Delayed MMR increases the incidence of post-vaccination seizures

Many counties in PHC’s network have low pediatric immunization rates. This may be due in part to access or transportation issues. But it is also due to misperceptions about the risks of vaccines. While these misunderstandings can be tough to change, it is incumbent on physicians to help parents understand the importance of vaccinating their children, and doing so on time.

A recent on-line article in Pediatrics by Hambidge SJ, et.al, compared the incidence of post-vaccination seizures in children who received their immunizations on time to those whose vaccines were delayed, an approach that is popular in some quarters. Some parents feel that having their children vaccinated on a delayed schedule, rather than the standard recommended schedule, is safer.

In their study, the authors found no association between the timing of infant vaccinations and post-vaccination seizures during the first year of life. However, in the second year of life, infants who received a delayed MMR after 16 months of age had a three-fold higher risk of seizures than children vaccinated on the standard schedule. For the MMRV vaccine (varicella included), delaying administration until after 16 months of age doubled the risk of seizures.

The study followed a cohort of 323,247 children from the Vaccine Safety Datalink (a collaborative project between the CDC and several managed care organizations across the U.S.) born from 2004 to 2008. The authors analyzed the occurrence of first seizures in a self-controlled series analysis of initial doses of vaccines received in the first two years of life.

There was no higher risk of post-vaccination seizures for immunizations administered on any schedule, as long as they were given in the first year of life. Acellular TDAP and other infant vaccines have not been associated with seizures in the US. Inactivated influenza vaccine and 13-valent pneumococcal vaccine have been associated with seizures, but the risk varies with the influenza season and is greatest if these two vaccines are given together – again in the second year of life.

I have never seen a case of congenital rubella syndrome, but with news of the recently-imported cases of MERS and measles, I can imagine some unfortunate, unimmunized pregnant woman will come in contact with an international traveler with rubella. The consequences could be significant.

Promote immunizations, on time.

Marshall Kubota, MD

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