Objective: This content analysis study explored how online news media communicates and frames vaccination rates and herd immunity (the effect where enough people are immune, the virus is contained).
Methods: We analyzed 160 vaccination-related news stories by nine highest-trafficked news websites in Serbia, published July-December 2017, around the start of the measles outbreak. We coded both the news story as a whole and every vaccination-rate mention (N = 339).
Results: News stories framed current vaccination rates and changes in them in a predominantly negative way (175/241 and 67/98 mentions, respectively) (e.g., "only 50% vaccinated", "fewer parents vaccinating their children"), especially when referring to the measles vaccine (202/262 mentions). A total of 23/86 of news stories mentioning vaccination rates did not provide any numerical values. Reference groups for vaccination rates were rarely specified. Out of the 32 news stories mentioning herd immunity, 11 explained the effect.
Conclusions: Even routine communication of vaccination rates can be biased through negative frames and imprecise descriptions. Lamenting low immunization rates could activate a negative descriptive social norm ("many people are not getting vaccinated"), which may be especially ill-advised in the absence of an explanation of the social benefit of achieving herd immunity through vaccination.
Keywords: descriptive norms; framing; health communication; immunization; mass media; vaccine.