and hosting information sessions for families.

“Our response cannot just necessarily always be punitive or to the letter of the law. We have to work with people,” he said.

Monique Braxton, a spokeswoman for Philadelphia public schools, said the district reaches out to parents whose children have not been vaccinated and that educational leaders try to provide access to the shots.

But without fully enforcing the state’s vaccine mandate, medical experts say school leaders are taking risks at a time when measles cases in other parts of the country have erupted to the highest levels in more than 30 years.

In South Carolina alone, nearly 900 children have been infected with the illness after an outbreak — currently the largest in the nation — raged through at least 30 schools since October.

In Pennsylvania, at least a dozen measles cases have been reported in three counties in the state’s first outbreak of the year.

Even more alarming to health experts are the state’s overall average vaccination rates, which were among the top 10 in the country less than a decade ago.

Now, Pennsylvania lags behind 20 other states — and is comparable to Texas, which was hit last year with the nation’s first massive outbreak of measles in decades.

More than half of the 762 cases — including the two deaths — swept through the Mennonite communities of Gaines County in Texas, where many of the sick children attended schools with average vaccination rates of 77%.

The same low rates are now emerging in schools in Lancaster County in southeastern Pennsylvania, where the state’s outbreak began.

State health officials would not specify the precise locations where the cases started, but at least 53 schools in the county are below herd immunity.

At Shalom Mennonite School in the rolling countryside, less than half the children are vaccinated and just 10 miles away at Ephrata Mennonite School, it’s the same.

Joshua Good, a longtime educator and head of the Ephrata school, said the rates in his school and others are spurred by a growing distrust of the shots.

Though he supports vaccinations — his own children are immunized — he said he believes parents have a basic right to make those decisions for their families.

“That overrides my concerns about mumps or chicken pox or measles or whatever,” said Mr. Good.

However, health experts say it’s not just the children who attend those schools who are vulnerable, but the larger community.

Dr. Roberts, the retired University of Pittsburgh professor, helped oversee a landmark study in Texas that showed plunging classroom rates were often leading signals of future outbreaks in the school and surrounding area.

The Texas study, published in the Journal of the American Medical Association in 2019, took in a range of data, including school rates and commuting patterns to identify several areas of Texas that were vulnerable — the same areas where cases later exploded in 2025.

“If you look at what happened in Texas over the last couple of years, it’s exactly what our paper predicted,” he said.

Dr. Roberts, who has tracked the illness for years, said he’s seeing some of the same patterns playing out in Pennsylvania, especially with falling school rates.

What’s not clear is just how far the levels have declined in hundreds of schools.

While the vast majority of rates in the state have been released, the data for 1,428 schools remains secret — and off limits to the public, the Post-Gazette found.

Citing privacy concerns, state health officials would not disclose the records because of the small size of the classrooms and the risk of identifying individual students.

Health experts say that without the data, parents are left without critical information about the safety of their children.

“That’s part of informed consent,” said Dr. Adalja. “What if your kid is immunocompromised? What if your kid needs an organ transplant?”

Another concern is the vast majority of the schools where the rates remain secret are private, religious institutions — the category of schools with the lowest immunization rates, a Post-Gazette analysis of the state data shows.

In Allegheny County, at least a quarter of the schools — nearly 100 in all — are in that category.

For Melissa Potthoff, who lives just miles from some of the Allegheny schools, the lack of disclosure could put more children in danger.

A nurse whose son has cystic fibrosis, a genetic disorder that leaves the lungs open to severe infection, Ms. Potthoff said parents have a right to know if the schools where their children are attending are below herd immunity.

“That should be public, so at least parents could make a choice,” she said.

Her son, Landon, 15, who is vaccinated against measles and other diseases, attends public school in the Pine-Richland district where most of the schools are above herd immunity.

But he lives in a county where a third of the elementary schools are below the safety threshold. And with his immune-compromised condition, the risks for him are higher if there’s an outbreak, say medical experts.

Clash with feds

The decline in rates comes years after state officials tried to launch sweeping changes in Pennsylvania’s vaccine rules that were aimed at bolstering rates and creating stronger enforcement guidelines.

In addition to the joint commission urging the state in 2016 to improve its enforcement system, the panel recommended that vaccine rates for school districts be easily accessible to the public.

But less than a decade later, parents still can’t search for the rates in their local schools — even as the vaccination numbers continue to fall.

