The Disneyland measles outbreak that began late last year spread to more than 150 people, hospitalized at least 17, and sparked a national debate about whether parents should be allowed to opt out of vaccinating their children. The outbreak helped lead to a new California law, passed by the state legislature and just signed by Gov. Jerry Brown, revoking personal beliefs as a justification for exempting children from vaccines required to attend public school. It’s a sweeping change that makes California’s school immunization law among the strictest in the country.
But this latest outbreak wasn’t the first cluster of measles cases associated with the happiest place on Earth. In 2001, a tour group of Japanese high school students visited Disneyland, bringing measles with them. Four were diagnosed with the disease and spent most of their stay in their hotel rooms. I was working as an epidemiologist for the California Department of Public Health at the time, tracking vaccine-preventable diseases. We were lucky that year. No other cases of measles were linked to those Japanese teenagers.
In 2001, Japan was struggling to control measles, a disease that is notoriously effective at getting human bodies to help it spread. Why did an industrialized nation in the 21st century have a measles problem? In the early 1990s, the mumps portion of Japan’s version of the MMR vaccine was linked to several cases of viral meningitis. Although the disease was mild and in all cases people recovered without any long-term consequences, the mumps vaccine was withdrawn from market in Japan. (The strain of the vaccine that was pulled from Japan was never used in the United States.) In response to the scare, Japan upended its immunization policy. All vaccinations were made voluntary, and vaccination rates dipped. In 2001, only about 80 percent of the country’s 2-year-olds were vaccinated against measles, nowhere near the 95 to 99 percent needed to maintain “herd immunity.” That year, Japan had more than 265,000 measles cases in children younger than 15. After people became aware of the measles danger and physicians started advocating for measles vaccinations, the rate rose dramatically, and, by 2007, 98 percent of toddlers were vaccinated. In recent years, the country has seen a few hundred cases a year.
In the same span of time, the United States has seen its measles rate grow. In 2002, the U.S. had a record low of 44 measles cases. In 2014, we had 644 cases. The rise has been driven by imported measles cases that spread within communities that have too few vaccinated people. Those communities are becoming more and more prevalent. Orange County, Disneyland’s home, is an example. The rate of personal beliefs exemptions from required vaccinations at kindergarten entry in 2001 was less than 1 percent. In 2014, that rate had tripled. Countywide, the measles vaccination rate remains high, but in many schools, about half the students have exemptions.
Parents who don’t vaccinate their children often cite worries about toxins in vaccines or the risk of autism—although the only study that supported this worry was a fraud that has been thoroughly debunked. Many of these parents furthermore think that the diseases prevented by vaccines are mild. But that’s simply not the case, especially for a disease like measles.
Measles is one of the most infectious diseases we know. The virus enters through our lungs, then quickly commandeers our own immune cells. It sneaks inside organs and tissues throughout the body. Eventually, it infects the cells of the throat and sinuses and new viruses come pouring out in coughs, sneezes, and runny noses. It causes a bad fever, conjunctivitis, light sensitivity, and diarrhea that can lead to dangerous dehydration. In rare cases, it causes encephalitis, or swelling of the brain.
“The idea that measles is not a highly pathogenic disease is totally untrue. It’s an irresponsible attitude,” Says Paul Duprex, a microbiologist at Boston University. What’s more, he says, “small kids are measles-making machines.”
Because measles tamps down our immune response, the disease leaves people at risk for secondary infections. People with measles seem to have fewer immune memory cells, which normally are poised to fight infections the body has encountered before. That reduced immune response can last for weeks or even months. “That means the patient has a temporary immunological amnesia, if you like,” Says Richard Plemper, an immunologist at Georgia State University. Nearly 150,000 people worldwide died from measles in 2013, many because of secondary infections that attacked their weakened immune systems.
One of my jobs when I worked for the health department was to field phone calls from parents with questions about the school vaccine law, including questions about getting a personal beliefs exemption. I lost count of the number of parents I talked to—answering questions, explaining the risks of choosing not to vaccinate. Early on, when some parents would hang up still convinced that vaccines were worse than the diseases they are supposed to prevent, my response was often angry frustration, so I understand the angry backlash since the Disneyland measles cases. But over time, I realized that parents who opt out of vaccinations aren’t making this choice lightly. It’s clear to me after hours spent on the phone with them that they are worried about their kids.
The best tool we have at our disposal for promoting vaccination is state-level health laws. The new California school immunization law leaves the medical exemption from vaccines intact, but if parents refuse vaccines for personal beliefs, including religion, their only option will be to homeschool their children. State Sen. Richard Pan, a pediatrician who co-authored the bill, links the rise in preventable diseases to parents choosing not to vaccinate. “That choice has consequences, especially when it impacts other people—not as a punishment for you, but to protect others from your choice,” he says.
The law has an unsettling loophole. It doesn’t eliminate all personal beliefs exemptions, just those for children entering kindergarten or seventh grade or changing schools. Kids in other grades won’t have to get immunized immediately. In communities with high personal beliefs exemption rates, the pool of susceptible kids will remain high for several years to come and may serve as the source of the next measles outbreak. That’s what’s happened in Japan in 2007. The cohort of toddlers in the 1990s who had low immunization rates had grown into teenagers and young adults. They were the source of a measles outbreak bad enough to shut down the entire university system.
The latest Disneyland outbreak has made an impression in the public imagination, something that years of public health officials’ warnings about dropping vaccination rates hadn’t. But it also demonstrates just how aggressively infectious measles is, and that it can easily break through weak spots in our first line of defense—communities with high vaccination rates.