When the naturopath Elias Kass testified before a Washington state senate committee on 20 February with a baby on his chest and a pacifier in his hand, he knew that his arguments would be unpopular with the anti-vaccine activists in the room. Amid a measles outbreak that has infected 66 people so far, legislators were considering a bill to eliminate personal and philosophical exemptions for childhood vaccinations, and Kass was one of several practitioners to speak in support of the measure.

Kass faced some anger in the hallway after the hearing, he said, with one person calling him “a disgusting liar”. But it wasn’t until several hours later that “the shit hit the fan”. That’s when Kass realized that his Facebook page was being flooded with one-star reviews calling him everything from a “disgrace” and a “pedophile” to a “Nazi pharma shill” and “scumbag shilling for infanticide”. Kass disabled the Yelp-like reviews feature on the page, but that didn’t stop the onslaught, which moved into the page’s comments and across the ecosystem of anti-vaxx Facebook pages and groups. By Monday, five days after his brief testimony, he had compiled a photo gallery with hundreds of screenshots of abusive comments.

Kass is only the latest pro-vaccine health practitioner to be subjected to an online harassment campaign by anti-vaxxers. Networks of closed Facebook groups with tens of thousands of members have become staging grounds for campaigns that victims say are intended to silence and intimidate pro-vaccine voices on social media. The harassment only exacerbates an online ecosystem rife with anti-vaccine misinformation, thanks in part to Facebook’s recommendation algorithms and targeted advertising.

“Their goal is to tell my patients what a bad person I am so I lose business,” Kass told the Guardian by phone, five days into his ordeal. “It’s made me reluctant to engage online.”

‘Facebook is their vehicle’

The harassment of Kass appears to have been at least partially instigated by Larry Cook and Erin Elizabeth, two anti-vaccine activists who have built large Facebook platforms.

Cook has gained notoriety as a full-time anti-vaxxer who has raised nearly $80,000 on GoFundMe to pay for fear-mongering anti-vaccine Facebook ads which he targets at mothers. His Facebook page, Stop Mandatory Vaccinations (SMV), has about 130,000 followers, while the related closed SMV Facebook group has more than 150,000 members.

The former executive director of a professional organization for naturopaths, Cook has various online ventures, including an anti-vaccine YouTube channel, a $99 online training course about social media activism and a $3.95 ebook advising tenants on how to take their landlord to small claims court. He specializes in emotive anti-vaccine propaganda videos featuring parents and babies.

Many are afraid of posting pro-vaccine material on Facebook, for fear of putting a bullseye on their backs Chad Hermann, Kids Plus Pediatrics

Erin Elizabeth runs the website Health Nut News, links to which have been banned by Pinterest for consistently promoting harmful health misinformation. Her Facebook page has more than 500,000 followers, and she also runs a closed Facebook group, Holistic Lives Matter, which has about 53,000 members.

These Facebook groups are highly active and entirely one-sided: because they are closed groups, members are screened before they are allowed to join, and pro-vaccine voices are quickly banned as “trolls”. Another group containing abusive comments about Kass, “VaccineChoices – Fact VS Fiction, Conversations & Research”, is run by the prominent anti-vaxxer Sherri Tenpenny and has more than 40,000 members.

Both Cook and Elizabeth posted links to Kass’s Facebook page with their group members on the evening of 20 February, along with posts or screenshots criticizing Kass for his testimony. On Health Nut News, Elizabeth also published a post about Kass, which included the directive, “CLICK HERE for the ‘naturopath’s’ Facebook page”. The message was implicit: go get him.

A post in Stop Mandatory Vaccination appears to encourage harassment. Photograph: Facebook

The Guardian contacted Elizabeth for comment. Cook responded to queries about harassment by email, saying: “My intent is to ensure that those who oppose vaccine mandates know who favor vaccine mandates just like your intent is to name those who are against vaccine mandates.”

“It shows what a linchpin Facebook is,” Kass said. “That is their vehicle. Without that vehicle, who knows what they would be accomplishing.”

Facebook does not “tolerate harassment”, a spokeswoman said in a statement. “We use a combination of AI, machine learning and manual review to proactively detect and remove harassment throughout our platform – including posts in groups and pages. And since we find that comments can be used for harm, we’re working to give people the ability to hide or delete multiple comments at once.”

But Kass, who has been honored by the Centers for Disease Control and Prevention and profiled in Time magazine for his approach to persuading hesitant parents to vaccinate, said that he is concerned the campaign will affect his ability to connect with parents, now that he has been labeled “an enemy”.

