Vaccine Refusal and Herd Immunity for Measles

Parents hesitant to vaccinate their children may delay routine immunizations or seek exemptions from state vaccine mandates. Recent outbreaks of vaccine preventable diseases in the US have drawn increasing attention to this phenomenon. Improved understanding of the association between vaccine refusal and the epidemiology of these diseases is needed. In our analysis of US measles outbreaks that have occurred since measles was declared eliminated in the US (after January 1^st, 2000), we selected 18 published measles studies (nine annual summaries and nine outbreak reports), which described 1,416 measles cases and more than half (56.8%) had no history of measles vaccination. Of the 970 measles cases with detailed vaccination data, 574 were unvaccinated despite being vaccine eligible and 405 (70.6%) of these had non-medical exemptions, e.g. exemptions for religious or philosophical reasons, as opposed to medical contraindications (41.8% of total).  Conclusion: A substantial proportion of the US measles cases in the post-elimination era were intentionally unvaccinated.  The phenomenon of vaccine refusal is associated with an increased risk for measles among people who refuse vaccines and among fully vaccinated individuals.