Reactions to the first shot of combined measles, mumps and rubella vaccine are fairly 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 a hospital emergency department, with visits peaking eight to 12 days after children got 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 PLoS One, a journal of the U.S. Public Library of Science.

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.

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

They found that in the period four to 12 days after the 12-month vaccination – especially eight to 12 days after a shot – there was a sharp increase 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 records, 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 said judging from the hospital records, children who were brought in with these symptoms were sick, but not seriously sick. And their symptoms were consistent with the signs of an immune system responding to a live virus vaccine, he said.

Dr. David Scheifele, a pediatric vaccine expert at B.C. Children’s Hospital in Vancouver, said the rate of reactions wasn’t surprising.

But he said the number of parents taking children to emergency departments for assessment of these symptoms suggests health-care workers who are giving toddlers their first MMR shots could be doing a better job explaining to parents what kind of reaction might occur and when it might occur.

“I think providers could probably do a better job describing what parents ought to expect, so when kids do develop fever in the second week they (parents) don’t panic,” Scheifele said.

Wilson said he plans to try to determine from the data whether first-time parents are more likely to bring their toddler to emergency departments when they react to an MMR shot.

He also said he thinks this problem may be contributing to parental anxiety about MMR vaccine, which has been singled out by anti-vaccination campaigners.

“The MMR vaccine is a very important vaccine. Measles is a highly infectious condition and you need a highly effective vaccine. And the MMR vaccine has been proven to be highly effective. And it’s been proven to be very safe. But we know people are quite anxious about this vaccine,” Wilson said.

“This may be contributing to a willingness to believe that the vaccine may be causing other problems which it has been shown not to cause.”

There have long been claims the MMR vaccine is linked to autism but multiple studies have found no association between the two.

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Scheifele praised the study, saying the methodology Wilson and his co-authors employed will be really useful in future to look for safety signals when new vaccines are introducted. “They should get a gold star for being able to do this.”

“Typically we look for vaccine side-effects that have already been described,” Scheifele explained. “And it’s very difficult to look for the unknowns, except by doing this sort of approach where you can identify who’s been immunized and then look in a completely openminded way at what happens in the following days and weeks.”

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