The harassment has also created paranoia, he said. “Now, when we’re at the grocery store, if someone is looking at me, I’m wondering, ‘Did you see a meme where I had an X over my face and was holding a bunch of aborted babies?’”

A video shared in Stop Mandatory Vaccination garnered more than 600 comments where users urged each other to harass a Pittsburgh pediatrics practice. Photograph: Facebook

Calling in the cavalry

The power of harassment campaigns to dissuade pro-vaccine voices from engaging online raises questions about Facebook’s approach to anti-vaccine propaganda. Despite pressure from public health experts and politicians, the company has not banned anti-vaccine misinformation, which it says can be better addressed with accurate counter-speech than with censorship.

“The idea that we can have counter-speech when [Facebook] groups become brigade mobs is ludicrous,” said Renee DiResta, an expert in online misinformation and co-founder of Vaccinate California. “It makes just participating as an everyday citizen a high-stakes ordeal.”

“We are at the point where doctors are creating their own anti-vaxx social media attack response teams to help other doctors,” DiResta added.

One such rapid response team is being organized by Dr Todd Wolynn and Chad Hermann, the CEO and communications director of Kids Plus Pediatrics (KPP) in Pittsburgh, Pennsylvania.

“If you’re being attacked, we’ll light the signal fires of Gondor, and you’ll have pro-science, pro-vaccine cavalry come to your aid,” Hermann said of the nascent project, called “Shots Heard Round the World”.

Wolynn and Hermann are self-described social media evangelists for pediatricians. For years, they have encouraged practitioners to invest in digital communications in order to fill the void of evidence-based medical information on social media sites.

In September 2017, their practice was targeted by what Hermann described as a “coordinated terrorist attack from inside an anti-vaxx Facebook group”, which woke them up to the need for a means to rally the troops behind science.

The harassment campaign was touched off by a video KPP had produced and published in August 2017 to encourage uptake of the HPV vaccine. The video was initially well received and effective, Hermann said, prompting an uptick in queries and appointments from parents.

But at 1.15pm on 15 September – three weeks after the video was initially posted on Facebook – the first negative comment came in: “Is this some kind of joke? This vaccine kills people.” By the end of the day, the practice’s Facebook page had received more than 250 comments from anti-vaxx accounts. By the time the troll storm leveled off a week later, Hermann says that he had banned more than 800 accounts and deleted more than 10,000 comments.

The anti-vaxxers also went after the practice’s Yelp ratings and Google maps ratings. Hermann and Wolynn believe that the campaign originated in Tenpenny’s closed Facebook group, based on screenshots shared with them in the aftermath.

Negative reviews of Kids Plus Pediatrics remain on their Google reviews. Photograph: Google

“I never suggest or encourage any Facebook group or individual members to engage in social media harassment schemes,” Tenpenny said by email. “As you logically know, it is impossible to control what a Facebook member chooses to do or how they choose to behave.”

Tenpenny said that she has asked people not to “personally attack anyone”, but does “encourage confident, articulate, intelligent and factual responses online or within comment threads” in order to provide a “counterpoint” to pro-vaccine messages.

The campaign against KPP also found support inside SMV. After the video was shared with the Facebook group, members left hundreds of comments urging each other to go to KPP’s Facebook page and sharing screenshots of the negative reviews and comments they left. As the comment thread stretched into the hundreds, members also shared links to the practice’s Yelp and Google pages, as well as the phone number for the practice.

“They’re down to 1.8 stars lol,” one user posted, with a screenshot of the practice’s Google review page. “Yes!” another user responded. “Let’s get them with yelp too. Bastards. Haha.”

While Yelp has a robust process for protecting businesses from fraudulent reviews, KPP’s Google rating remains depressed by one-star reviews referencing the HPV video, something Wolynn noted could create “real financial harm” for medical practices.

After weathering the storm, Wolynn and Hermann decided to fight back. They worked on an observational study of the harassment campaign. And they’ve doubled down on their evangelism, working on a toolkit to battle the anti-vaxx backlash.

“Many providers and even whole hospitals are afraid of posting pro-vaccine material on Facebook simply for fear of putting a bullseye on their backs,” Hermann said. “When they stop posting that information, it leaves a vacuum, and we all know who is going to fill that.”

Hermann said that he does not think Facebook should ban groups like Sherri Tenpenny’s or SMV, but the social network should provide pages with tools to protect themselves from coordinated attacks.

“It seems to me that it would take Facebook about 10 minutes to allow us to ban or block every member of Sherri Tenpenny’s group,” he said. “I have no problem with them wanting to stay in their echo chambers, but this is the digital equivalent of 39,000 people storming into our waiting room and screaming at us and our patients.”