A study out this month shows a combination vaccine that protects against four diseases in one shot may increase the risk of fever-induced seizures in toddlers.

The MMRV, or Priorix-Tetra vaccine, used in Canada gives infants immunity against measles, mumps, rubella and varicella, or chickenpox. Traditionally the varicella shot was given separately, but this four-in-one injection was designed to reduce the number of shots infants and toddlers have to receive.

In a study conducted by the University of Calgary in Alberta, one-year-olds who received the MMRV vaccine were twice as likely to develop a fever-induced, or febrile, seizure. A 2010 study revealed similar results for a MMRV vaccine called ProQuad that was used in the United States.

That study prompted the U.S. Advisory Committee on Immunization Practices to recommend doctors give the first round of MMR and varicella shots separately. Parents now have to specifically request the MMRV vaccine.

It’s unclear why the MMRV shot is more likely to cause a febrile seizure, but some researchers surmise the combination vaccine triggers a stronger immune response and higher fever in some children.

The study published in the Canadian Medical Association Journal looked at children ages 12 months to 23 months. It found there were about six seizures for every 10,000 doses of the MMRV vaccine compared to only two seizures with the two separate shots. However, Dr. Nicola Klein with the Kaiser Permanente Vaccine Study Center in Oakland, Calif. says children are much more likely to suffer a high fever and seizure if they actually get the measles. Klein points out we are currently experiencing a 20-year high in measles cases.

News, Vaccine Injury News Study Shows Combined MMRV Vaccine Increases Toddlers’ Seizure Risk