LANDRUM, South Carolina (AP) — With baby Arthur too young for the measles vaccine and a sibling due in June, the Otwells grew nervous when the threat of the highly contagious virus started factoring into their grocery run.

"We go to the Costco that was kind of a hotbed," said John Otwell, who knew about the state health department's warnings of public exposures at the store. "A lot of people just don't get it; they think it's just a cold. It's not."

READ MORE: A parent's guide to preventing measles infection and what to look for

By Arthur's 9-month checkup, the South Carolina outbreak had exploded into the nation's worst in more than 35 years, surpassing last year's in Texas. That meant that under state guidance, Arthur could get his first dose of the MMR vaccine — for measles, mumps and rubella — earlier than the usual 12 to 15 months old. Their new baby won't be able to get the shot until at least 6 months — a prospect that worries parents of infants wherever measles spreads.

Babies too young to be vaccinated are among the most vulnerable in a measles outbreak. The disease can wreak havoc on their fragile bodies, making them so sick they stop eating and drinking. They can develop pneumonia or brain swelling, and sometimes die.

Babies depend entirely on herd immunity — at least 95% of a community must be vaccinated to prevent measles outbreaks. But dropping vaccination rates have eroded protection in South Carolina and across the nation. In Spartanburg County, the outbreak's epicenter, less than 90% of students have gotten required vaccines.

"Babies become sitting ducks," said Dr. Deborah Greenhouse, a Columbia pediatrician. "The burden is on all of us to protect all of us."

WATCH: What to know about South Carolina's big measles outbreak and who is most at risk

But increasingly, some policymakers and officials push a view of vaccination as an issue of individual freedom and parents' rights, rather than one of public health to safeguard the population as a whole.

At the federal level, Health Secretary Robert F. Kennedy Jr., a longtime anti-vaccine crusader, has sought to remake vaccine policy and oversaw billions in public health cuts. And though a temporary ruling from a federal judge has slowed his momentum, a raft of bills has been introduced in states, including South Carolina, that threaten to further reduce vaccination rates.

South Carolina's measles outbreak, totaling about 1,000 cases, has slowed. But measles is spreading in many states, with 17 outbreaks this year and 48 last year, and the U.S. on the verge of losing its status as a country that has eliminated measles.

Doctors work to protect the youngest against measles

Dr. Jessica Early never thought she'd have to deal with measles, but the pediatrician feared for her patients and her own baby when it popped up in her Greer community. She and other doctors began offering an approved infant MMR dose as early as 6 months old. Her practice also started giving the second MMR dose — usually for ages 4 to 6 years old — early.

To the chagrin of many doctors, no one knows how many South Carolina infants have gotten measles or been hospitalized by it.

State officials will disclose only that 253 of the 997 cases were among children 4 and younger; they say they won't break cases down further for confidentiality reasons. It's not uncommon to group statistics this way.

Officials also don't know exactly how many infants were hospitalized with the virus because, as in some other states, hospitals aren't required to report measles-related admissions.

READ MORE: The U.S. is on the verge of losing its measles elimination status

Across the state, doctors said they got many questions about whether it was safe to bring infants to waiting rooms or day care.

Thomas Compton — regional director of Miss Tammy's Little Learning Center, a child care network operating across the outbreak region — said 18 parents pulled children out of his facilities, though they had no confirmed cases. Some abandoned deposits days before their kids were scheduled to start, forcing the company to lay off a teacher.

Although licensed day cares must require vaccines under state law, families can easily get religious exemptions. About a fifth of Miss Tammy's 300 children have vaccine waivers.

When measles surged, Compton said state officials gave little guidance. His staff scrubbed down surfaces, as they did when COVID-19 was raging; tracked local measles cases on Facebook; and relied on Google for information about the disease.

"A lot of parents were really stressed out," Compton said. "Anytime that we had a little sickness going on or something, they were like, 'Do you think it's the measles?'"

State legislation would prohibit vaccines for children under 2

Last year, an Associated Press investigation found that Trump administration officials were directing activists to push anti-science legislation in statehouses. Nationally, around 350 anti-vaccine bills were introduced as of late October, AP found, including at least eight in South Carolina.

This year, a state bill would prohibit requiring vaccines for children under 2.

"In other words, it would get rid of those requirements in the day cares," pediatrician Greenhouse said. "And for people like me, that is a gut punch that is terrifying."

In a subcommittee discussion, Republican State Sen. Carlisle Kennedy said his bill aims to protect parents' rights. His baby was born in August without working kidneys and got vaccines on a personalized schedule, in coordination with doctors.

"We didn't want to put vaccines in his body before his body was able to survive them," he said.

Opponents countered that herd immunity protects children in these situations.

READ MORE: Do adults need a measles booster? An epidemiologist explains who is immune

The Senate subcommittee advanced the legislation. Greenhouse fears it has momentum.

"In the climate that we are currently living in, I think any bill potentially could have legs," she said. "It is our job to do our absolute best to make sure that those legs don't go anywhere."

Whether the bill becomes law, doctors say this sort of legislation fuels vaccine skepticism and confusion. While the American Academy of Pediatrics advises giving babies all the vaccines they've gotten for years, some parents tell Greenhouse they know the government has called for fewer.

"They don't actually know who they can trust," she said.

South Carolina, like other states, has made nonmedical vaccine exemptions easier to get, noted Dr. Martha Edwards, president of the state's American Academy of Pediatrics chapter. In the outbreak's epicenter, religious exemptions have more than doubled since 2020. Statewide, 4% of school-age students have such exemptions in 2025-26.

"Parental choice is a big buzzword in a lot of the Southern states," Edwards said. But the choice not to vaccinate, she said, impacts other parents' rights to keep their children safe.

Nationwide, protection fades as measles spreads

Doctors expect things will only get worse.

In the first three months of 2026, the U.S. logged 1,671 measles cases. That's 73% of the total from 2025, the worst year for the virus in more than three decades. In November, international health officials will determine whether measles is still considered eliminated in the U.S.

WATCH: What the overhaul of U.S. vaccine guidance means for public health

National MMR vaccination rates – which dropped to 92.5% among kindergartners in the 2024-25 school year, from 95.2% in 2019-20 – obscure much lower rates in certain communities. At one Spartanburg County school, 21% of kids received all required vaccines.

Doctors worry it's just a matter of time before all sorts of vaccine-preventable diseases threaten lives like they did a century ago.

"The whole concept of immunization is one of the best things that has ever happened to medicine," Greenhouse said. "To see that we are actually going backwards is just confounding."

Helen Kaiser, who lives in the outbreak area, vaccinated her twin 2-year-old boys early to protect them and the community.

"I would never forgive myself," she said, "if I knew that my son had gotten another baby very sick and it was something I could have prevented."

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Ungar reported from Louisville, Kentucky.