(Jeff J. Mitchell/Getty Images)
(Jeff J. Mitchell/Getty Images)

Although vaccines are among the safest, most effective ways to protect children from major communicable diseases, many parents, for reasons that range from ill-informed to infuriating, still doubt this. As a result, many choose immunization schedules that defy science or refuse to vaccinate altogether.

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

Now, in a study published Monday in the journal Pediatrics, researchers have (perhaps not surprisingly) found the same phenomenon among parents of infants and toddlers. These younger children face the highest risk of dying from whooping cough and other vaccine-preventable diseases.

The study has come out as the state is grappling with a measles outbreak linked to people who visited Disneyland in mid-December.

Dr. Tracy Lieu, director of Kaiser’s Division of Research, led the research team. They knew that places like Marin and Sonoma counties had higher rates of personal belief exemptions, she says. But those parents probably refused those vaccines some years ago, Lieu says. Her team hoped to spot clusters of refusal earlier, when recommended vaccines are due, so they could take steps to ward off outbreaks.

Early warning signs

To look for at-risk communities, Lieu and her colleagues analyzed the medical records of 155,000 children in Kaiser’s system who lived in 13 Northern California counties and were born between 2000 and 2011. They were looking both for children who had received no vaccines and for children who had been “under-immunized,” meaning they had missed one or more shots by age three.

Researchers then matched these children’s vaccination records to their addresses, to see if these children were clustered geographically.

Across the 13 counties analyzed, the proportion of children who’d missed one or more shots increased from an average of 8 percent at the beginning of the study period to 12.4 percent at the end.

But that’s a broad geographic range. When the researchers drilled down to the county level, they found pockets of even higher rates of under-immunization ranging from 9.2 percent in Santa Clara County to 17.9 percent in Marin County between 2010 and 2012.

Five hot spots stood out between 2010 and 2012, including a 1.8-mile area in Vallejo, where 22.7 percent of kids were under-vaccinated. More than 10,000 toddlers lived within these five clusters.

The team also identified five clusters where all vaccines were refused for the babies and toddlers in the study:

  • 10.2 percent of kids 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 Northeastern Sacramento County and Roseville
  • 13.5 percent of kids in a small area south of Sacramento

Altogether, nearly 9,000 young children lived in these clusters.

In nearly every case, vaccine-refusal clusters overlapped with large areas of under-immunization.

When Lieu’s group analyzed vaccination against specific diseases, they found that under-vaccination rates for the MMR vaccine – which protects against measles, mumps and rubella – were 1.69 times higher for children living in Marin and Sonoma counties compared with other areas.

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

Vaccination Refusal Helps Disease Spread

The Disneyland measles outbreak is a stark reminder that pathogens can gain a foothold where vulnerable people congregate. That’s why states require children to be fully vaccinated before entering kindergarten. The easier it is to opt out of these mandates, the more likely disease will follow.

“Not surprisingly, areas that are under-immunized directly correlate with those areas at greatest risk of infections when they occur,” says Dr. Paul Offit, who directs the Vaccine Education Center at Children’s Hospital of Philadelphia and was not associated with the study. When enough people stop vaccinating, he says, measles and whooping cough, among the most contagious diseases, rapidly exploit holes in community, or herd, immunity.

Last year, when a record number of California parents claimed personal belief exemptions, health officials reported the most measles cases seen here since 1995 and the most whooping cough cases since 1947.

Marin, Napa and Sonoma counties — where Lieu and her team found under-immunization clusters ranging from 17.5 to 18.1 percent between 2010 and 2012 —  had the highest rates of whooping cough in the state in 2014. During the same time, a review of state immunization records shows, vaccination rates for whooping cough at nearly two-thirds of Marin schools, a third of Napa schools and 37 percent of Sonoma schools fell below targets to halt disease spread.

Prof. Saad Omer at Emory University earlier found that clusters of personal belief exemptions contributed to the 2010 California whooping cough epidemic that killed 10 babies. Omer says that by using electronic medical records, Lieu and her team have developed a tool that can estimate risk earlier, without having to wait until children enter school.

The main problem with this clustering behavior, says Omer, 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. (In the Disneyland measles outbreak, at least four of the cases had been vaccinated.)

Children on chemo or who have other genuine medical reasons for exemption are particularly vulnerable when they interact with unvaccinated people, whether at school or places like Disneyland.

