In recent years, the United States has been confronting a resurgence of vaccine-preventable diseases—most notably, a measles outbreak that began in California's Disneyland in December, spurred on by the state's low vaccination rates. Fed up with the altogether avoidable outbreaks, several states have introduced legislation to eliminate the philosophical opt-out, a way for parents to get around public schools' vaccine requirements for their kids. As early as Tuesday, the California State Assembly will vote on SB-277, a law that would ban the so-called personal belief exemption. School boards, medical associations, and community leaders support the law.

But a small group of vocal anti-vaxxers is fighting hard to keep it from passing. This group, which leverages the power of social media, has launched a full-scale attack on the bill as it travels through the legislature. Each day, leaders craft tweets and instruct followers to disseminate them. Several senators who voted in favor of the California legislation have found themselves receiving extensive attention from the group—one, Senator Hannah Beth Jackson, has been @-mentioned (often unfavorably) in a particular Twitter hashtag more than 2,000 times since casting her vote in favor of the legislation.

One small, vocal group can have a disproportionate impact on public sentiment and legislation. Welcome to Anti-Vax Twitter.

This anti-vax activity might seem like low-stakes, juvenile propaganda. But social networking has the potential to significantly impact public perception of events—and the power to influence opinions increasingly lies with those who can most widely and effectively disseminate a message. One small, vocal group can have a disproportionate impact on public sentiment and legislation. Welcome to "Anti-Vax Twitter."

Since anti-vax activists lose on the science and are small in number, they have increasingly begun to rely on social media to inflate their presence. Twitter hashtags are particularly powerful because they transcend organized groups and the standard friend or follower relationships. More than any other social network, Twitter helps citizens to connect and organize in the real world even if they aren’t part of the same physical communities—anyone can participate in a conversation simply by following and using a hashtag.

A network graph showing the relationship between anti-vax, conservative, and autism clusters on Twitter. Scale Model

To take a closer look at how anti-vax Twitter organizes its attacks, specifically against the California legislation, we analyzed the hashtags used by people in the network. In network graphs like the one above, circular nodes are Twitter handles; larger nodes indicate accounts with more followers within the network, making their tweets more likely to be seen and acted upon. That's called "high centrality." The lines between the nodes represent follower relationships. Different colors represent communities of people who are sharing a similar message, and the distance between regions is based on common ties—the closer a colored group is to another, the more shared connections its members have, and the more likely information is to spread amongst the groups.

Much like any other group with a message, anti-vaxxers have leveraged Facebook, Twitter, and YouTube for quite some time. In the early days, the majority of their activism focused on propagating the myth of the autism-vaccination connection. In August of 2014, anti-vax activity began to coalesce into one primary hashtag: #cdcwhistleblower, a reference to an anti-vaxxer conspiracy theory that claims the CDC is concealing information that proves a link between the MMR vaccine and autism in African American children.

Within two days, autism-vaccine proponents had looped in celebrity sympathizers such as Donald Trump and Rob Schneider to amplify their message. Andrew Wakefield, author of the original fraudulent study linking MMR and autism, got involved. CDC scientists responded, but the idea of the conspiracy—and the hashtag—continued to grow in popularity. There were 250,000 #cdcwhistleblower tweets between August 18 and December 1 of 2014. A whopping 63,555 of these came from 10 prominent anti-vax accounts.

Like any good brand, that messaging has evolved to tie to current events, including legislation to increase vaccination rates.

In terms of massive events and national conversations, 250,000 tweets is rather small (there were 19.1 million tweets sent in a 12 hour period about the Oscars). But it became apparent to anti-vax leaders that this Twitter hashtag had attracted a loyal following among true believers. And so, in December 2014, “hashtag organizers” began to publish nightly “Trends and Tips” (TaTips) instruction videos on YouTube, containing instructions on what to tweet to advance the cause, and to improve the SEO of “vaccine questioning” websites. There are over 150 of these videos now—a testament to how much the anti-vax movement prioritizes Twitter.

Like any good brand, that messaging has evolved to tie to current events—including legislation to increase vaccination rates. As the California personal exemption bill progressed, tweets with the #cdcwhistleblower hashtag started to shift to include #sb277, the number of the bill.

When we dig into the activity in the #sb277 hashtag over time, trends and ties emerge. If you look at the first network graph, several distinct clusters emerge based on the hashtags and topics tweeted by each account. In other words, we didn't search for the group labels deliberately; they revealed themselves. Pink is tweeters focused on anti-vax content; orange is the autism community. These groups tweet about many of the same topics, but don't share the exact same material or use the same hashtags. Blue are social conservatives, many of whom use popular Tea Party hashtags. The long strands between the blue groups and the anti-vaxxer group indicates minimal overlap among accounts following each other; the networks are not tightly connected.

