MMR vaccine reactions usually mild

Reactions to the first shot of combined measles, mumps and rubella vaccine are common among young children, but serious reactions are actually rare, a new study looking at data from Ontario suggests.

The study found that about one in 168 toddlers who got the 12-month MMR shot was taken to hospital in the four-to-12-day period after getting the shot. While records indicate most were suffering from real symptoms of illness, such as fever, few were sick enough to require hospitalization.

The lead author of the study interprets the findings this way: The spike in hospital visits about a week after vaccination means the vaccine was doing its job. "This is the vaccine working," said Dr. Kumanan Wilson, a researcher with the Ottawa Hospital Research Institute who has done a number of studies looking at parental attitudes towards vaccination.

"It's expected and necessary. This is the immune system working."

The study was published Tuesday in the journal PLoS One, a journal of the U.S. Public Library of Science.

Flu-like symptoms common

The measles, mumps and rubella vaccine is made with live but weakened viruses. It's known that live-virus vaccines tend to induce more short-term reactions in the period after vaccinations than inactivated vaccines. Those reactions include fever and flu-like symptoms.

Wilson's team used administrative data in Ontario to study the question. They took data on more than 271,495 kids who got the 12-month shot and 184,312 children who got the MMR booster shot at 18 months and compared it to hospital visits data.

They found that in the period four to 12 days after the 12-month vaccinations — especially eight to 12 days after a shot —there was a spike in hospital visits for symptoms commonly seen after vaccinations. There was not a statistically significant increase in children being admitted to hospital as a result of those emergency department visits, however.

The same trend was not seen with the 18-month booster shot, but that was no surprise. The first exposure to the vaccine virus — the 12-month shot — would be expected to produce the greatest response, Wilson suggested.

Because they were working from data, not individual case reports, the researchers cannot say with certainty that reactions to the vaccination were what sent these children to the hospital. But the large sample size and the pattern seen is suggestive.

Wilson suggested more work may be needed on communicating to parents what to expect in a child who is getting the first shot of MMR vaccine at 12 months. "The MMR vaccine is a very important vaccine. Measles is a highly infectious condition and you need a highly effective vaccine. And it's been proven to be very safe. But we know that people are quite anxious about this vaccine," Wilson said.

There have long been claims the MMR vaccine is linked to autism but multiple studies have refuted that there is any association between the two.