AMNA NAWAZ: A second child died from measles-related causes in Texas last week.

It's part of a growing outbreak that's infected at least 505 people in Texas and has spread across 22 states.

Until this year, the U.S. had reported no deaths from measles in more than a decade.

Over the weekend, Health and Human Services Secretary Robert F. Kennedy Jr. visited Gaines County, the epicenter of the West Texas outbreak, and spoke with families of both children who died.

Kennedy has previously resisted calling for widespread vaccinations, but, on Sunday, he posted on X -- quote - - "The most effective way to prevent the spread of measles is the MMR," or measles, mumps, and rubella, "vaccine."

The measles vaccine is considered one of the 20th century's major advances in public health, and it saved an estimated 94 million lives in the last 50 years.

The "News Hour"'s Deema Zein recently sat down with someone who had a front row-seat to its creation.

DAVID EDMONSTON, Blood Donor For Measles Vaccine: Measles was something that almost every child came down with at some point.

I was pretty miserable for a better part of a week.

DEEMA ZEIN: In 1954, when he was just 11 years old, David Edmonston would have never guessed that his very first blood test would change the world.

DAVID EDMONSTON: I was attending a boarding school in Massachusetts, and I came down with measles.

DEEMA ZEIN: David and other students recovering from a measles outbreak were approached by a physician from Boston Children's Hospital, Thomas Peebles.

DAVID EDMONSTON: He told me that he was a member of a team who were developing a vaccine for measles, and would I like to participate in that by giving some blood and throat washings?

And I was thrilled with the thought of being able to help thousands, millions of people.

DEEMA ZEIN: A few weeks later, Dr. Peebles returned to tell David that they had isolated measles virus cultures from his blood.

Did you realize how monumental it was going to be?

DAVID EDMONSTON: No, it didn't impress me as a very important thing that I was doing, because I felt the chances of this actually developing into a vaccine that would help millions of people were kind of astronomically small.

This was the launch of the vaccine.

There was a TV show.

DEEMA ZEIN: In fact, the team led by Nobel Prize winner John Franklin Enders would eventually go on to develop the world's first measles vaccine from the Edmonston-B strain, named after David.

DAVID EDMONSTON: This became a joke in our family.

DAVID EDMONSTON: Because anything that came up that was stressful, we would always say, oh, that's the Edmonston strain.

NARRATOR: The real story of a vaccine is told in the fears and smiles of a little girl, who today is given greater protection than ever before against the infectious diseases of childhood.

DEEMA ZEIN: Before the shot, the highly contagious disease was endemic in the United States, with an estimated four million cases and 48,000 hospitalizations a year.

The vaccine helped reduce measles deaths from 500 per year in the U.S. to near zero.

But years after helping to create the medical miracle, David's stance on that same vaccine would change.

When he and his late wife, a public health teacher, had a son, they decided not to give him the measles vaccine.

DAVID EDMONSTON: She had read an article by a vaccine researcher who wrote that the measles vaccine in particular may be dangerous to small children.

Unfortunately, it was a wrong decision because it was the wrong information.

DEEMA ZEIN: Now 82 years old, he considers himself a vaccine advocate.

DAVID EDMONSTON: There's an awful lot of misinformation out there, and you need to be careful who you listen to.

DEEMA ZEIN: David has been a decades-long member of a spiritual group founded in India.

During the pandemic, when the COVID-19 vaccine became available, he urged other members to get it.

DAVID EDMONSTON: One lady I talked to told me that she ate healthy food and did yoga.

She thought she was too healthy to need it.

People just didn't trust vaccines at all and didn't want to get involved in it.

DEEMA ZEIN: But, for David, his early experience with the researchers behind the measles vaccine made a difference.

DAVID EDMONSTON: I had a sense for how ethical and careful these people were.

So I assumed that the developers of the COVID vaccines were similarly ethical and careful.

DEEMA ZEIN: Today, he worries about the impact noted vaccine skeptic Secretary of Health and Human Services Robert F. Kennedy Jr. could have on public health.

DAVID EDMONSTON: They're not experts in the medical field.

It's not what the country needs.

DEEMA ZEIN: I want to talk a little bit about why you have decided to speak out.

DAVID EDMONSTON: I feel that vaccines are very important not just for the individuals who take them, but for our society as a whole.

And I feel that anything I can do to help our society as a whole is important.

DEEMA ZEIN: David says he hopes speaking out will humanize the measles vaccine, the people who created it, and the people it protects today.

For the "PBS News Hour," I'm Deema Zein in Bowling Green, Virginia.