delayed MMR and seizures

 a new study looked at 323K children in the US (from the Vaccine Safety Datalink) born between 2004 and 2008, assessing the timing of the array of first doses of childhood vaccines  and the development of seizures, with self-control -- ie, noting the timing of seizure after vaccination in children who never had a prior seizure (see vaccine MMR delay and sz peds 2014 in dropbox, or DOI: 10.1542/peds.2013-3429).  the seizure risk window varied a bit by different vaccines, but was 7-10 days for MMR. results:

    --5667 children had seizures. of these 49.7% received the vaccines on-time in the first 2 years of life.
    --in the first year of life, no diff in seizures with vaccine timing (includes DTaP, PCV, HIB, IPV, rotavirus)
    --in the second year of life, the IRR (incidence rate ratio) for seizures after the first MMR given at the recommended 12-15 months was 2.65. if given between 16-23 months it was 6.53!!  in those given the combo vaccine or MMR plus varicella, the IRR at 12-15 months was 4.95, increasing to 9.80 if given between 16-23 months. these numbers were worse with longer delays (eg, if MMR given between 16-18 months, IRR was 5.09; if given between 19-21 months it was 8.75)
    --although the old DTP vaccine used until the 1990s was associated with increased seizure, the subsequent DTaP vaccine is not associated with seizures

this study reinforces the importance of timely vaccine. some parents have been hesitant in this regard, anticipating an older child might tolerate the vaccine better. sometimes it is because of barriers to immunization (lack of insurance, transportation, accessible care). although this is an observational study, and it is possible that vaccine delay was related to some parental choice reflecting an increased risk of seizure, the large scope of this study makes that bias unlikely, and the more likely explanation having to do with maturity of the immune system with age and higher likelihood of febrile reaction prompting seizures.

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