Inadequate vaccine coverage is to blame for the ongoing U.S. measles outbreak linked to Disneyland in California, according to calculations out Monday by a research team at Boston Children's Hospital that confirmed previous understanding of the highly contagious disease.
Their report, based on epidemiological data and published online by the U.S. journal JAMA Pediatrics, found that the measles vaccination rate among the exposed population where secondary cases occurred 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.
"Clearly, MMR (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," said the researcher, led by Maimuna Majumder and John Brownstein of Boston Children's Informatics Program.
Measles is so contagious that an infected individual in a population fully susceptible to the disease will spread the virus to between 12 and 18 additional people. By comparison, the so- called basic reproduction rate for Ebola, the virus currently circulating in West Africa, is only 1.5 to 2.5.
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.
The researchers calculated that the virus's effective reproduction rate in the Disneyland outbreak is between 3.2 and 5. 8.
Currently, the United States is experiencing a multi-state measles outbreak that started at Disneyland in December 2014, although the index case has not yet been identified. As of March 13, there have been 133 confirmed measles cases reported in California residents