Although epidemiologists have not yet identified the person who brought measles to Disneyland, a new analysis shows that the highly contagious disease has spread to seven states and two other countries thanks to parents who declined to vaccinate their children.
Using some simple math, a team of infectious disease experts calculated that the vaccination rate among people who were exposed to the measles during the outbreak was no higher than 86%, and it might have been as low as 50%.
In order to establish herd immunity, between 96% and 99% of the population must be vaccinated, experts say.
“Even the highest estimated vaccination rates from our model fall well below this threshold,” the researchers reported Monday in the journal JAMA Pediatrics.
… In other words, the only way to explain how the measles spread from a single person at Disneyland to 145 people in the U.S. and about a dozen others in Canada and Mexico is that a substantial number of parents have not had their children fully immunized with the measles, mumps and rubella vaccine.
“Clearly, MMR vaccination rates in many of the communities that have been affected by this outbreak fall well below the necessary threshold to sustain herd immunity, thus placing the greater population at risk as well,” the researchers concluded.
Fortunately, there are bills in the California legislature (and hopefully in other states as well) to make it harder for parents to “opt-out” their kids.