By Isaac Saul

“I felt like I owed it to everyone to explain how I was wrong."

For most parents, protecting their children with the modern magic of vaccines makes perfect sense. But for the thousands of parents who have been swept up in the anti-vaccination movement, vaccines are dangerous, life-threatening products that they're determined to avoid — and make sure you avoid, too.

In recognition of World Immunization Week, we reached out to three parents who fell into the anti-vaxxing movement, which is gaining traction in the United States. A study in the found that pediatricians who encountered anti-vaxxer parents grew from 74.5 percent to 87 percent between 2006 and 2013. But what makes these three moms special is what they did after joining the anti-vaxxer movement: they changed their minds. Now, they're vaccinating their kids and speaking out about how important it is for all parents follow in their footsteps.

"When I was an anti-vaxxer myself, I was loud and proud about it," Megan Sandlin, a 24-year-old mother of two said in an email. "I felt like I owed it to everyone to explain how I was wrong and how I came to that conclusion. I also care deeply about keeping children safe, so when I see someone start to doubt vaccines, I like to let them know that I do understand the fears, but also give them factual information so they hopefully don't go down the road of not vaccinating."

For all three parents, fear was at the heart of their journey into the anti-vaxxer movement. Fear that if they gave their children vaccines, they might contract an illness as a result or be ostracized.

"I'm not stupid, but something kind of happens when you get pregnant and you're a first-time mom," said Carli Leon, a 35-year-old mother of two. "You're scared."

Leon was nannying for a wealthy couple when she became pregnant with her first child. The wife talked often about vaccines, and one day the husband asked her what she was going to do about her child. She had never considered not vaccinating her kids, but he pressed her to speak to his wife about it. When she did, the wife pointed her to Sherri Tenpenny, a close friend of hers who happens to be one of the most well-known anti-vaxxers on the internet.

If you Google Tenpenny's name, you'll be bombarded with blogs that ask, "Are vaccines necessary?" Other Tenpenny blogs lay out how to legally refuse to vaccinate your kids. A former emergency room nurse, Tenpenny's HuffPost blogging profile calls her "one of the country's most knowledgeable and outspoken physicians regarding the negative impact vaccines can have on health."

"She falls into the category of a conspiracy theorist," Leon said. "But I went right down the rabbit hole. And I became a super loud advocate for the anti-vax movement."

Conversely, Kristen O'Meara might be the most well-known pro-vaxxer in the United States. Family lore about negative reactions to vaccinations and her own affinity for alternative medicine sent her into motherhood with a skeptical attitude toward vaccines.

"When I got pregnant, I already had that bias," said O'Meara. "I went into it wanting to hear the other side. That led me down the rabbit hole of alternative takes on vaccinating… I wanted professional medical information, but I found the doctors that I now kind of see as quacks."

O'Meara learned her lesson the hard way.

In a blog post on the website Voices for Vaccine, she wrote about how shortly after her twin daughters were born, her entire family was infected with rotavirus, a vaccine-preventable disease that left her three daughters crying in pain and hunched over the toilet. O'Meara's blog post was spotted by The New York Post, where she ended up writing about how the experience changed her mind about vaccines for good. She quickly became a viral sensation, appearing on Good Morning America and in newspapers across the country to tell her story.

While news outlets latched onto to her kids' illness as the impetus for her change of heart, O'Meara says her doubts about not vaccinating her children were planted before they ever got sick. First, it was a well-respected colleague who had been a high school biology teacher that reacted angrily when O'Meara told her she wasn't planning to vaccinate her daughters. Then, news stories about a measles outbreak in California shook her confidence. Her own grandmother had gone deaf in one ear because of measles. She began to realize that these diseases were not a thing of the past, but were actively being prevented by vaccines. And she recognized that her own assurances that her children were safe was based largely on a herd immunity — the idea that other kids around her children were vaccinated, so her child's chances of getting sick were lower precisely because vaccines worked.

"I took the path of passive risk," O'Meara said. "Not doing anything, I felt, was less of a risk than taking the active chance of vaccinating my children and running some kind of risk of side effects."

