The doctors shifted nervously in their seats as the sharp-tongued questioner scanned the room.
Dr. Paul Offit, a University of Pennsylvania pediatrician and the nation's most outspoken childhood vaccine proponent, had come to the UCLA lecture hall to subject several dozen physicians to a faux parental grilling.
He wanted to give them the kind of pushback doctors have come to expect in affluent parts of Los Angeles and California, where increasing numbers of parents are refusing to inoculate their kids against contagious, even life-threatening diseases for fear of complications.
For many of the pediatricians in the audience, taking a hard line on the immunization schedule can mean potentially alienating well-intentioned, if misinformed, parents.
If Offit, a rock star in his field, could give these doctors more factual ammunition — and a little practice on their delivery — could they help convince resistant parents that science is simply not on their side?
The salt-and-pepper-haired Offit slipped straight into character and zeroed in on one young doctor.
"I know you doctors keep telling me that vaccines don't cause autism. If that's true, then why is it on this package insert?" he asked, playing the role of a parent who had read the blogs and heard the celebrities who connect the two.
Shifting in her seat, the designated victim shot Offit an unsure look.
Then she began citing studies and said that drug packaging inserts include many "temporally associated symptoms" that weren't necessarily caused by the vaccine.
"Why?" Offit pressed. "Why would they put that there — just to scare me?"
The doctor kept trying. "They're required by law," she said. "I actually didn't know the answer."
Offit broke character to explain: Drug companies must list any condition known to have occurred within six weeks of a vaccination, whether the medication caused the condition or not, and even if it occurs at the same level as with a placebo.
Package inserts are legal documents, not medical documents, he said, calling them "the bane of [his] existence."
"If you look at the original package insert for chicken pox vaccine, it says, 'Broken leg has been associated with this drug,'" he added.
Studies have firmly debunked the notion that vaccines cause autism. Yet that is one of the most common claims made by a persistent national anti-vaccination movement that treats Offit as public enemy No. 1.
Some brand the doctor as a mouthpiece for the pharmaceutical firms that sell vaccines, in part because he co-invented one to prevent rotavirus, which causes severe diarrhea in infants and children and kills nearly half a million around the world each year.
Here in Southern California, physicians said, parents become concerned about vaccines for all kinds of reasons.
"In this area, we have a lot of families who don't want to get their children immunized," said Dr. E. Richard Stiehm, a UCLA pediatric immunologist and the organizer of the event. "Many parents will say, if everyone else is vaccinated, my child will be protected. That's true — but it's shortsighted and self-centered."
Doctors who showed up for the recent Q&A session — a first for UCLA — said they were eager for advice.
"I want to listen to Dr. Offit because he's a world-renowned expert," Dr. Lisa Stern said before the event. "I know whatever he talks about, there'll be something I'll take away from it."
Stern said parents who reject vaccines outright generally don't seek care at Tenth Street Pediatrics in Santa Monica, where she has spent 20 years treating a patient base that includes the wealthy and well-traveled. She sees a lot of families who are "sort of on the fence" about immunizations, she said.
"There's a lot of misinformation at baby groups and on the playground," Stern said. "The more educated people are and the more free time they have, the more they take a position."
Parents ask her about additives in injections, and whether giving babies "bundled vaccines" — shots that protect against more than one illness — will overwhelm their young immune systems. They ask whether a baby who's going to stay home with Mom really needs protection. They question why their child can't just skip being immunized against polio, since the disease has been eradicated in the U.S.
"I say, 'Your kid is going to travel. You're going to take your child to Africa. You can't take an unvaccinated child to Africa,'" Stern said. "I have stock answers for all of the questions."
But other doctors in the audience didn't have as ready an information arsenal, which was apparent as Offit continued to pepper his audience with questions.
A simulated parental inquiry about the Roman Catholic Church's opposition to four vaccines made using cells derived from aborted fetuses elicited uncomfortable squirms.
Offit told the group that, in fact, the Catholic Church had ruled that use of these vaccines is acceptable to protect the health of children and pregnant women. (The church's official position does, however, call on researchers to develop alternative immunization options.)
Regarding worries about the inclusion of the preservative formaldehyde in some vaccines, Offit told the group that humans have been "making formaldehyde in our cells since we crawled out of the ocean onto land."
Audience members deftly answered "bread and butter" questions about the slower-going approaches to vaccination promoted by television talk-show host Dr. Mehmet Oz and Orange County author Dr. Bob Sears, among others.
"With vaccine schedules, it's not an opinion," said Dr. Julie Omid, a pediatrician in the audience who practices in Irvine, not far from Sears' Dana Point home turf. "Dr. Sears writes lots of books and makes lots of money, but he's frankly wrong."
Omid said she tells parents that her own children, ages 6 and 3, get immunizations "on the first date that it is allowed."
Stern said Offit's presentation had armed her with useful information to take back to her "overeducated parents," and that she would "make more of an effort to push" those who sought to delay shots.
But she also said she thought it might be easier for Offit to draw a hard line than for pediatricians practicing day to day in L.A.'s tonier ZIP Codes.
"I waver with my families," she admitted. "But I have to. I have to keep their trust. If I send them away, they won't come back" and could end up in an office where doctors don't vaccinate.
In an interview, Offit said there also was risk in not pressing a resistant parent.
"If you do that," he said, "if you say: 'OK, we're going to make this decision together' — which is to say that they're going to be a major force in the decision — then you have to be willing to watch them make a bad one and stand back."