"The Happiest Place On Earth" is ground zero for a recent measles outbreak centered in California. Now, unvaccinated people are being warned to avoid visiting Disneyland parks.

Image Credit: Wonkblog + Associated Press

No Infants In Disneyland

There are now 67 confirmed cases of measles in an ongoing outbreak centered in California. According to the California Department of Public Health, 59 of the cases are in-state. Among the 34 California patients for whom vaccination status is known, 28 were unvaccinated and one had received partial vaccination. Only five were fully vaccinated.

Forty-two of the California cases have been linked to an initial exposure at Disneyland or Disney California Adventure Park, and while cases were originally tied to people who visited the park in mid-December, state health officials now note other cases visited Disney parks in January. According to the CDC, the majority of measles cases reported so far during 2015 have been part of the "large, ongoing outbreak" connected with these parks.

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Last year, there were 644 measles cases documented in 27 states – the biggest annual number in close to a quarter century. For those hoping to avoid seeing similar infection rates in 2015, the year is off to an inauspicious start.

Above: Cumulative number of new measles cases by month, for each year from 2001 to 2014 | Via Wonkblog

Unvaccinated people are now being warned to avoid visiting Disneyland parks. The reasoning is simple: Most people who get measles are unvaccinated, and the disease spreads easiest when when it reaches a community where large groups of people are unvaccinated. Limiting the number of unvaccinated people in the park therefore not only protects them from themselves, it protects the immunized visitors, as well.

It also protects those too young to be immunized. Of the measles patients who have been hospitalized in this recent outbreak, six cases have been in infants too young to be vaccinated, whether their parents want them immunized or not.

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"I would recommend that infants are not taken to places like Disneyland today," said Gil Chavez, deputy director of the California Department of Public Health's Center for Infectious Diseases, in an interview with the LA Times. Yesterday, Chavez implored parents to take action to protect not only their own children, but other children who might be affected by their decisions.

"I am asking unvaccinated Californians to consider getting immunized," Chavez said. "We have a particular responsibility to protect all of our infants in the state until they are old enough to be vaccinated."

This is how the anti-vaccination movement ruined Disneyland, not just for those who would actively refuse vaccines, but for everyone else. The cause for measles's resurgence is as unambiguous today as it has been in recent months. Last May, Dr. Anne Schuchat, assistant surgeon general and director of the CDC's National Center for Immunization and Respiratory Diseases stated unambiguously that "the current increase in measles cases is being driven by unvaccinated people." Yesterday, pediatric infectious disease specialist James Cherry told the New York Times that the Disneyland outbreak was "100 percent connected" to the anti-vaccine movement. "It wouldn't have happened otherwise — it wouldn't have gone anywhere," he said.

When Anti-Vaxxers Cluster

Of course, the ill-effects of the anti-vaccination movement are not limited to Disneyland. The consequences of refused vaccinations are felt anywhere that people gather, allowing diseases like measles to spread vast distances very quickly. Venues like theme parks and airports are considered potential flashpoints, because they see a lot of international travelers, who may originate from countries where diseases like measles have yet to be eliminated.

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Schools also pose a serious challenge. While state officials have not gone so far as to ban unvaccinated people from visiting Disneyland altogether, such measures have recently been taken at California schools. Health officials in Orange County this week issued more than 20 letters to parents stating that students who could not prove they had received a measles vaccine could be barred from class. (The Journal of the American Medical Association has published research this week on legal strategies for combating the growing danger of nonmedical vaccine refusal.)

The major concern of California health officials is that school vaccination rates remain above 95% – a threshold critical to maintaining herd immunity. Statewide, the vaccination exemption rate among California kindergartners was 3.1% for the 2013–2014 school year, but there are pockets across the state where exemption rates have crept into the double digits.

A newly published study on anti-vaccination patterns is the latest to highlight some of these pockets. The study, which was led by Kaiser Permanente Division of Research Director Tracy Lieu and appears in this week's issue of the journal Pediatrics, finds that parents who opt out of vaccinating their children tend to cluster, creating geographic hot-spots where large percentages of children receive no vaccines or are under-immunized. The findings could explain how a disease like measles – which was officially "eliminated" from the U.S. in 2000 – has managed to acquire so firm a foothold within the American population.

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NPR's Liza Gross reports on the study, and an ironic consequence of this clustering effect:

If these parents were distributed randomly, their decisions would be less likely to harm others, especially babies too young for vaccination. But parents who use personal belief exemptions to avoid school vaccination requirements often live in the same communities, studies have found.

And parents of children too young to go to school do, too... These younger children face the highest risk of dying from whooping cough and other vaccine-preventable diseases.

...The main problem with this clustering behavior, says [Saad Omer, a researcher at Emory University who found that clusters of personal belief exemptions contributed to the 2010 California whooping cough epidemic that killed 10 babies], is that every child's risk for disease depends on what others do. That's because no vaccine is 100 percent effective, so even a vaccinated child could get sick if exposed.

Lieu's team also identified five clusters where all vaccines were refused for close to 9,000 babies and toddlers in the study:

  • 10.2 percent of children in an area from El Cerrito to Alameda
  • 7.4 percent in northeastern San Francisco
  • 6.6 percent in Marin and southwest Sonoma counties
  • 5.5 percent in northeastern Sacramento County and Roseville
  • 13.5 percent of kids in a small area south of Sacramento

"These are early signals," says Lieu. "These kinds of clusters can be associated with later epidemics."

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In an interview with the NYT, Jane Seward, deputy director of the viral diseases division at the CDC, echoes Lieu's sentiment:

"The problem is that there are these pockets with low vaccination rates... if a case comes into a population where a lot of people are unvaccinated, that's where you get the outbreak and where you get the spread."

[ NPR | CDPH | WaPo | LAT | CDC | JAMA | NYT]