Concerns about vaccine safety have led up to 40 percent of parents in the U.S. to delay or refuse some vaccines for their children in hopes of avoiding rare reactions. Barriers to health care access can also cause immunization delays. But delaying some vaccines, in addition to leaving children unprotected from disease longer, can actually increase the risk of fever-related seizures, according to a new study.

The U.S. Centers for Disease Control and Prevention annually updates its recommended childhood immunization schedule, the only schedule endorsed by the American Academy of Pediatrics and other medical organizations. Following the CDC guidelines means children may get as many as five vaccines at one visit. But some parents space out vaccines, leading to delays in shots such as the first measles-mumps-rubella (MMR) dose, recommended when a child is between 12 and 15 months old.

The new study, published in the May 19 Pediatrics, found that administering the MMR shot or the less frequently used MMRV one (which includes the varicella, or chickenpox, vaccine) later, between 16 and 23 months, doubles the child’s risk of developing a fever-caused, or febrile, seizure as a reaction to the vaccine. The risk of a febrile seizure following the MMR is approximately one case in 3,000 doses for children aged 12 to 15 months but one case in 1,500 doses for children aged 16 to 23 months “This study adds to the evidence that the best way to prevent disease and minimize side effects from vaccines is to vaccinate on the recommended schedule,” says Simon Hambidge, lead author of the study and the director of general pediatrics at Denver Health. Otherwise, he says, an undervaccinated child is left at risk of infectious disease for a longer period. “Delaying also makes for increased visits to the doctor’s office,” he says, “along with the time and hassle and risk of exposure to other infectious diseases in the doctor’s office.” Hambidge’s previous research found that pediatric office visits might increase the risk of gastrointestinal illness (symptoms then potentially misinterpreted as a vaccine reaction).

It's not clear why the MMR and MMRV vaccines increase febrile seizure risk in the older children, but it may be simply that they receive the vaccines when they are already more susceptible to the seizures. Hambidge says evidence shows the immune system may still be maturing during the second year of life, and febrile seizures caused by viruses naturally peak around 16 to 18 months. Vaccines administered during this interval may increase the risk of fever, and therefore febrile seizures, because the vaccines rev up the immune system to mount a better immune response. These seizures do not cause any long-term health effects. “Even though they’re scary for parents, these seizures are temporary events. They don’t recur and don’t cause epilepsy,” Hambidge says.

Why parents delay vaccines

Children receiving delayed vaccinations tend to fall into one of two groups: those whose parents intentionally delay vaccines and those whose families have difficulty getting vaccines on time. Anna Saporito, a family physician who works at the Institute for Family Health in New York City, sees more of the latter. “Many of my patients are working two jobs, are in the shelter system, are in abusive relationships, have psychiatric disorders or all of the above,” she says. “That can make their lives too chaotic to get their children in for vaccines.” But she is increasingly hearing from parents who are uneasy about vaccine safety, some of whom believe vaccines are unnecessary if they are breast-feeding, even though breast milk cannot adequately protect children against vaccine-preventable diseases.

Hambidge points out that misinformation on the Internet often frightens parents away from following the CDC schedule. A 2011 study found that parents who delayed or refused some vaccines were less likely to believe that vaccines are necessary to protect children’s health, that their child might get a disease while unvaccinated or that vaccines are safe. In a separate study, even among parents who did vaccinate on time, more than a quarter believed delaying would be safer than following the CDC schedule.

But these beliefs are not grounded in evidence, says Paul Offit, director of the Vaccine Education Center at The Children’s Hospital of Philadelphia. “No vaccine can be put onto the schedule unless there is data showing it doesn’t interfere with the other vaccines’ [effectiveness] or safety,” he says. “When you choose to spread out the vaccines, you’re making something up that hasn’t been tested. You don’t know how well that schedule will work whereas the CDC schedule is well-tested.”

Delaying vaccines has other drawbacks, too, Saporito says. “You are increasing the amount of sticks your child must endure—and if you delay them enough, your child can develop a real fear and distrust of the doctor because as they get older they have more of a memory of the shots,” she says. “We also worry that your child will not get all the shots they need because it is hard logistically to come back so often.”

No benefit to waiting to vaccinate

No evidence to date reveals any benefits to delaying vaccines. A study in 2010 showed that children who received delayed vaccinations performed no better at ages seven to 10 on behavioral and cognitive assessments than children who received their vaccines on time. “There was not a single outcome for which the delayed group did better,” observes Michael Smith, the pediatric infectious disease specialist at the University of Louisville who led that study. He notes that delaying vaccines leaves children at risk for disease longer, and that many parents have little firsthand experience with those diseases. “In this context, any potential side effect—real or perceived—may be enough to convince a parent that it’s safe to defer vaccines,” he says. “However, that is not a safe choice, especially as vaccine-preventable diseases like measles are making a comeback.”

A measles outbreak currently underway in Ohio, for example, is now the largest state outbreak since 1996, keeping the U.S. on track for the worst measles year in nearly two decades. Universities in Wisconsin, Ohio, Illinois and Virginia have had recent mumps outbreaks, and pertussis, or whooping cough, has been increasing for several years. Although national vaccination rates for school-aged children are high—typically above 95 percent for most vaccines—rates for younger children are considerably lower, and they are particularly susceptible to diseases such as Haemophilus influenzae type b and measles. “Ten years ago if you chose to delay vaccines, it wasn’t as big a deal as it is now, but that is a more dangerous choice now,” Offit says. “We’re seeing far more cases of measles, mumps, pertussis and bacterial meningitis. If you walk out of that office now unvaccinated, you are walking into a more dangerous world.”