Sport activism and advocacy have received substantial attention in both academic and popular literature. However, recent years brought controversy, and corresponding advocacy, around a new topic—COVID-19 vaccines—which offers an interesting research opportunity since fans may view athlete engagement with health topics differently from other causes. Thus, this study explored the effect of athlete health-related advocacy on the corresponding fan attitudinal and behavioral changes directed toward both the athlete and the athlete’s team. A quasi-experimental design was utilized and both National Basketball Association (NBA) fans (n = 210) and Women’s National Basketball Association (WNBA) fans (n = 218) with a variety of perspectives on vaccines were recruited. Quantitative analysis demonstrated that the interplay between fan vaccine perspectives and athlete messages on the topic stimulated a variety of changes in fan attitudes and consumption intentions toward the athlete and, in some cases, the athlete’s team. Collectively, the findings have important practical and theoretical implications within sport marketing.