written by Tara Haelle

Delaying certain vaccines is one of the more common trends among parents who are uneasy about vaccinations but still recognize their value and effectiveness. If they hold off until their child is older, the parents reason, then perhaps the possible side effects will be less likely, or perhaps the child’s immune system will be “stronger” and better able to cope with the vaccine.

A recent study in JAMA Pediatrics, however, may make some parents think twice about this rationale for at least the MMR vaccine. In fact, kids who get the MMR at the recommended age are about half as likely to have a fever that leads to a seizure (“febrile seizure”) as kids getting the vaccine closer to age 2, the study found.

The first dose of the MMR, which stands for measles-mumps-rubella, is currently recommended by the CDC (pdf) when a child is between 12 and 18 months old. Most pediatricians include it in the 12-month well-child visit. I have at least a handful of friends who have delayed the MMR for one or more of their children well past 18 months, even up to 3 or 4 years old (the second dose is recommended between 4 and 6 years old). These parents often have good reasons: their child may have had a reaction to a previous vaccine which they believe may make a reaction to the MMR more likely. The MMR is already known to be more likely than the vaccines to cause a fever in the week and a half after it’s given: according to the CDC, about one in every six children are likely to experience a mild to moderate fever afterward.

Of course, the downside to delaying the MMR, as in delaying any vaccine, is that the child is left unprotected against those diseases for a longer period of time. For measles, a delay has become particularly riskier lately because of the recent measles outbreaks in the US. (These outbreaks led at least one friend of mine to get her child the MMR, which she had been delaying, sooner than she had wanted to.)

But this new study offers another reason to get the MMR on time: children may actually be LESS likely to develop a high fever reaction. The researchers used data from 840,348 children, aged 12 to 24 months, enrolled in the Vaccine Safety Datalink database. They all received either the MMR or the MMRV (MMR plus varicella, or chickenpox, vaccine) between 2001 and 2011.

The researchers compared the children’s risk of febrile seizures in the 7 to 10 days after vaccination to their risk in the subsequent month (days 11 to 42 after vaccination). They also compared risks between the MMR and the MMRV for febrile seizures. First, though, a quick word on febrile seizures: they look scary as hell, but they do not cause long-term damage. They tend to last a minute or two and occur primarily with rapidly increasing fevers that reach 102 F or higher. Studies put the risk of febrile seizures after the first MMR shot at about 1 in 3,000 vaccinations (which approximately matches up with this study’s findings as well). The second MMR dose, between ages 4 and 6, does not appear linked with febrile seizures.

So, what did the study find? Kids getting the MMR when they were 12 to 15 months old were just over three times more likely to have a febrile seizure in the week or so after vaccination than in the following month. That equated to about 4 in every 10,000 children experiencing febrile seizures in the 7 to 10 days after the MMR shot.

But the risk doubles for kids just a little older: kids getting the MMR between 16 and 23 months old were 6.5 times more likely to have a fever in the week after the vaccine than in the subsequent month. That risk equates to 9.5 children having febrile seizures for every 10,000 getting the MMR.

As other studies have shown, this one also found a higher risk for high fevers with the MMRV vaccine compared to the MMR on its own. Getting the MMRV combination vaccine made febrile seizures about 1.4 times more likely than getting the MMR vaccine and the varicella vaccine in different shots at the same visit. The risk for a febrile seizure with the MMRV was twice as high as the the risk from just the MMR, minus the varicella at the same visit.

So, bottom line, what does all this mean? First, above all, the medical recommendations are still to vaccinate all children without medical counter-indications against measles, mumps, rubella and varicella. Second, if one goal in doing that is to reduce your child’s likelihood of a high fever reaction that leads to a febrile seizure, then you should A) get the MMR instead of the MMRV, and B) vaccinate your child on time, between 12 and 15 months, rather than delaying the shot.