Pockets of Low Immunization Contributed to Measles Outbreak

Lara C. Pullen, PhD

March 16, 2015

A preliminary analysis suggests that substandard vaccination compliance contributed to the 2015 measles outbreak that began in Disneyland in December 2014.

Maimuna S. Majumder, MPH, from Massachusetts Institute of Technology in Cambridge, and colleagues present their analysis of outbreak data in a research letter published online March 16 in JAMA Pediatrics. They report that the rapid increase in cases suggests that a substantial percentage of the exposed population was susceptible to measles infection. The team also evaluated whether suboptimal vaccine coverage may have contributed to the outbreak.

"This article confirms a lot of what we have already suspected previously for the measles outbreak associated with the theme park in California," Litjen Tan, PhD, from the Immunization Action Coalition, explained to Medscape Medical News when asked to comment on the analysis.

In the absence of vaccination, measles is highly contagious, with a basic reproductive number (R 0) of 11 to 18. The team used nonlinear optimization to calculate the effective reproductive number (R E), which reflects the number of secondary infections that occur when a portion of the population is immune to disease.

They estimated that measles, mumps, and rubella vaccination rates in the exposed population with secondary cases might be as low as 50%, and probably no higher than 86%.

"Clearly, [measles, mumps, and rubella] 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 authors write. They estimate that a vaccination rate of 96% to 99% is required to preserve herd immunity from measles.

The problem may not just be with children, however. According to the California Department of Public Health Data, the outbreak included many adults older than 20 years.

"This would be supported by this article's suggestion that we have a concerning number of persons who are not immune to measles either through vaccination or through exposure. These adults were likely insufficiently immunized, perhaps receiving only one dose of vaccine, or not immunized at all. With the highly contagious nature of measles, it is not surprising that these adults became a focal point for the outbreak," elaborated Dr Tan.

Dr Tan also explained that vaccine coverage rates are typically tracked and reported at the state level, yet this approach may not be sufficient for predicting risk for an outbreak. This is especially the case for an extremely infectious disease such as measles, where high coverage is necessary to maintain community immunity.

Majumder and colleagues reveal that when coverage rates decline, the community becomes vulnerable to outbreaks, despite the fact that many states report their coverage levels to be higher than 90%. This is because state-wide coverage levels leave open the possibility of pockets of underimmunized individuals who are vulnerable to outbreak.

The recent measles outbreak occurred within such a pocket because, as described in the research letter, the coverage rates were significantly lower than the reported state average.

"This analysis draws our attention [to the fact that,] when you factor in other variables such as the presence of a large population of less immunized people such as international tourists, and combine them with an underimmunized community, outbreaks are very likely. We need better methods to survey vaccination coverage rates within states," concluded Dr Tan.

The authors have disclosed no relevant financial relationships. Dr Tan has served on scientific advisory boards within the past year for Pfizer, Merck, Novartis, Temptime Corp, and TruMedSystems.

JAMA Pediatrics. Published online March 16, 2015. Abstract

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