As anti-vaccination groups fight back against public health campaigns to promote immunization in the face of measles outbreaks, some Canadian doctors say the battle has escalated beyond social media platforms to personal threats and attacks.

"The pitchforks are coming out," said Dr. Anna Wolak, a family physician in Vancouver who has publicly spoken out on social media about the importance of vaccination — both as a doctor and the mother of three children.

Although most patients and many people in the community have been supportive, Wolak said she has also been subjected to furious comments — both in person and through social media.

"Patients have come in and told me that they can't believe I would deliberately poison my children," Wolak said in an email to CBC News.

"Some of those have threatened to report me to the [provincial regulatory] college because they consider me a threat to children."

Promoting false claims

Anti-vaccination groups promote false claims linking vaccines to childhood illnesses or injury. Some of those beliefs stem from a research paper in the 1990s that wrongly suggested vaccines could cause autism. That study was debunked years ago and the author stripped of his medical licence. Other adverse reactions to vaccines have been proven to be very rare.

Wolak worries about what people with anti-vaccination views will say in front of her children. She remembers an especially hateful comment someone made at an event she was attending.

"[They said] I really hope that your kids get a vaccine injury and then you'll know exactly what sort of poison you're talking about," she recalled.

Although those interactions have been hurtful, Wolak said she has not received any "overt threats" of physical harm — something that has happened in recent months to at least two other Canadian doctors.

I WILL make sure you NEVER support vaccines EVER AGAIN! - Email threat to two Canadian doctors

One of the recipients of the threats is in Toronto. The other is in Eastern Canada. They say they have both received a steady stream of emails ranging from harassment to threats — about 200 of which came from the same email address — since the fall.

CBC News has agreed to protect the identities of both doctors because they are afraid they will be the target of further threats.

One email sent to both of them said: "Come at my son with another vaccine and I WILL make sure you NEVER support vaccines EVER AGAIN! This email isn't even CLOSE TO LISTENING TO ME IN PERSON!"

"Signed, A Momma With Claws OUT!"

The eastern Canadian doctor received additional emails and voicemails. The doctor said they include threats of killing and dismemberment.

'Deeply unpleasant'

That doctor has received angry emails before from anti-vaccination advocates and ignored them, but said these were different.

"These were not trivial threats," the doctor said. "I can't change the science ... and to be unable to stand up and tell what the science is and not have somebody threaten you in a very nasty way is deeply unpleasant."

The Toronto-based public health doctor said the emails she received were upsetting — but the fact that people are trying to silence those promoting life-saving immunization is even more concerning.

"It's not about me," she said. "It's emblematic of the larger system under attack. It's putting kids at risk."

Dr. Gigi Osler, president of the Canadian Medical Association, says threats against doctors by people opposed to vaccination are 'not acceptable' and says it's vital for Canada's physicians to keep educating people about vaccine effectiveness and safety. (Dennis Cleary/CBC)

The threats to both doctors were reported to police.

Both physicians said police told them the IP addresses for the emails were in the U.S.

CBC News contacted Toronto Police — which confirmed a report had been filed — as well as the RCMP and the municipal police force in Eastern Canada where the death threats occurred. None of them would confirm whether there is an ongoing investigation or provide any details.

There are more established anti-vaccination groups in the U.S. than in Canada. The most prominent organizations characterize themselves as advocating for "vaccine choice" or representing people who have been "vaccine injured," rather than being against vaccinations.

Recently, measles outbreaks have prompted some government officials in the U.S. to call for mandatory vaccinations, which has enraged anti-vaccination groups, prompting rallies and increased outcries on social media.

Demonstrators at the 'March for Medical Freedom' in February protested a bill that would eliminate non-medical exemptions from vaccination in Washington state. (Lindsey Wasson/Reuters)

American physicians, including pediatricians, have received threats from anti-vaccination believers, although the American Academy of Pediatrics was unable to confirm how many cases they are aware of.

The goal is to shut you up. - Dr. Paul Offit , pediatrician, Children's Hospital of Philadelphia

Pediatrician Dr. Paul Offit, director of the Vaccine Education Center at the Children's Hospital of Philadelphia, said he has received harassing and threatening emails for years, ranging from messages calling him evil and accusing him of conspiring with "big pharma" to death threats.

"The goal is to shut you up," he said. "And it certainly works. There are a number of people who choose not to stand up in this arena because they know that it means that they're going to be personally targeted."

Offit continues to put himself in that difficult position, he said, because the consequences of children not being vaccinated against preventable diseases, including measles, is worse.

"We're getting to the point now where children could once again die from measles," he said. "You just have to do the right thing."

Del Bigtree, a prominent anti-vaccine activist in the U.S., told CBC News he would "absolutely ... discourage any sort of aggressive talk or violence of any kind."

Bigtree also said he didn't believe the threats would have come from anyone who was affiliated with his movement.

Offit doesn't believe anti-vaccination groups are making threats against doctors as part of a co-ordinated strategy — but he does think that they create the fervour that may set certain individuals off.

"They just whip up the base, you know? They use these really mean-spirited terms, they make it personal. They directed that, essentially, by setting you up as an evil person," he said. "But I don't think it's an organized effort."

Threats 'not acceptable'

Regardless of where the threats against the Canadian doctors originate, they are "extremely concerning and a matter for the police," said Dr. Gigi Osler, president of the Canadian Medical Association.

"Threats to personal safety for anybody is not acceptable and in particular towards doctors who are trying to speak out on behalf of the health of their patients and the health of Canadians."

Dr. Nadia Alam, a family physician and outgoing president of the Ontario Medical Association, worries that threats against doctors will silence important conversations about vaccination. (Ontario Medical Association)

Dr. Nadia Alam, a family physician in Georgetown, Ont., and outgoing president of the Ontario Medical Association, said she's concerned about the possible chilling effect such threats could have on getting accurate information about vaccines to parents.

"I ... worry that this may impact physicians from speaking out. And it shouldn't. Because at the end of the day, educating the public is a core part of our job," she said.

"Parents in my clinic are more confused than ever about the risks and benefits [about vaccines]. And they're coming in with information that's frankly incorrect."

The Toronto-based public health doctor who received the threats shares Alam's concern, noting that the return of measles after the disease was eliminated through vaccination in many European countries is "horrifying."

"Easily we could end up in the same boat," she said.