Sam Bernstein, Daily Briefing
On April 17, California health officials declared an end to a high-profile measles outbreak that sickened 147 people in six states. But public health officials still worry that measles—once thought to be completely eradicated from the United States—could stage a permanent comeback because of declining rates of vaccination in certain communities.
How one sick visitor at Disneyland became a multi-state measles outbreak
State immunization rates vary, but remain generally high
The World Health Organization (WHO) estimates that between 83% and 94% of community members must be vaccinated to achieve heard immunity against measles. By that metric, all 50 states have immunization rates for the mumps, measles, and rubella (MMR) vaccine that should protect young children from the disease.
Variation in states' vaccination rates is driven in large part by variations in government policy.
Although every U.S. state requires children to receive the MMR vaccine, data compiled by the Immunization Action Coalition, which is funded in part by CDC, show that only two states have limited vaccine exemptions for the MMR vaccine to medical reasons: Mississippi and West Virginia. As you can see in the map below, most states also allow exemptions for religious or personal reasons—or both.
However, experts say state-level immunization rates only tell part of the story. It's variation at the community level that's actually putting kids in danger.
Drew Harris, a physician in New Jersey, recently obtained school-level immunization records for his state.
Overall, he found that only 615 of 5,775 schools had fewer than 94% of their students fully vaccinated. But within that group, 111 schools had fewer than 83% of their students vaccinated—placing them below the WHO standard for heard immunity against measles.
Harris says schools with low rates of vaccination tend to cluster. "It's a social-network phenomenon," he says, explaining, "Bad ideas spread like a virus… Part of what we must do is inoculate people against the idea that a vaccine is worse than the disease itself."
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Notably, there were almost two times more private schools than public schools with rates of vaccination below 83% in New Jersey. The school with the lowest rate of vaccination in New Jersey was the Waldorf School of Princeton, which has an annual tuition of over $19,000 dollars. Only 59% of its students were vaccinated.
Researchers in California have seen a similar trend, nothing that schools with low vaccination rates cluster together. In one of the wealthier regions in the Los Angeles area, rates of vaccination exemptions for preschoolers increased by 26% from the 2011-2012 school year to the 2013-2014 school year, reaching 9.1%. By comparison, exemption rates in Los Angeles County overall were about 2.2% during the 2013-2014 school year.
Experts say that low vaccination rates at the community level are putting children at risk of contracting the disease. "Immunization is one of the best ways parents can protect children from serious childhood diseases. Through immunization, we have made great strides in drastically reducing infant death and disability in the United States," says Donna Leusner, a spokesperson for the New Jersey Department of Public Health.
New data suggest the calls for vaccination during the multi-state measles outbreak may have paid off: Merck says sales of its measles vaccine in Q1 2015 were 24% higher than in Q1 2014.