On-Time Vaccines Better than Late

Seizures after MMR vaccines twice as likely if the shot is delayed

(RxWiki News) Some parents may feel it's better to delay their children's vaccines instead of following the officially recommended schedule. But that could present more risks from side effects.

A recent study found that delaying the measles-mumps-rubella (MMR) vaccine, in the second year of life, doubled the risk of a seizure occurring after the vaccination.

Seizures following some vaccines are nearly always a result of a high fever reaction from the vaccine. The seizures do not cause long-term damage or cause epilepsy, but they can look scary to parents.

"Discuss vaccinations with your child's pediatrician."

The study, led by Simon Hambidge, MD, PhD, of the Institute for Health Research at Kaiser Permanente Colorado, looked at the risk for seizures after vaccinations given on time or later than recommended.

The researchers looked for everyone who had experienced a seizure out of 323,247 children, born between 2004 and 2008, whose records are in the Vaccine Safety Datalink system.

The Vaccine Safety Datalink provides researchers with a way to look for possible safety concerns related to vaccines.

The researchers located 5,667 children who had experienced a seizure in their first two years of life and did not have a seizure disorder.

Then the researchers compared the timing of these children's seizures to the dates they had received various vaccinations and considered whether the vaccines had been received on time or not.

Vaccines received on time are those given within the windows of time recommended by the US Centers for Disease Control and Prevention.

The analysis revealed that there was no relationship between receiving vaccines and experiencing seizures for children in the first year of life.

However, in the second year of life, children who received the MMR vaccine between 12 and 15 months old, when it's recommended, were about 2.6 times more likely to experience a seizure than they would be without being vaccinated at that time.

This number translates to about one seizure out of every 4,000 children receiving the vaccine.

Yet, if parents delayed the MMR vaccine until any time between 16 and 23 months, the risk of a seizure was 6.5 times greater than when not being vaccinated.

That means that waiting to get the MMR vaccination more than doubles the risk of a seizure occurring in the one to two weeks after vaccination.

Similarly, the risk of a seizure from the MMRV vaccine, which includes the chickenpox (varicella) vaccine, is about 5 times greater than not receiving that vaccine when given between 12 and 15 months.

Yet when given between 16 and 23 months, the risk of seizure following that vaccine is 10 times greater than not receiving it.

The seizures the children experience may look scary to parents but do not have long-term effects and do not cause epilepsy.

In fact, febrile (fever-caused) seizures among children in general are not uncommon; about one in 25 children will have one at some point in childhood.

However, parents can reduce the risk of a febrile seizure in their children following the MMR vaccine if their child receives the vaccine when it is recommended.

"This study is a fairly powerful one indicating the importance of following the recommended schedules," said Tom Seman, MD, a pediatrician at North Shore Pediatrics in Danvers, Mass. "Often parents do not recognize that the schedules have been carefully and intensely researched and evaluated for the safest and most effective timing for the vaccinations."

He said that this side effect of febrile seizures is fairly rare and therefore not something he sees much in his practice.

"However, I do seem to see more complications when the parents extend the vaccine schedule," Dr. Seman said. "The standard schedule is designed to follow standard physical exam office visits and so when this schedule is changed, there can be issues with catching up. This information is a very strong argument on the pro side when discussing the schedule with the parent."

The study was published May 19 in the journal Pediatrics. The research was funded by the CDC through American’s Health Insurance Plans.

Two authors receive royalties from textbooks unrelated to this study. One author has received research funding from GlaxoSmithKline, and another has received research funding unrelated to this study from GlaxoSmithKline, Sanofi-Pasteur, Merck, Novartis, Pfizer. and Protein Science.

Review Date: 
May 18, 2014