A new research letter published in JAMA Pediatrics is the first to positively link low vaccination rates to the Disneyland measles outbreak that emerged in California late last year. The new research also shows how frighteningly fast measles can spread in a population that's insufficiently immunized against the highly contagious virus.

Calculations made by a research team at Boston Children's hospital strongly suggests that substandard vaccination compliance is a likely driving force behind the ongoing Disney measles outbreak. The new report, which was based on epidemiological data, indicates that vaccine coverage among those in the exposed populations is far lower than what's required to prevent the virus from spreading.


"The ongoing measles outbreak... shines a glaring spotlight on our nation's growing antivaccination movement and the prevalence of vaccination-hestitant parents," write the authors in the new research letter.

Compromising Herd Immunity

The research team, led by by Maimuna Majumder, MPH, and John Brownstein, PhD, of Boston Children's Informatics Program, estimate that the measles vaccination rate among the case clusters in California, Arizona, and Illinois is somewhere between 50 to 86%. That's far below the 96 to 99% vaccination rate required to create a so-called "herd immunity" effect. The researchers gathered the data by examining case numbers reported by the California Department of Public Health and current and historical data acquired by the HealthMap disease surveillance system.


(Jama Pediatrics)

Measles is incredibly contagious; a lone infected person in a population fully susceptible to measles has the potential to spread it to between 11 and 18 additional people, i.e. the virus's basic reproduction rate (R0). In a population where there's at least some immunity conferred by vaccinations, the virus spreads much more slowly, i.e. the virus's effective reproduction rate (RE). According to the calculations performed by Majumder and Brownstein, the virus's RE in the Disneyland outbreak was between 3.2 and 5.8. This range is what allowed the researchers to calculate the vaccination rate estimate for the exposed population.


It's important to note that this estimate is not a reflection of immunization rates across the United States, California, or even among the population of Disneyland visitors at the onset of the outbreak. Rather, this figure represents the vaccination rate among the exposed populations in each cluster of cases linked to the outbreak thus far.

As Majumder put it in a statement, "It's as though you took everyone exposed to measles in the areas with case clusters, put them in a room and measured the level of vaccine coverage in that aggregate population."


A New Model

Interestingly, the HealthMap team used the same data to compile an interactive model showing how different rates of vaccine coverage can affect the spread of measles over time.


According to the model, one case of measles in a population fully vaccinated against the virus will give rise to two additional cases over a span of 70 days. Some 30 cases will appear over 70 days when there's 90% immunity. Contrast that with a population in which only 60% of people are immunized; in such a scenario, more than 2,800 cases will appear over the same period.


"Our data tell us a very straightforward story — that the way to stop this and future measles outbreaks is through vaccination," added Brownstein. "The fundamental reason why we're seeing the number of cases we are is inadequate vaccine coverage among the exposed. We hope these data encourage families to ensure they and their loved ones are vaccinated and help local public health officials in their efforts to control this outbreak."


Read the entire research letter at JAMA Pediatrics.

Graphs: HealthMap; top image: Cheryl Casey/Shutterstock.

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