Sen. Bill Cassidy (R-LA), a physician before he began his career in politics, tore into so-called anti-vaxxers on Tuesday during a hearing on vaccines and public health.

“Let me give some color to what Sen. Paul said,” Cassidy said at the start of his remarks during a hearing of the Senate Committee on Health, Education, Labor and Pensions, the beginning of a fiery response to Sen. Rand Paul (R-KY)

The hearing, “Vaccines Save Lives: What Is Driving Preventable Disease Outbreaks?” featured public health professionals and a high school student who got vaccinated against his parents’ wishes when he turned 18.

Paul, himself a former ophthalmologist, stressed that he had personally vaccinated himself and his kids, “but I still don’t favor giving up on liberty for a false sense of security.” In the past, Paul has questioned whether vaccines should be required, and he’s parroted anti-vaccine conspiracy theories, such as in 2015, when he said he’d “heard of many tragic cases of walking, talking normal children who wound up with profound mental disorders after vaccines.”

Rand on Tuesday brought up a hypothetical in which the federal government mandated flu vaccines, “yet, the current science only allows for educated guessing when it comes to the flu vaccine.”

Cassidy appeared to take issue with the remarks.

“You may or may not know I am a physician,” Cassidy said. “I have seen people who have not been vaccinated who have required liver transplantation because they were not, and who have ended up with terrible diseases for no other reason than they did not understand vaccination was important.”

Even if flu shots are not completely effective, he said, “they do mitigate, and so there is a cross benefit that will decrease the severity.”

And further, he said, “hospitals commonly require their employees to be immunized, because they understand that herd immunity is important, and if a nurse’s aide is not immunized, she can be a Typhoid Mary, if you will, bringing disease to many who are immunocompromised.”

There is no federal vaccine requirement, though all states require certain vaccines for students, with state-by-state exemptions for medical, religious, and in some cases, philosophical reasons.

“The requirement is just that you cannot enter school unless you’re vaccinated,” Cassidy said. “Now, if you’re such a believer in liberty that you do not wish to be vaccinated, then there should be a consequence, and that is that you cannot infect other people.”

Watch the beginning of Cassidy’s remarks below: