A Combo Vaccine May Cause Seizures in Kids. But Why?

A young boy receives his MMR shot. Jeff J Mitchell / Getty Images

Another day, another controversy about the vaccine that protects against measles, mumps, and rubella. Parents don't need to worry that the injection will increase their children's risk of autism—as we've written many times before, that supposed link has been roundly discredited—but if their kids are getting ProQuad, the Merck-manufactured "MMRV" vaccine that also includes protection against varicella (chicken pox), they should be vigilant a week or so after the shot. A new Kaiser Permanente study shows that in days seven to 10 post-injection, children given the MMRV vaccine have a slightly higher risk of developing febrile seizures—the scary if ultimately harmless phenomenon that can accompany a bad fever—than children given the plain MMR and varicella shots separately on the same day.

In one sense, this isn't a surprise. The Centers for Disease Control has known for a while that the combo vaccine causes more high fevers than what we'll call the "MMR+V," and it has advised parents accordingly. The Kaiser group discovered the link to febrile seizures in 2007; the new data simply confirms it.

Here's the weird thing about the new findings, though: Nobody knows why the MMRV vaccine causes more fevers and thus more seizures. It doesn't contain more "live, attenuated" measles viruses than the plain MMR shot. "The main difference between the vaccines is that the manufacturer needed to increase the varicella component of the combination," says Nicola Klein, co-director of Kaiser's Vaccine Study Center. "Whether that affects the other components is unclear." The Kaiser scientists checked to see whether kids given the chicken-pox vaccine alone had a higher-than-normal risk of a fever. They didn't.

At the same time, the timing of the uptick in seizure risk—which only occurs between days seven and 10—corresponds to the point at which the viruses from the MMRV vaccine are multiplying in the body. So "perhaps [the MMRV] stimulates the immune system a little bit more" than the separate MMR and V doses, says Klein.

The MMRV vaccine's seizure risk is still very low—less than 1 in 1,000—and no one's talking about taking the shot off the market. (It's been largely unavailable for the past few years anyway; Merck ran into manufacturing problems in 2007 and reintroduced the vaccine just a few weeks ago.) The CDC looked at a draft of the Kaiser data last July and decided it was OK for pediatricians to give parents the choice between the MMRV combo and the separate MMR + V shots. Doctors don't seem overwhelmingly committed to either regimen, though there is an apparent preference: According to the San Jose Mercury News, "a CDC-sponsored study found that 59 percent of pediatricians and 67 percent of family physicians surveyed said they would recommend the split regimen of MMR+V for children aged 12 and 15 months."

Still, given the new findings, it's a little hard to see why parents would choose the MMRV shot. Yes, it's one fewer poke with the needle, but what would you rather your child have—a fleeting sting and a few tears, or a seizure brought on by a fever of 104 degrees? (As the mother of a child who has had two febrile seizures, I can attest to the fact that even when you know they have no long-term effects, they're awful.)

Parents should keep a thermometer and some infant Motrin or Tylenol handy during days seven to 10 if they do decide they want the MMRV shot. What they absolutely shouldn't do is forgo vaccination, because whatever the seizure risk, getting the MMRV vaccine is still much safer than getting the diseases it prevents.