A few days after an uncomfortable confrontation, I invariably wish I had explained things differently.

The most recent example: I was to be interviewed about my latest book,  Bad Advice: Or Why Celebrities, Politicians, and Activists Aren't Your Best Source of Health Information, which in part describes some of the fringe elements of the anti-vaccine movement. National Public Radio's Alison Kodjak moderated the discussion at the National Press Club in Washington on Oct. 29, with about 120 people in the audience.

When I walked into the room, Del Bigtree, the writer and producer of a virulent anti-vaccine movie, handed me a letter from Robert F. Kennedy Jr., demanding that I answer a series of questions about his thoroughly discredited belief that vaccines have caused autism.  More activists, some from as far away as Oregon and Washington state, were also in the room.

I should have expected this. Recent events have largely frustrated the anti-vaccine movement, causing a small but vocal group to become angrier.  For example, during the presidential debates, Donald Trump declared that vaccines caused autism, raising activists' hopes that he  was their man in the White House. It hasn't worked out that way. Since Trump became president, state bills designed to overturn vaccine mandates have largely failed. Further, most parents of children with autism don't believe that vaccines are the cause — a far cry from how parents perceived vaccines 10 to 15 years ago.

Now, anti-vaccine activism has entered the mean season. Billboards claiming that vaccines kill children have popped up in West Virginia, New York, Missouri, and elsewhere. Groups of anti-vaccine activists now routinely appear at federal vaccine advisory committee meetings to confront public-health officials who recommend vaccines.

Before the interview began, Kodjak took the precaution of asking the audience for written questions that she would ask me in order to keep the conversation civil; unfortunately, it didn't work out that way. Some activists started shouting at me; one asked how much the pharmaceutical industry was paying me.

I think I did a reasonable job of handling the shouting. But here's how I could have made my point clearer:

"Let's take a step back.

"In 1976, 40 million people were inoculated with flu vaccine. About 400 (0.001 percent) developed a disorder called Guillain-Barré syndrome, which can be severe and occasionally fatal. The vaccine-disease link was detected by scientists and  published in the New England Journal of Medicine.

"Currently, autism occurs in 1 of 59 children, or about 2 percent of births. If retrospective studies like the one performed following the 1976 flu vaccine could detect a problem that affects 0.001 percent of a population, they also should work in the case of a far more common condition.

Seventeen studies involving hundreds of thousands of children have shown that the measles-mumps-rubella (MMR) vaccine doesn't cause autism. And, contrary to Kennedy's  claims, seven studies involving thousands of children have shown that thimerosal, the mercury-containing preservative once used in several vaccines, also didn't cause autism.

"So one of two things must be true.

"One: Vaccines don't cause autism.

"Two: A vast international conspiracy involving thousands of researchers, academicians, and public-health officials — all deeply in the pocket of the pharmaceutical industry — is hiding the truth. Remarkable, given that these same 'conspirators' vaccinate themselves and their own children or grandchildren.

"Sadly,  a small but vocal fringe element of the anti-vaccine movement has chosen the second option. Yet none of their shouting, lawsuits, hate mail, harassment, and even occasional death threats has led to a decline in autism rates.

"Many parents came here for some answers about their children and have had trouble being heard above the shouting.  Sitting in the audience tonight is Alison Singer, director of the Autism Science Foundation and the mother of a child with autism. Her organization has spent tens of millions of dollars on studies that have identified some of the genes responsible for autism and how those genes affect brain cell structure and communication. Learning the details of these and other promising studies would be far more constructive than following those who continue to proffer false hope and false cures."

I just didn't think quickly enough to say any of this. Maybe next time.

Paul A. Offit, M.D., is a professor of pediatrics and the director of the Vaccine Education Center at Children's Hospital of Philadelphia.