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Home » Delaying measles vaccination may increase risk of seizures

Delaying measles vaccination may increase risk of seizures

Last updated on August 24th, 2019 at 12:31 pm

keep-calm-get-shots-bigThere is an unscientific myth, pushed by some parts of the vaccine deniers (more accurate, vaccine delayers), that parents should delay vaccinations based on the unsupported belief that “too many vaccines” could overwhelm the child’s immune system. This belief is utterly unscientific and thoroughly debunked

That belief is unfounded, as Paul Offit summarized in Addressing Parents’ Concerns: Do Multiple Vaccines Overwhelm or Weaken the Infant’s Immune System?:

Current studies do not support the hypothesis that multiple vaccines overwhelm, weaken, or “use up” the immune system. On the contrary, young infants have an enormous capacity to respond to multiple vaccines, as well as to the many other challenges present in the environment. By providing protection against a number of bacterial and viral pathogens, vaccines prevent the “weakening” of the immune system and consequent secondary bacterial infections occasionally caused by natural infection.

A new study concluded that children who received their measles vaccination on time had a lower risk of certain adverse events than those who received the vaccine later than the schedule recommended by the Centers for Disease Control and Prevention (CDC) (pdf). According to the article, children who received their first dose of a measles-containing vaccine at ages 12 to 15 months, the timeframe recommended by the CDC, had a lower risk of experiencing fever or febrile seizures (a convulsion associated with a significant rise in body temperature, which is usually resolved quickly and without medical intervention) shortly after vaccination than those who received the vaccine at ages 16 to 23 months.

Previous studies have provided evidence that measles vaccines may be linked with a small increased risk of febrile seizures or fever around 1-2 weeks after vaccination. However, the febrile seizures are extremely rare and are not associated with later neurological deficits. Although there is no definitive hypothesis of what might cause this increase, some scientists suspect that it is a response to an increase in the replication of the weakened virus (from the vaccine) that occurs during this 1-2 week period of time. In other words, it appears to be a natural adaptive immune response to the virus. 

This new study retrospectively analyzed data from 840,348 children (a huge study by any measure), ages 12-23 months who had received a measles-containing vaccine, either MMR, (for measles, mumps and rubella), or MMRV, (for MMR plus chickenpox). Here are some of the key results from the research:

  • Children ages 16-23 months were 6.5X more likely to experience a febrile seizure in the 7-10 days following vaccination than on other days.
  • For every 10,000 children vaccinated, this risk was equivalent to about 9.5 extra seizure cases during the weeks following vaccination than would be expected without vaccination.
  • For children ages 12-15 months (in other words, those vaccinated on schedule), the risk was lower. These children were about 3.4X more likely to experience a febrile seizure in the 7-10 days following vaccination than on other days. F
  • For every 10,000 children vaccinated according to the CDC schedule, this risk translated to about four extra seizure cases during the weeks following vaccination, which was about 50% lower than the risk for those who were vaccinated later.
  • The researchers highlighted that the child’s risk of experiencing a febrile seizure that would require medical attention is tiny, less than 1 out of every 1,000 vaccinations.
  • The researcher found that the rate of febrile seizure following vaccination is seven times lower than the rate following natural infection with measles. In other words, setting aside the data that those get vaccinated on time experience fewer febrile seizures than those who wait, but the risk of it is 700% lower than those who don’t get vaccinated. 

The authors, from the Kaiser Permanente Vaccine Study Center, concluded that the “findings of this study that focused on safety outcomes highlight the importance of timely immunization of children with the first dose of measles-containing vaccines.”

Drs. Paul Offit and Kristen Feemster, of the University of Pennsylvania School of Medicine, in an accompanying article in JAMA Pediatrics, stated that,

The implications of these results are significant. First, it reinforces the well-established safety of the current recommended schedule, which is based on many years of prelicensure and postlicensure safety and effectiveness data, including concomitant use studies. The requirement for these studies exceeds those of any other medication. The safety of the schedule was also recently endorsed by a report from the Institute of Medicine, whose review of both mechanistic and epidemiological data concluded that the current schedule does not increase risk for autoimmune diseases, asthma, seizures, developmental disorders, hypersensitivity, or attention-deficit disorders.

Additionally, the current schedule is also informed by disease epidemiology and immunogenicity data. Vaccines are recommended at certain ages and intervals to optimize the immune response, ensure protection when a child is most at risk for disease acquisition, and minimize adverse events. The recommended schedule incorporates consideration of special situations like receipt of immunosuppressive therapy to ensure safety and efficacy. A choice to delay vaccination without considering disease risk, efficacy, and safety could impact any one of these outcomes.

If vaccine safety is a concern, the currently recommended schedule is the best choice for preventing disease and minimizing adverse events.

So what can we conclude? Arbitrarily delaying the measles vaccination, because of an unfounded belief in “too many vaccines”, increased the risk of febrile seizures. Not vaccinating increased the risk of febrile seizures from measles by around 700%. Which means, vaccinate against measles, and don’t delay it. Simple, isn’t it?

If you need to search for accurate information and evidence about vaccines try the Science-based Vaccine Search Engine.

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Michael Simpson

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