By Brenda Goodman, MA
WebMD Health News
Reviewed by Arefa Cassoobhoy, MD, MPH
Feb. 6, 2015 -- The current measles outbreak, which has topped 100 cases so far, is believed to have started at two Disney theme parks in California.
But it's not the first time measles has been spread through a big theme park.
Two years ago, measles passed through a theme park in Orlando, FL. The grand total of cases in that outbreak? Five, including a tourist from Brazil.
Those cases got little notice. The California outbreak, on the other hand, has parents scared and angry, politicians taking stands on vaccination, and public health officials in 14 states bracing for a second wave of infections.
What made the difference? How did one outbreak flame while the other fizzled?
"Part of it, of course, is just luck. When you have patients who are infectious, it all depends on how many non-immunized people they come across. Of course, we try to make the best luck we can," says Kenneth Alexander, MD, chief of the Division of Infectious Diseases at Nemours Children's Hospital in Orlando.
Public health officials agree.
"I can only speculate, but I think that we have a higher vaccination rate in Central Florida compared to the areas of California affected by the outbreak," says Tania Slade, epidemiology program manager at the Florida Department of Health. "Herd immunity likely prevented the further spread of measles in our community."
Herd immunity means that since many people in a group, or "herd," are vaccinated against a disease, the few people in the group who aren't immunized also get protection from the illness. The disease won't spread in their group since most people can't catch it.
Slade declined to name the theme park involved since the outbreak is over, and she doesn't want to spook visitors. Tourism is the area's biggest industry, after all.
But as public health officials in Florida have watched the California case count climb, they've realized they were lucky. "We're very relieved we dodged that bullet," she says.
Comparing the two outbreaks is instructive. Both are believed to have started when an unvaccinated person visiting the U.S. from another country came into contact with the measles during a trip to a theme park. Both began over the winter school holidays in mid-December. And both sent investigators scurrying to tamp down the disease. Both were even in counties named for their famed orange groves.
In Orange County, FL, the health department determined that four sick children from one unvaccinated family -- who had claimed a religious exemption -- exposed 528 students, 50 school faculty and staff, 24 kids and adults on a sports team, 15 people who visited the same pediatric urgent care clinic they went to, 67 teachers and kids at a day care, and an unknown number of church parishioners.
After tracing all those possible chains of infection, they only found just three others who were unvaccinated and possibly exposed. Three.
In California, on the other hand, at least 36 infected people are unvaccinated, according to Gil Chavez, MD, state epidemiologist. And those unvaccinated people have carried the measles to dozens of other unprotected kids and adults.
For example, a secondary infection from the Disneyland outbreak at a single high school in Huntington Beach, CA, sent 20 unvaccinated kids home from school for 3 weeks to help keep them, and everyone around them, safe, says pediatrician Marc Lerner, MD. He's the medical officer for the Orange County Department of Education in California.
And two school districts in that county, Laguna Beach and Capistrano, have measles vaccination rates so low that more than 20% of kindergartners start school without their measles, mumps, and rubella (MMR) shots.
Ease of Exemption Plays a Role
How did the vaccination safety net get to be so full of holes in California?
One answer lies in the ways parents are allowed to opt out of vaccination.
Florida, like all states, requires kids to be vaccinated before they can enroll in public school. But there are two exceptions. The first is for kids whose health could be compromised by vaccines, perhaps because they have an underlying medical condition like cancer, or a severe allergy to one of the vaccine ingredients. The second allows people to opt out if vaccination conflicts with their religious beliefs.
Religious exemptions like Florida's are available in 48 states.
California and 16 other states allow for a slightly different kind of exemption, called a philosophical or personal belief exemption.
In general, personal belief exemptions have been easier for parents to claim than religious exemptions. Until last year, when California toughened its requirements for personal belief exemptions, all parents had to do was flip over the form for vaccination records and check a box indicating they didn't believe in the shots.
In Florida, on the other hand, in order for parents to claim a religious exemption, they have to drive to their local county health department, pick up a specific form, and get it signed by the county director. Officials aren't allowed to question parents or do anything more than take their word that their spiritual beliefs conflict with immunization.
Still, the extra effort involved appears to make a difference.
A 2012 study published in The New England Journal of Medicine found that in states that allow parents to opt out for personal beliefs, rates of vaccine exemptions were more than twice as high as in states that only allow exemptions on religious grounds.
In Orange County, CA, about 3% of kindergartners had personal belief exemptions this school year.
The year measles came to Orange County, FL, the percentage of kids starting school with a religious exemption was about half that, just 1.4%.
Protecting the Herd
Experts say these exemptions have eaten away at decades of work by public health officials to protect the public from infectious threats.
Measles is a highly contagious disease. Ninety percent of people who are exposed will get it if they aren't already immune, either because they've had it or because they've been vaccinated against it. So the percentage of people who need immunity to protect the entire group has to be substantial -- somewhere between 92% and 94%, the CDC estimates.
And that protection has to be extremely local to be effective. High state or even county vaccination rates can mask pockets of people who are unprotected. Clusters of unvaccinated people, within churches, schools, and day cares, are what allow infections to spread.
"Vaccine refusal is a local issue. It doesn't matter what the state rate is, or the national rate is, if 1 out of 3 or 1 out of 4 kids that your child comes into contact with is unvaccinated, then your child is at risk and that community is at risk," says Daniel Salmon, PhD. He's the deputy director of the Institute for Vaccine Safety at Johns Hopkins Bloomberg School of Public Health in Baltimore.
"If the national coverage is great, but your community is not, that's a problem," Salmon says.
Both Orange County, CA, and Orange County, FL, were in the lower end of the herd immunity range when measles struck their theme parks. They each recorded vaccination levels around 92%.
What probably made the difference, experts say, is that the vaccination coverage in Florida was more uniform than it was in California.
"There are pockets, sometimes deep pockets, of unimmunized kids behind these numbers. It comes as no surprise that families that don't immunize tend to send their kids to the same school, or they go to the same church or they get together socially," Alexander says.
"This is what's keeping state officials awake at night, is that measles is going to get into some private school where they don't have an immunization requirement, and it's going to spread like wildfire," he says.
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