Inadequate vaccination coverage was to blame for the ongoing, multistate outbreak of measles linked to Disneyland Resort in California, according to a research letter published online in JAMA Pediatrics.
Rates of MMR vaccination in affected communities fell well short of the level needed for herd immunity, wrote Maimuna Majumder of Children’s Hospital Boston and the Massachusetts Institute of Technology, Cambridge, Mass., and associates. The ongoing outbreak “shines a glaring spotlight on our nation’s growing antivaccination movement and the prevalence of vaccination-hesitant parents.”
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Measles is highly contagious, and adequate vaccine coverage is essential to prevent outbreaks. The current outbreak began at Disneyland Resort in December 2014, with secondary cases reported elsewhere in California and in Arizona, Colorado, Nebraska, Oregon, Utah, and Washington, according to the Centers for Disease Control and Prevention. The rapid rise in cases suggested that much of the exposed population was susceptible to measles, the investigators wrote (JAMA Pediatrics 2015 Mar. 16 [doi:10.1001/jamapediatrics.2015.0384]). To test that hypothesis, they used the IDEA (incidence decay and exponential adjustment) method to analyze publicly available outbreak data from the California Department of Public Health and Health Map media alerts. Based on their model, MMR vaccination rates in communities with secondary cases might have been as low as 50% and did not exceed 86%. “Given the highly contagious nature of measles, vaccination rates of 96%-99% are necessary to preserve herd immunity and prevent future outbreaks. Even the highest estimated vaccination rates from our model fall well below this threshold,” they wrote.
Estimating vaccination coverage among Disneyland visitors was challenging, not only because vaccination data are usually available only at the state and county level, but because the exposed population likely had diverse vaccination rates, the researchers said. Nonetheless, they concluded, “MMR vaccination rates in many of the communities that have been affected by this outbreak fall below the necessary threshold to sustain herd immunity, thus placing the greater population at risk as well.”
The National Library of Medicine funded the work. The authors declared no relevant conflicts of interest.