“I don’t know how many Make-a-Wish Foundation kids were (at Disneyland),” Omer said in reference to the foundation that grants wishes to children with life-threatening illnesses. “But parents of kids with all sorts of illnesses like to give them an opportunity to have fun like other kids. … And they depend on herd immunity.”

Bad Decisions

Some parents think measles isn’t such a big risk, Offit says. And compared to the pre-vaccine days, he says, when every year millions of children got sick, 48,000 were hospitalized and 500 died, that’s true. But as a veteran of the 1991 Philadelphia measles epidemic that infected 1,400 and killed nine children who weren’t immunized, Offit cautions that playing the odds is a dangerous game.

But creating rules that make that game safer is challenging in a society that cherishes individual rights. “We’re open-minded in the U.S.,” Offit says, “even to the point where we let parents hurt their children.”

But increasingly, pediatricians, including Offit’s wife, Bonnie, refuse to help parents do that.

The alternative is to sit back and let parents make bad decisions.

Not long ago, a one-and-a-half-year-old boy was admitted to Offit’s hospital. He’d been seen at the hospital’s outpatient clinic at two, four, six and 12 months — all the times when kids get the pneumococcal vaccine. But his parents chose not to vaccinate and the staff didn’t push it, Offit says. “He came in with pneumococcal meningitis,” and then suffered a devastating brain herniation, Offit said. “He will never see, walk or speak again and probably won’t live past five. It’s not OK.”

Liza Gross is an independent science journalist based in the San Francisco Bay area who writes frequently about science and society.

  • RobertChase

    Fear of modern vaccines is irrational and threatens the lives of the children of those who suffer from it, as well as others.

  • Damiana

    We need to create laws to prevent this dangerous behavior.

    • Black Labs

      Hey Damiana, I’ve been looking for you and wanted to have a discussion OFF that other thread. I’d appreciate your responding when you see this. Thanks a lot.

  • oregonmom

    No, the alternative is to provide us with safer vaccines with better efficacy!!!

    • Dale

      Vaccines that are used to prevent stuff like the Measles and Polio are already safe.

      • bouncedancer

        No vaccine is safe! The biggest cause of polio nowadays in industrialized countries is the polio vaccine!

        Because my husband’s nephew is paraplegic after the polio vaccine, we had our firstborn get the dead polio vaccine (in a shot). Our son got so deathly ill that I said over my dead body does he get any more.

        • Gay_for_Jesus

          You literally are retarded

    • DSS

      Read from sources that are backed or written by scientists, not fake doctors, moms, or celebrities.

      • Xielur

        How dare you anonymously expose MY child to life-threatening disease? How dare you do it to your child either?Home-school your non-immune kid if you like, but keep him well away from mine.

    • Gay_for_Jesus

      You are a shitty mother but thankfully your ugly stupid get kid with measles and whooping cough and polio is banned from Disney Land forever because your stupid shitty mommy instincts decided you know better than people with Phds and MDs you dumb fuck

  • Mike

    I think the government should take everyones kids away so they can be raised right.

    • bouncedancer

      Spoken like a true troll.

  • DrangUndSturm

    I encourage this kind of clustering of anti-vaxxers… if it happens on an island in the pacific.

    Remember leper colonies? Anti-vax colonies. We can’t force a family to vaccinate their children, but we can sure prevent them from endangering everyone else’s children by refusing to allow them live with the rest of us.

    I haven’t really decided if I’m joking.

  • Doctor Jack

    What the public and medical school graduates believe causes what’s described as ‘measles’, ‘whooping cough’, ‘the flu’ and so forth and what really causes these symptoms ARE NOT THE SAME. And it is impossible to prevent disease by artificial means. So called vaccines don’t prevent anything; on the contrary, they are a contributing influence. The parents who refuse vaccinations are making the physiological correct decision but they still probably don’t know how to raise their kids so that they are completely disease free because they haven’t found the right information on how to properly take care of their kids so that there’s no biological necessity for them to “get sick”.


Liza Gross

Liza Gross, an award-winning independent journalist and senior editor at the biomedical journal PLOS Biology, writes mostly about conservation and public and environmental health. She was a 2013 recipient of the NYU Reporting Award, a 2013 Dennis Hunt Health Journalism fellow and a 2015 USC Data Journalism fellow.

Read her previous contributions to QUEST, a project dedicated to exploring the Science of Sustainability.

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