A single-shot vaccine used for toddlers across Canada doubles the risk of fever-induced seizures compared with using two separate vaccines, according to a University of Calgary study published Monday.

While the findings may fuel anti-vaccination advocates, the lead researcher says being honest with their research can help prevent another measles outbreak.

“We are continually monitoring vaccine safety, and when we discover an issue we share it in the most public forum possible,” said Dr. Shannon MacDonald, a fellow at the University of Calgary’s Faculty of Medicine who led a four-person team studying six years of Alberta vaccine records.

The study shows a slightly higher risk of febrile seizures in Alberta children given the combined measles, mumps, rubella and varicella (MMRV) vaccine used across Canada, versus the previously separate two vaccines, known as MMR+V. Febrile seizures usually affect children aged 6 months to 5 years old with a high body temperature and temporary lack of consciousness, though long-term symptoms are rare.

The study, published in the Canadian Medical Association Journal, looked at 227,774 one-year-old Alberta children who received either the MMRV or MMR+V vaccines between 2006 and 2012. With MMRV, 3.52 additional seizures occur for every 10,000 doses in the 7-10 days after vaccination — double those that occur after an MMR+V vaccine.

“Although we found the risk of febrile seizure is slightly higher, it shows that the risk is still considerably less than the risk if you contract measles disease,” she said.

“The risk of a febrile seizure for a child with measles is tenfold higher.” This statistic comes from data from the United States Centers for Disease Control and Prevention. Calgary declared a measles outbreak in April.

Health officials in Canada, Australia the European Union implemented the combined vaccine to reduce the number of needles children need. While that may simply sound convenient, MacDonald says many parents skip both injections because their children are afraid of needles.

MMRV was introduced in 2010, and in the following two years, 86 per cent of the infants studied got that form of the vaccine, versus the 0.2 per cent who received MMR+V.

While the study looks at a correlation and not a cause, MacDonald said a common view is that administering both doses in one arm could be triggering a stronger immune response, which is not necessarily a bad thing.

“Most likely is it something to do with fact that all vaccine components going into one localized spot means higher immune response from the body,” she said, adding that studies show a higher level of the vaccine in the blood means a stronger reaction.

“It increases the fever, and it also increases the body’s ability to fight the disease.”

This study follows a U.S. study of ProQuad — a drug similar to Priorix-Tetra, which is used instead in Canada — published in 2010. The study similarly found a higher rate of febrile seizures, prompting doctors to switch to an opt-in system where parents have to request the single shot MMRV vaccine. Monday’s study proposes doctors inform parents of the benefits and risks of both choices, and let them make the decision. The study notes ordinary fever medication can alleviate symptoms in toddlers.

Alberta Health, which supplied the data to the research team, has not yet indicated any change in policy.

“Parents who do not want the combined vaccine and wish to have the vaccines as separate components may be accommodated,” spokesman Timothy Wilson wrote in an email.

MacDonald acknowledged that her team’s findings will likely rile anti-vaccination parents, but she says making these findings public can help soothe their suspicion of public health officials.

“A common misconception among people who don’t support immunization is that they think there are risks not shared with the public, and this study counters that argument,” she said. “We’re openly sharing the studies.”

drobertson@calgaryherald.com

Twitter.com/dcrHerald