The "vast majority" of people commenting, sharing and liking anti-vaccination information on Facebook are women, a new study finds.

Researchers dug into the world of anti-vaccination people — better known as anti-vaxxers — by looking at data from six of the largest, public anti-vaxxer pages on Facebook. By analyzing two years' worth of data from these pages, the researchers determined that these communities are extremely active, negative in tone and primarily female.

"Social media plays an important role in making anti-vaccination beliefs durable and persistent," study lead researcher Naomi Smith, a lecturer in sociology at Federation University Australia, told Live Science. "Sharing posts is an important way to spread this type of information, so be careful before you share any post that claims vaccines make people sick." [5 Dangerous Vaccination Myths]

Vaccination works

Over the past two decades, childhood vaccines have saved the lived of 732,000 U.S. children and prevented more than 300 million kids from getting sick, a 2014 study from the Centers for Disease Control and prevention (CDC) found, Live Science previously reported.

Yet despite these impressive statistics, vaccines have faced opposition from the time they were invented in the late 1700s. This opposition continues today, including from people who suspect the measles, mumps and rubella (MMR) vaccine causes autism (it doesn't) or that the aluminum salts added to some vaccines are dangerous (these salts enhance the body's immune response, and studies show they're safe, according to the CDC). Other people delay vaccinations because they mistakenly believe that the recommended vaccination schedule can overwhelm a child's immune system, Live Science previously reported.

Digging deeper

To get a better idea of how anti-vaxxer Facebook communities function, Smith and co-researcher Tim Graham, a postdoctoral research fellow at the Australian National University, who has a joint appointment in the Research School of Social Science and the Research School of Computer Science, dug into the groups' posts, likes, shares and comments. They found the following:

Anti-vaxxer posts are highly shared, meaning that people frequently "shared" posts on their own Facebook pages or on their friends' pages, Smith said. In all, there were more than 2 million shares across the six groups during the two-year period, she said. "This means that the page's reach is much greater than the number of people who 'like' it," Smith said.

Participants were moderately active across several anti-vaccination Facebook pages, "suggesting that users' activity on anti-vaccination is more than just a product of Facebook's recommender system" — a system that recommends like-minded groups to people, Smith said.

Despite their large size and high levels of activity, anti-vaccination groups are relatively loose-knit. "That is, they do not necessarily function as close-knit communities of support with participants interacting with each other in a sustained way over time," Smith said. [Top 10 Golden Rules of Facebook]

Even though they are "loose," these groups show features of "small-world" networks. "In small- world networks, information diffuses quickly and easily through the network, in this instance through user-generated comments," Smith said. However, it's difficult to say whether these small-world effects are due to the nature of the anti-vaxxer movement itself, or are an artifact of Facebook, a platform that can help spread information quickly, Smith said.

The sentiments expressed in these Facebook pages were "quite negative in tone, suggesting that users of the anti-vaccination pages feel not only morally outraged about the practice of vaccination, but structurally oppressed by seemingly tyrannical and conspiratorial government and media," Smith said. Moreover, many posts had conspiracy-style beliefs placing blame on the government and media, Smith said. A 2011 survey found that conspiracy-style thinking is common among the general public and more pronounced in anti-vaxxers, a 2014 study in the American Journal of Political Science found.

Anti-vaxxers had concerns about state-sanctioned harm and interference with their autonomy. "In particular, anti-vaccination Facebook pages commonly compare vaccination to the Holocaust, illustrating a strong sense of persecution," Smith said.

Despite the paucity of evidence supporting anti-vaxxer beliefs, "it’s important to not make fun of anti-vaccination attitudes," she said. "This will likely reinforce any sense of persecution they feel."

The study was published online Dec. 27 in the journal Information, Communication and Society.

Original article on Live Science.