There is little to no risk that the four-in-one measles-mumps-rubella-chickenpox vaccine (MMRV) causes fever-related seizures in children 4 to 6 years old, a new CDC-funded study confirms.

In the U.S. parents can decide to give their child the four-in-one combination MMRV vaccine (measles, mumps, rubella, chickenpox) or two separate vaccines: MMR (measles, mumps, rubella) and a chickenpox vaccine at age 1 to 2.

A booster four-in-one MMRV vaccine, or MMR vaccine and a separate chickenpox vaccine, at age 4 to 6 is also recommended. 

In 2008, a related study found that babies who got measles, mumps, rubella, and chickenpox in one shot had twice the risk for fever-related convulsions as those who got two separate shots, with measles-mumps-rubella components in one and chickenpox, or varicella, in the other.

Even though the convulsion risk was quite small with the four-in-one vaccine -- about one additional seizure for every 2,300 MMRV doses given -- the CDC recommends that parents be educated about the increased fever-related seizure risk with the MMRV vaccine at age 1 to 2.

Childhood Illnesses Every Parent Must Know
MMRV: Seizure Risk Very Small

The four-in-one vaccine, called ProQuad, was first licensed by the FDA in 2005, but manufacturing issues led to repeated shortages even before safety questions arose.

In the new study, researchers from the Kaiser Permanente Vaccine Study Center once again examined MMRV-related seizure risk, but this time in older children receiving booster doses of the vaccine.

The study included nearly 87,000 children between the ages of 4 and 6 who got the MMRV vaccine, and about 67,400 who got separate MMR and chickenpox immunizations on the same day.

Since fever-related seizures occur most often in children under the age of 2 and are rare in those over the age of 5, researcher Nicola Klein, MD, PhD, says she was not surprised to find very few seizures and even fewer fever-related seizures in the 4 to 6 age group.

Just one fever-related seizure occurred within a week to 10 days after vaccination in the MMRV group and no fever-related seizures were recorded in the MMR plus varicella group.

Klein tells WebMD that even if these vaccines were responsible for all fever-related convulsions that occurred during this time period, there would be only one such seizure for every 15,500 doses of MMRV and one seizure for every 18,000 two-shot doses administered.

The study will be published in the May issue of the journal Pediatrics.

“This is a very reassuring finding,” Klein says. “We didn’t really see any evidence of an increased risk for these seizures in this age group for either of these combinations.”
Febrile Seizures Usually Harmless

Fever-related convulsions, known medically as febrile seizures, can occur with any condition that causes a spike in temperature, including illnesses like ear infections and, rarely, vaccinations.

While they can be terrifying for parents, they usually have no lasting effects, says infectious disease specialist Bruce Hirsch, MD, of the North Shore University Hospital in Manhasset, N.Y.

“Febrile seizures can be very disturbing and scary in very young children, but they don’t lead to long-term problems and they are not associated with epilepsy,” he tells WebMD.

Hirsch agrees that the new study should reassure parents, especially given its large size.

“You rarely see a vaccine safety study with such large numbers,” he says. “This gives us the ability to understand what is happening with the MMRV vaccine in this age group with a high degree of certainty.”