A network graph showing the emergence of the “parental choice,” or “vaccine choice” subset from within the anti-vax cluster. This messaging appeared after anti-vaxxers lost a legislative committee vote. Scale Model

But as you look at this second network graph, you can see how antivax political strategy has shifted. A new group emerges in the space between “Antivax Twitter” and “Conservative Twitter”—we call it “vaccine choice” Twitter. The tweeters are the same individuals who have long been active in the autism-vaccine #cdcwhistleblower network. And originally, much of the content shared in #sb277 focused on the same anti-vax pseudoscience underlying #cdcwhistleblower. However, as bad science and conspiracies repeatedly lost in legislative votes, anti-vaxxers updated their marketing: They are now “pro-SAFE vaccine” parental rights advocates. Instructions to the group now focus on hammering home traditionally conservative “parental choice” and “health freedom” messaging rather than tweeting about autism and toxins.

Twitter activity around #sb277 is part of a multipronged strategy that takes place alongside phone, email, and fax campaigns, coordinated by well-funded groups including the Canary Party and the NVIC. The net effect is that legislators and staffers feel besieged on all fronts. In one unfortunate video, a movement leader encouraged supporters to use Twitter to harass and stalk a lobbyist, who has since filed police reports. In a very recent creation, that same leader excoriates her “Twitter army” for diluting the power of the #cdcwhistleblower movement by creating their own hashtags rather than using the ones they’ve been assigned. She also requests that the entire network tweet at Assembly representatives to inform them that their political careers will be over if they vote in favor of SB277. Much like Food Babe leverages her #foodbabearmy to flood corporations with demands for change, the goal of anti-vax twitter is to dominate the conversation and make it look as if all parents are vehemently opposed to the legislation.

A zoomed-in view of the online antivax community, which includes @DrWakefield, author of the original fraudulent study linking MMR and autism. Scale Model

The other finding from observing anti-vax network graphs is that despite the vast majority vaccinating—national vaccination rates remain above 90 percent—there is no offsetting pro-vaccine Twitter machine; most parents simply vaccinate and move on with their lives. People don't organize in groups around everyday life-saving measures; there is no pro-seatbelt activist community on Twitter. The recent emergence, lack of central coordination, and weak connections seen among pro-vaccine Twitter users, who often use the hashtag #vaccineswork, means that the pro-vax message (in green) is not amplified to the same degree.

“Tweetiatrician” doctors, lawyers, and pro-vaccine parents often do attempt to join the conversation around the antivax hashtags. Unfortunately, many of the most active accounts experience the same attention received by the legislators: They become the target of harassment that includes phone calls to their places of employment, tweets posting identifying information or photos of their children, or warnings that they are being watched. Pro-vaccine activists and legislators alike often encounter paranoia when they attempt to engage the anti-vax community. They face accusations of being shills paid by Big Pharma to sway the narrative and keep “vaccine choice” activists from spreading The Truth.

The broader public health implications of propagating these memes and articles make anti-vax activities more than a bizarre online curiosity.

Like many fringe communities, while the group is extremely well-organized and passionate they are largely tweeting into an echo chamber. Twitter users who don’t look for these hashtags would likely not know that they exist. Besides occasional celebrity amplification, very little percolates out to a mainstream audience or trends at a meaningful level. And even theoretically-sympathetic affinity groups that they occasionally reach out to, such as anti-GMO, conservative, and autism groups, are not amplifying their hashtags.

However, the broader public health implications of propagating these memes and articles make anti-vax activities more than a bizarre online curiosity. Most of the material that the #cdcwhistleblower accounts tweet are designed to erode confidence in vaccination. The goal is to make new parents question everything, says anti-vax leader Dana Gorman in one of her instructional videos. Public health officials are concerned. "It is important to remember that today, the vast majority of people follow the recommended vaccine schedule—they take the advice of their doctors, supported by professional medical bodies and the WHO," says Gary Finnegan, editor of Vaccines Today. "However, it is essential that when people go online for information they are left with the clear impression that vaccines are safe and effective." If that's going to change, the people fighting misinformation need to understand how it gets spread in the first place.

Renee DiResta works in technology in San Francisco and invests in startups. She volunteers with Vaccinate California. Gilad Lotan is the chief data scientist at betaworks, making products including Scale Model