Rebecca Estepp, parent support and media relations manager for the advocacy group Talk About Curing Autism (TACA) -- of which McCarthy is a spokeswoman -- says the star's comments were out of line.
"I guess she's calling me a parasite, because I did not vaccinate my second son because of what I saw happened to my first son," says Estepp, who adds that her first child developed autism shortly after his vaccination.
While celebrity advocacy of certain health causes is not a new phenomenon, having McCarthy and Peet squaring off over the vaccine debate has taken public interest in the issue to another level.
Dr. Paul Offit, chief of the section of infectious diseases at the Children's Hospital of Philadelphia, says much of this has to do with the fact that the public, as a whole is, in many ways, more eager to listen to celebrities than to researchers.
"I think when Jenny McCarthy appears on "Oprah" and says, 'I think vaccines harm our kids,' that has an effect. When Amanda Peet comes out and says, 'I believe the scientists on this,' that also has an effect," Offit says. "It's a culture of celebrity, and we tend to trust what celebrities tell us."
Offit, a leading advocate of the current vaccine schedule, is the doctor whose advice Peet sought early in her search for information. Peet met with Offit through her brother-in-law, who is a fellow in the Children's Hospital of Philadelphia's division of infectious diseases. And Offit says that what Peet learned about the risks of skipping vaccinations is what drove her to take action.
"It angered her," he says. "She felt like this, at some level, put her children at risk."
As for McCarthy, Estepp says the former model approached TACA in 2005 after her own son had been diagnosed with autism.
"When [her son] got better, she said, 'I want to help,'" Estepp says, adding that McCarthy's advocacy grabbed the attention of other celebrities. "It looks like Jenny's story kind of sparked like wildfire through the Hollywood community."
Offit, too, notes that Hollywood has become a major battleground in the vaccination debate. And he says that while scientific evidence on the importance of vaccination is irrefutable, celebrity culture goes a long way in shaping public discourse on the topic.
"[Peet] lives, to some extent, in the California culture and knows that people like Jenny McCarthy are not the minority there," he says.
But Peet says she bases her arguments for vaccination on the expertise of doctors and medical groups.
"I don't think parents should be taking medical advice from actors," she told ABCNews.com via email. "I take medical advice from several pediatricians, other doctors, the CDC, and the American Academy of Pediatrics."
Following Peet's appearance on "Good Morning America," the American Academy of Pediatrics issued a statement further clarifying its position on the matter.
"We know vaccines protect children against potentially deadly and disabling diseases," the statement reads. "Vaccine side effects are usually minor and include fever, mild swelling and pain at the injection site. In unusual cases, such as when children are ill or have certain chronic conditions, a pediatrician will advise delaying or skipping certain vaccines.