The #DoctorsSpeakUp hashtag, which was designed by physicians to collectively promote vaccines on Twitter in Spring of 2020, was hijacked by anti-vaccine advocates. Through the lens of counterpublic sphere theory, thematic analysis revealed that the hashtag hijacking by a scientific counterpublic was successful, and the majority of #DoctorsSpeakUp tweets were oriented against vaccines. Five overarching themes emerged in anti-vaccine hijacked tweets, including: personal experience with vaccine injury, profits over people, lack of liability, perception that doctors are uninformed, and 'We are the Herd.' In contrast, fewer than 17% of tweets originated from pro-vaccine doctors who openly identified themselves as such in their tweets or profiles. Thus, using the #DoctorsSpeakUp hashtag, anti-vaccine advocates dominated the discourse, which speaks to the communication dynamics afforded within the information ecosystem of a social network. Hashtag activism can connect individuals and promote grassroots movements but may also backfire, allowing a vocal minority of individuals to shout the loudest through the digital megaphone.