There was a moment at a Congressional committee hearing on Tuesday where the entire conversation about health risks and vaccines was laid bare. Sen. Elizabeth Warren asked a top Centers for Disease Control immunization expert whether vaccines are safe – eight different ways.

“Is there any scientific evidence that vaccines cause autism?” “No,” Dr. Anne Schuchat said. “Is there any scientific evidence that vaccines cause profound mental disorders?” Warren asks. “No, but some of the disease we vaccinate against can,” Schuchat answers. “Is there any scientific evidence that vaccines have contributed to the rise of allergies or autoimmune disorders among kids?” Warren asks. “No,” she said.

The exchange continues, with Warren asking an additional five times about the dangers of vaccines, hearing the same answers: no, no, no, no, and no again.

“Vaccines are safe,” Schuchat said.

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The congressional hearing on vaccines was pre-scheduled, but it comes amid the largest Measles outbreak the country has seen in decades. Fifteen years after it was determined that the vaccine had eradicated homegrown Measles, 121 Americans have measles and hundreds of others are being isolated or tracked during the incubation period, in case they develop the highly contagious and potentially deadly disease. Much of the outbreak has occurred in the California area, where immunization rates have been falling for years as parents frequently choose not to immunize their kids.

“The increase in measles cases should be seen as a wake-up call,” Schuchat testified.

But the outbreak has become more than a public health crisis, it’s also become a political lightning rod as conservatives struggle to reconcile their personal views with an ongoing emergency. Last week, Gov. Chris Christie stumbled, saying while he’d vaccinated his own kids, he wanted parents to have a choice on the matter.

Sen. Rand Paul, who has a background as a physician, went farther and said he’d seen vaccines cause “profound mental problems.” The pair of potential 2016 candidates were hit with significant political blowback; Paul recanted and and got a booster vaccine to emphasize it. (Paul’s a member of the committee that held today’s hearing, but he wasn’t present. A spokesman said he was a classified Foreign Relations committee hearing at the exact same time.)

Tennessee Republican Sen. Lamar Alexander, however, didn’t sympathize with the anti-vaccination movement, instead pinpointing parental exemptions as a health risk.

“What is standing between healthy children and deadly disease? It ought to be vaccinations, but too many parents are turning away from science,” he said at the start of the hearing.

Experts on the panel agreed. “It’s this philosophical exemption that’s causing problems,” Dr. Sawyer said.

Asked by Louisiana’s Sen. Bill Cassidy if immigrants were bringing measles into the country, Schuchat noted that these children were being vaccinated (or had already been vaccinated), saying “it’s just these new communities where parents are opting out that we’re quite worried about.”

The committee hearing was arranged by Alexander and Washington Democrat Sen. Patty Murray, who jointly selected guests to testify and both argued in favor of vaccinations. Alexander particularly lampooned the “near panic” expressed by the country on Ebola, all the while our own vaccination rate of Measles, an even-more contagious disease, slipped.

“I think we need to decide do we want to protect the most vulnerable or do we want to offer personal choices,” Tennessee Department of Health Immunization Program Director Kelly Moore said.

Moore and others emphasized particular the need and importance of herd immunity—the idea that if everyone else is immunized against these deadly disease, the collective protections will keep children too young to be immunized and those battling diseases that compromise their immune systems from contracting the disease. Dr. Tim Jacks, a father and pediatrician, also testified at the hearing. Jacks, whose letter to the parents of the unvaccinated child who exposed his infant son—too young to be vaccinated—and daughter—who is immune-compromised due to cancer treatments—went viral.

“As immunization rates go down, herd immunity breaks down,” Jacks said. “This herd immunity is the only thing that’s protecting my two young children.”