The combined measles-mumps-rubella-varicella vaccine (MMRV) has a slightly increased risk for seizures in children compared with vaccines administered separately for MMR and varicella (MMR+V), according to a new Canadian study.

The combined MMRV vaccine, used to protect children against measles, mumps, rubella and varicella (chickenpox), was developed to reduce the number of shots required for young children.

A version of the combined MMRV vaccine called Priorix-Tetra is currently used in Canada. But new research shows that the combined vaccine is associated with slightly higher rates of febrile seizures.

Febrile seizures are seizures that usually occur in children, accompanying high fevers. Most febrile seizures last between one and two minutes. They are not usually associated with ongoing health issues, such as epilepsy.

The study's lead author, the University of Calgary's Dr. Shannon MacDonald, said that although febrile seizures are not typically associated with long-term health problems, they are still stressful for parents.

"Febrile seizures are typically self-limiting and rarely have long-term effects, but they can be extremely distressing for parents, may precipitate acute care visits and may undermine confidence in immunization programs," she said in a statement.

Vaccination still 'the best option'

However, MacDonald notes that the absolute risk of febrile seizures from the vaccine is still very low, and the seizure risk from measles is much higher.

"We know from this study that the risks of febrile seizures from the disease are tenfold higher than the risk from this vaccine," she told CTV News.

"If you look at the absolute risk, we're talking about three to four seizures for every 10,000 doses of the vaccine given, so the absolute risk is very small. But it's something that parents should be aware of. Parents need to know what the benefits and risks of the vaccines are."

The study, published Monday in The Canadian Medical Association Journal, looked at data from 227,774 Alberta children between the ages of 12 to 23 months, who had received either the combined MMRV vaccine or the separate MMR+V vaccine between 2006 to 2010.

The researchers found a small increase in relative risk for febrile seizures with the MMRV vaccine compared to the MMR+V vaccine.

The risk of seizures for children who received the combined vaccine in the first seven to 10 days after vaccination was double the risk for children who received the separate vaccine, according to the study. This amounted to about one extra seizure for every 2,841 doses administered in the first seven to 10 days after vaccination.

The researchers said that while the risk for children who receive the combined vaccine is double, the absolute risk is still relatively small.

The findings of the Canadian study are consistent with a similar American study, which examined the seizure risk associated with the combined vaccine ProQuad. After that study was published in 2010, the U.S. Advisory Committee on Immunization Practices withdrew its preference for using the combined vaccine for the child's first dose, the Canadian study notes.

Now, the committee advises doctors and parents to consider the benefits and risks of each option before selecting which vaccine to use. If there is no stated parental preference, the committee recommends using the separate vaccine for children between 12 to 47 months of age.

The Canadian researchers suggest that a similar recommendation might be adopted in Canada.

MacDonald said it will be up to each provincial and territorial jurisdiction to determine whether to continue to use the combined vaccine or start using the separate vaccine. However, she stresses that the benefits of getting a vaccination still outweigh the small risk of febrile seizures.

"The evidence is very clear that parents should continue to vaccinate their children, that is absolutely the safest course of action," she said. "The risks of a febrile seizure from the disease are tenfold higher than the risk from the vaccine, and in addition to that, the disease results in other adverse reactions like pneumonia and brain-swelling.

"The vaccine is absolutely the best option."