“We’re definitely losing the battle,” said Dr. Wolynn, the Pittsburgh pediatrician. “I think it’s sad and I think it’s tragic, but it’s already happening.”

The state health department said it would be costly to create a reporting system that displays all the district rates to the public, and that the numbers are “always available upon request.”

“Expanding school vaccination rate reporting to the district level would require reallocating DOH personnel and staff time, as well as an overhaul of system reporting,” the state said.

With the skepticism of vaccine safety growing, the rise of national leaders who have opposed immunization policies like Mr. Kennedy, who is U.S. secretary of Health and Human Services, will place more pressure on the districts that are trying to keep their rates above herd immunity.

Under his leadership, the Centers for Disease Control and Prevention earlier this year scaled back the vaccine regimen from 17 to 11 shots — a move that was blocked last month by a federal judge, who ruled the government was not permitted to alter the schedule of shots without consulting established scientific bodies.

Mr. Kennedy also directed the CDC to change a statement on its website last year to suggest the link between vaccines and autism has not been fully ruled out, despite extensive evidence to the contrary.

Some state leaders are pushing back against the latest federal policies, saying that they threaten people’s access to vaccines and endanger the health of children.

Because insurance companies that pay for the shots take their guidance from the CDC, state Democratic leaders pitched legislation last year to force insurers to continue to cover the vaccines.

In an executive order last year, Gov. Josh Shapiro followed up with his own directive requiring the insurers to follow the guidelines of the leading medical organizations, and for the Department of Health to create a website for residents that provides firm “evidence-based recommendations” about the vaccines.

The actions are designed to provide access to the shots, but that’s not enough to reverse the decline, said Dr. Adalja, who is also a UPMC physician.

The state is failing to take basic steps to enforce its own vaccine policy, like putting more pressure on district leaders to improve their rates, he said.

Because the agency collects all the vaccination data from the schools, it should take a lead role and alert parents and communities when rates fall to dangerous levels, said Dr. Adalja.

“It’s the state’s responsibility,” he said. “I think they should be naming the schools. Parents need to know.”

He said the health department can target vulnerable schools by bringing in mobile units to help children get the shots, and to educate families about the need for vaccines.

Reach the “people that have just slipped through the cracks,” he said.

In response, the health department said it partners with health care providers to increase access and awareness about the importance of vaccines, including the launch of a media campaign that created television, radio, digital and outdoor ads from 2023 to the end of last year.

“The Department works directly with schools, health care providers, and community organizations to ensure they have the necessary information to properly educate parents about the importance of vaccinations,” the state health agency said.

To take more forceful action, the state could invoke its own emergency authority like it did during the pandemic, when it ordered schools and businesses to shut down, said Dr. Lori Handy, a vaccine expert at the Children’s Hospital of Philadelphia.

But she added that the state likely wouldn’t take such action unless there was a significant outbreak.

Dr. Adalja said the burst of measles cases in the country already constitutes a crisis.

“It’s only a matter of time that a measles case lands in one of your schools, where, if the vaccination rate is not high enough, you have an outbreak,” he said.

For Annette Hall, that’s one of her deepest fears.

Every week, she attaches two thin tubes to her daughter’s leg, which are hooked up to a small device the size of a paperback book.

Over the course of an hour, the machine pumps life-saving antibodies from donated plasma into her 9-year-old child in a regimen designed to help her fight off disease.

The morning after the treatments, the 43-year-old mother watches as Maddie boards the bus to school.

“Every day we live with that fear: Is she going to come home with something new that can’t be treated?” she said.

“The threat of measles scares the crap out of me. I don’t know what would happen. And Maddie’s not the worst case. There are kids that have worse case scenarios than us. It can kill them.”

Just miles from her home is the office of Dr. Wolynn, who leads a group that counsels health care workers on how to talk to parents about the need for vaccines.

After years of treating children, he said schools remain central to the spread of diseases, and that district leaders need to enforce the state mandate.

Before he co-founded the Trusted Messenger Program, he provided care to children whose weakened immune systems made it nearly impossible to fight off infections.

“There’s nothing that’s as horrible as watching a kid die who didn’t have to die, or get really sick and be permanently harmed," Dr. Wolynn said. “It tears your heart out.”

With rates falling in the schools, he said he fears the trend will continue until people see the kind of illnesses that he and other pediatricians have treated.

“It’s already getting worse,” he said. “We’re literally going to have to watch kids get infected, suffer and die. That’s the path we’re on until the pendulum swings the other way."

Sean D. Hamill contributed to this story.