Collaborative health promotion campaigns are advantageous because they extend the resources and reach of any single organization. Yet, they can be challenging because they require partner commitment and compromise. On social media, however, these campaigns are especially beneficial due to the high demand for ongoing content that facilitates user engagement. This study is a content analysis of an annual collaborative campaign, Preteen Vaccine Week, conducted by the California Department of Public Health (CDPH) to promote preteen immunizations. Campaign partners are encouraged to use creative assets provided by CDPH and to follow a themed content calendar. Message characteristics and audience engagement were evaluated for 2 years of the campaign (2019-2020). Results indicate that when there was a specific health issue scheduled as the daily theme, 85% of posts reflected that health topic. However, when the theme was general awareness, only 15% of posts aligned. Furthermore, the majority of posts included supplemental audiovisual assets of which nearly half were provided by CDPH. These findings suggest partners attempted to work together. Analyses of message characteristics indicate there was little effort to encourage online user engagement; however, a majority of messages included a call-to-action. These findings indicate that health organizations continue to use social media much like they use mass media: for information dissemination and behavioral recommendations.