FILE: Nancy Brajtbord, RN, (L) administers a shot of gardasil, a Human Papillomavirus vaccine, to a 14-year old patient (who asked not to be named) in Dallas, Texas. REUTERS/Jessica Rinaldi

A recent and disturbing trend in California is making headlines in the popular media: An increasing number of parents are opting out of vaccinating their school-aged children.

California, like all states, mandates that children receive certain vaccinations before they enter school. However, like all but two states, California also has a provision for voluntary exemptions (called Personal Beliefs Exemptions, or PBEs, in California).  Statewide, the rate of voluntary exemptions has been rising steadily for almost two decades—from 0.5 percent of kindergarteners in 1996 to 3 percent as of the most recent data.

This pattern reflects a growing number of parents who worry that vaccines are unsafe, ineffective, or unnecessary. These increases are troubling, especially in the face of recent outbreaks of childhood diseases such as measles and pertussis (whooping cough)—diseases that used to cause widespread illness and even death in the United States, until they were all but eliminated by successful immunization programs. Now those diseases are back: there have been 593 cases of measles to date this year in the U.S.  Compare that to only 119 cases per year on average in 2009-2013.

But statewide exemptions data don’t reveal the true severity of this trend.  Infectious diseases don’t operate at the state level—they are transmitted from person to person. We need to look at local data to fully understand the risk of outbreaks.  And it is the rates of vaccine exemptions in some localities—counties, school districts, individual schools—that are really making epidemiologists’ hair stand on end.  As articles in The Atlantic and the The Hollywood Reporter have recently highlighted, some affluent Los Angeles schools currently report that upwards of 60 or 70 percent of their students have PBEs on record.

The threshold varies by disease, but as a rule of thumb about 90-95 percent of a population needs to be immune (either through vaccines or prior exposure to disease) in order to produce what is known as “herd immunity”.  What is herd immunity, exactly?  When most people in a population have immunity to a disease, an infected person is less likely to bump into a susceptible person (someone with no immunity) —so the disease doesn’t readily spread.   One susceptible person may get it, but will be very unlikely to pass it along.  Herd immunity doesn’t provide direct protection to individuals who have no immunity and are exposed to disease, but it does make it very hard for a disease outbreak to start, which protects everyone from being exposed in the first place.  Certain vulnerable populations rely on this statistical protection—especially babies too young to be vaccinated and people with immune system problems.

The 3 percent statewide PBE rate in California is troubling because it’s starting to bump up against those herd immunity thresholds for diseases like measles; and the fact that it’s increasing steadily each year is even more worrying. But large clusters of exempted kids in close proximity to one another, for example in a classroom—that’s truly scary.  These micro-communities have rates of immunity far, far below herd immunity thresholds.  One sick kid in one of these schools can readily spread disease to the other unvaccinated kids at the school (as happened in 2008 in San Diego). 

And of course the outbreaks may not be confined to school.  Baby brothers and sisters at home are at risk from declining herd immunity (babies aren’t fully protected from pertussis until their 6-month vaccinations, and don’t get their first measles shot until they are 12 months old).  And sick kids go to the doctor.  The same doctors who have allowed their patients to go unvaccinated may be fostering another hotspot where herd immunity is in trouble—their own waiting rooms.  Even if most of the kids in any given practice are protected, there are always a few kids who can’t get vaccinated—either because they are too young or they have an immune system problem.  With so many unvaccinated kids in affluent communities like Santa Monica and Malibu, it’s easy to see how a serious epidemic could take hold.  And when we’re talking about an epidemic of measles or whooping cough, we’re talking about very sick kids.

How do these clusters of exempted kids form? We think that the idea of vaccines being unsafe or ineffective or unnecessary spreads—like a disease itself—from person to person within social networks.  Parents talk to their friends, other parents at their child’s school, and to their neighbors.  Normal social processes produce clusters of vaccine refusers in the social landscape.  When it’s time to enroll in school, like-minded parents are drawn to certain schools—and this is especially pronounced when parents have the resources to be choosy about their child’s education.  Soon, there are schools and communities where vaccine refusal is not an anomaly; it is the norm.

State and local public health officials across the country are understandably worried about this situation.  In January 2014, new legislation known as Assembly Bill 2109 went into effect in California.  The law is intended to make it more difficult to get an exemption by requiring that parents submit a signed statement from a healthcare provider stating that the parent has received information about the risks of forgoing immunization. We have our doubts about how effective the law will be at addressing the problem, however.  Kindergarten enrollment occurs well after most of the recommended vaccines on the childhood schedule, and parents who truly want to opt out of vaccines have probably already negotiated this with their doctor, and will have no trouble meeting the new exemption requirements.

We do think AB2109 will reduce the number of exemptions that are obtained for “convenience” rather than for “conviction” –parents who file an exemption at the time of enrollment because they can’t produce up-to-date vaccine records for their kids. But guess what? Kids with these “convenience” exemptions have probably received most or even all of their shots anyway. It’s the “conviction” exemption kids who are likely have to received fewer shots and therefore undermine herd immunity. We’ll be watching closely to see how California’s new law affects exemptions and disease outbreak.

Malia Jones is a Postdoctoral Fellow at the Department of Preventive Medicine, Keck School of Medicine, University of Southern California.

Alison Buttenheim is an Assistant Professor at the Department of Family and Community Health, University of Pennsylvania School of Nursing.