Low
vaccination rates likely fuel the 2015 measles outbreak, calculations show
Boston Children's Hospital, Science
Daily
Inadequate vaccine coverage is likely a driving force behind the
ongoing Disneyland measles outbreak, according to calculations by a research
team at Boston Children's Hospital. Their report, based on epidemiological data and published online by JAMA Pediatrics, indicates that vaccine coverage among the exposed populations is far below that necessary to keep the virus in check, and is the first to positively link measles vaccination rates and the ongoing outbreak.
By examining case numbers reported by the California Department of Public Health and current and historical case data captured by the HealthMap disease surveillance system, the researchers--led 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 between 50 and 86 percent, far below the 96 to 99 percent necessary to create a herd immunity effect.
Measles is highly contagious. It's estimated that an infected
individual in a population fully susceptible to measles will spread the virus
to between 11 and 18 additional people. This number is called the virus's basic
reproduction rate, or R0.
In a population where at least some individuals are immune to measles, the virus spreads from person to person more slowly. The rate of spread in an immune population is called the virus's effective reproduction rate, or RE.
In a population where at least some individuals are immune to measles, the virus spreads from person to person more slowly. The rate of spread in an immune population is called the virus's effective reproduction rate, or RE.
Using case data, R0 and measles' serial interval (the length of
time for each successive wave of transmission to follow the one before),
Majumder and Brownstein calculated that the virus's RE in the Disneyland
outbreak is between 3.2 and 5.8. From there, the pair calculated their
vaccination estimate.
The researchers are quick to note that their estimate does not
reflect vaccination across the United States, the state of California or even
among the population of Disneyland visitors at the outbreak's start. Rather, it
reflects the vaccination rate among the exposed populations in each cluster of
cases linked to the outbreak so far.
"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," says Majumder.
Using the same data sources, the HealthMap team has separately
released an interactive model illustrating how differing rates of vaccine
coverage could affect the growth of a measles outbreak over time. The model,
available athealthmap.org/measlesoutbreak,
puts the effects of vaccination into stark relief.
If a population is fully vaccinated against the virus, the model predicts that one case of measles will give rise to only two additional cases over 70 days. By contrast, if only 60 percent of a population is vaccinated, more than 2,800 cases will occur over the same time period.
If a population is fully vaccinated against the virus, the model predicts that one case of measles will give rise to only two additional cases over 70 days. By contrast, if only 60 percent of a population is vaccinated, more than 2,800 cases will occur over the same time period.
"Our data tell us a very straightforward story--that the
way to stop this and future measles outbreaks is through vaccination,"
says Brownstein, a digital epidemiologist and co-founder of HealthMap and
VaccineFinder, an online service that allows users to search for locations
offering a variety of vaccinations, including the MMR vaccine that protects
against measles.
"The fundamental reason why we're seeing the number of cases we are is inadequate vaccine coverage among the exposed.
"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," he continues, "and help local
public health officials in their efforts to control this outbreak."
Story
Source:
The above story is based on materials provided by Boston
Children's Hospital. Note:
Materials may be edited for content and length.
Journal
Reference:
Maimuna S. Majumder, Emily L. Cohn, Sumiko R. Mekaru, Jane E.
Huston, John S. Brownstein. Substandard
Vaccination Compliance and the 2015 Measles Outbreak. JAMA Pediatrics, 2015 DOI: 10.1001/jamapediatrics.2015.0384
Cite
This Page:
Boston Children's Hospital. "Low vaccination rates likely
fuel the 2015 measles outbreak, calculations show." Science Daily,
16 March 2015. <www.sciencedaily.com/releases/2015/03/150316121933.htm>.