The deliberate campaign against the vaccine has already begun. Within 48 hours of the first people in the U.S. receiving the Pfizer vaccine, anti-vaccine activists were amplifying stories of allergic reactions and sharing claims about friends of friends whom the vaccine had supposedly injured or killed.

Public-opinion polling indicates that tens of millions of Americans are what physicians call “vaccine hesitant,” and stories of people who experience harsh side effects from the injection, or die for entirely unrelated reasons after receiving it, will inevitably find an audience. But many vaccine horror stories will originate in the well-established echo chambers of anti-vaccine true believers, including many of the same people who actively reject the scientific evidence that consistently affirms the safety of childhood immunizations against measles and other diseases. Some, such as Robert F. Kennedy Jr., leverage pseudoscience and attempt to drive public focus to outlier adverse events. Others bypass even the attempt at creating a veneer of scientific legitimacy and straightforwardly embrace conspiracy theories instead. Since 2018, anti-vaccine communities have cross-pollinated extensively with QAnon and other paranoid fantasies. Participants in anti-vaccine groups online frequently see posts claiming that the government is using COVID-19 vaccines to secretly implant microchip identifiers in people, or that ingredients in vaccines will turn people into 5G antennas.

In today’s networked form of activism, determined groups—as varied as Beyoncé’s Beyhive, QAnon adherents, or the K-pop stans who commandeered pro–Donald Trump Twitter hashtags during the 2020 campaign—leverage the entirety of the social-media ecosystem to promote the things they believe in. If they succeed in getting a meme or a hashtag to trend online, they’ll often get some news coverage in broadcast or print media, elevating the message to a far larger audience. This is how bottom-up narratives spread.