Then she got on the Internet.

Smoot downloaded the recommended immunization schedule from the CDC website. She looked up each vaccine on the schedule. Although she says she tried to look at a variety of trustworthy websites, she can only remember reading any information supporting vaccination on the CDC website. Smoot, however, says she does not trust doctors and scientists. “I know they’re just going to tell me they’re safe, and they’re recommended, and this is what you’re supposed to do,” she says.

Dr. Neal Halsey is a professor at the Johns Hopkins Bloomberg School of Public Health and the School of Medicine. He runs the Institute for Vaccine Safety, providing independent assessment of and education on vaccine safety. “The vaccines we have available that are recommended for routine use in children are very safe vaccines,” he says.

Dr. Diane Griffin, a virologist and chair of the molecular biology and immunology department at Johns Hopkins, agrees. “Oh, yes, vaccines are very safe,” she says.

According to the CDC, vaccines in the United States are the most safe and effective immunizations in history. The CDC’s website says severe reactions to vaccines “occur so rarely that the risk is difficult to calculate.”

“I probably find more information that says vaccines aren’t safe,” she says. “I think it’s only because …” She pauses. “Well, I don’t really know. It could be maybe what I’m paying attention to more.”

Smoot also says she trusts the experiences of other parents more than data from a scientific study. “Right now,” she says, “the people telling their personal stories influence me more. I feel like the data could be flawed for one reason or another, but I feel like someone’s story, because they’ve gone through something, and they don’t want other people to go through it, I feel like I trust that more.”

The CDC website describes the years of required testing that vaccines have to pass before they are licensed. The CDC’s Immunization Safety Office constantly monitors reactions to vaccines, as do independent researchers. These experts test and retest vaccines, then confirm their results with more research. They replicate the results of other scientists to affirm their conclusions. If the data is flawed, they want to discover the flaw.

Dr. Kristin Hendrix, a professor of pediatrics at the Indiana University School of Medicine, researches how parents make decisions about their children’s healthcare, including vaccinations. “It’s a combination of pretty complex psychological factors,” Hendrix says. “Some folks are very predisposed to trust information about others’ personal experience.” She emphasizes that a story is even more likely to trump scientific data when the story comes from a friend or family member.

“Even if the situation that a person hears about didn’t actually happen to their friend or family member, but is being relayed by them, they trust that more than a face-to face conversation with a physician,” Hendrix says. “That information, anecdote, narrative, personal account, rare instance that may or may not be true, tends to carry more gravity and weight when it comes from someone they know.”