When her family contracted rotavirus, though, the cloud was finally lifted. O'Meara was never a "loud and proud" anti-vaxxer like Sandlin or Leon, who both leveraged the internet to try to convince other parents to avoid vaccines. When she had a change of heart, she didn't lose an online community. She did temporarily lose one of her closest friends, who told her point blank that she didn't know how to react to O'Meara vaccinating her children. Her husband's family, though, was relieved.

O'Meara says she knows the best way to get through to anti-vaxxers.

"The approach is absolutely not name calling, it's absolutely not, 'you're crazy, you're stupid,'" she said. "It's definitely the place for having some empathy… moms are under a lot of pressure, and I think that pressure has only increased over time. We only have one chance to get it right. We have this fear that if I screw up, the world is done for and I'm going to feel guilty for the rest of my life."

Leon, who at one point was the administrator of an anti-vaxxer Facebook group, said her doubts about the movement grew when she saw her peers misinterpreting a study on the DTaP vaccine. People in her anti-vaxxer circle misunderstood the vaccine to be capable of "shedding," a term anti-vaxxers use to describe a vaccine that can infect people with its live virus. Leon, though, had read the study closely and understood that DTaP couldn't shed, yet she struggled in earnest to get her fellow anti-vaxxers to stop spreading misinformation.

"I would bring up these points to other anti-vax people and it was just met with criticism or name calling," Leon said. "Instead of defending their camps, they just completely attacked me."

Leon, was pregnant with her second child when an outbreak of measles in the Amish community of Ohio further shook her confidence about what she was doing. Ultimately, she sat down with her husband and said she felt like they had been given poor information. They reversed course, vaccinated their kids and tried to convince others to join them.

It didn't go well.

"I got threatening messages," she said. "I had to completely get a new Facebook page because of the harassment. I was called a troll. One of the admins in this group was a real life friend who lived in the same city as I did and when she found out I was going start vaccinating my kids, she stopped being friends with me."

Now, Leon says she tries to point out inherent hypocrisies in the anti-vaxxer movement to get people to second guess themselves.

For example, a popular anti-vaxxer tactic is to insist new parents read the vaccine package insert before administering it to their children. Inside they'll find a list of scary sounding chemicals and warnings from government agencies about the vaccine. Those same people, though, typically say that you can't trust big pharmacy companies, the Center for Disease Control or the Federal Drug Administration, which all advocate for vaccinating your children.

"So then I would say, 'why are you telling to me read a document that's circulated by pharmaceutical companies and regulated by the FDA? Do you always just cherry pick this stuff to fit your own biases?'" she said. "And it gets them thinking, 'Oh shit, that is kind of being a hypocrite.'"

It was that rampant distrust for government that sparked Sandlin's exit from the anti-vaxxer movement. Her doubt sprung up when she realized how many people in the anti-vaxxer movement bought into wild conspiracy theories like chemtrails.

"A huge part of realizing I was wrong (besides reading studies for myself) was realizing that in order to believe vaccines are purposely being made with harmful ingredients that damage our kids, I'd have to believe every single government in the world and every scientific organization were all in on this together," Sandlin said. "Even governments at war with each other."

O'Meara, Leon and Sandlin are not alone. The website Voices for Vaccines has an entire section dedicated to blog posts from people who went from anti-vaxxer to pro-vaxxer, and there are dozens of stories. In states like California, people are taking the legislative route. Legislation was passed in 2015 that prevented parents from declining to vaccinate their children for "personal beliefs." As a result, 95.6 percent of the children enrolled in reporting child care facilities received all required immunization, a significant improvement from just five years earlier.

"One thing I wish pro-vax people would recognize is we're not trying to hurt our kids, we were just given the wrong information," Leon said.

She insists people should heed the advice of her friend Dorit Rubinstein Reiss, a professor of law at UC Hastings who is one of the most outspoken advocates of vaccines in the country.

"She always told me, 'debate the facts, not the person.'"