New study: yep, vaccines are still safe.

Earlier this week the journal Pediatrics released a new research report on vaccine safety. It is actually an update to an earlier report issued by the Institute of Medicine in 2011: a comprehensive study with a report-out of almost 900 pages. This update focuses specifically on vaccine safety for children (not adolescents or adults).

Both the original 2011 report, and the 2014 update, are “literature reviews” in which scientists systematically look at all of the primary research done on the topic, cull out those that are either off-topic or having sub-standard procedures, and analyze the remaining ones for relevant results and agreement across studies. Just to give you a flavor for the enormity of this task: the 2014 update found 20,478 candidate papers just since the 2011 report came out; they eliminated 17,240 based upon title and abstract alone; of the 3208 remaining, the full text review of the articles narrowed the list down to 67 that were actually studies of vaccine safety for children. This update includes information on a few vaccines that were not studied in the 2011 report: Hib, Hepatatis A, IPV, PCV13, and rotavirus.

While this kind of study is very useful in general, it is of course particularly valuable now given the high level of misinformation and panic being spread about vaccine safety, and particularly about links between vaccines and autism.

So the big news first: there is no link between the MMR vaccine, or any other vaccine, and the onset of autism. Also, there is no link between any vaccines and childhood leukemia, or between the DTaP vaccine and type 1 diabetes. There was no believable evidence of a link before 2011, and in the last three years all the research continues to find no link.

Beyond the headlines, it’s important to note that all vaccines can potentially cause side effects or “adverse events.” In the vast majority of cases, those “AE’s” (as they are called in the literature) are extremely rare: so rare that establishing the incidence can be difficult. For example, suppose a particular AE happened in 1 in 100,000 children receiving the vaccine. To see that, you would need to have at least 100,000 participants in the study that you ran; in fact, you would need many multiples of that, since the statistical significance of running a study with 100,000 participants and seeing one positive case is very low because that one case could just be a fluke unrelated occurrence, or the real rate may be 1 in 200,000 and you happened to catch the “1” in the first 100,000 people you randomly selected. The point: doing these kinds of studies well requires an extremely large sample size because the AE rate is so low.

Fortunately in the U.S. there is a comprehensive system for collecting large-scale information, called the Vaccine Adverse Event Reporting System. This, in addition to data that is collected about the number of people who are vaccinated every year, gives us a larger picture of the incidence of AE’s across tens of millions of people. This invaluable data is one source that forms the basis for the primary research that is done on vaccine safety (and that the literature reviews compile).

So what does the literature say about vaccines and AE’s? First and foremost, it confirms that AE’s continue to be extremely rare. The vast majority are minor: short-term fever, rash, etc. Of the more serious AE’s, the three most often cited are :

  • Febrile seizures: convulsions associated with a high fever. Though it’s important to note that many children have febrile seizures independent of their vaccination status.
  • Purpura: red or purple blotches under the skin.
  • Anaphylaxis: a sudden and serious allergic reaction.

Here’s a quick summary of the major vaccines and any moderate or strong connection to AE’s:

  • DTaP: no known causal relationship to AE’s.
  • Hib: redness and swelling at the injection site, but no other AE’s.
  • Hepatitis A: purpura associated for children aged 7 to 17, but not children aged 2-6.
  • Hepatitis B: relationship with anaphylaxis in yeast-sensitive individuals.
  • Inactivated Polio Virus (IPV): no confirmed associations with AE’s.
  • Influenza vaccines: a moderate association with febrile seizures (12.5 cases per 100,000 vaccinations);. It slightly increases if the influenza vaccine and a pneumonia vaccine (PCV13) are given at the same time.
  • MMR: Convincing relationship between MMR and “measles inclusion body encephalitis” in children with compromised immune systems, MMR and anaphylaxis in patients with certain allegies, and MMR and febrile seizures. A moderate association with purpura shortly after vaccination. Note that occurrences of these AE’s are extremely rare.
  • Meningococcal: Convincing relationship with anaphylaxis in children who may be allergic to ingredients in the vaccine.
  • Rotavirus vaccines (Rotateq and Rotarix): moderate relationship with intususception, but extremely rare (1.5 cases per 100,000 of Rotateq and around 5 cases per 100,000 of Rotarix).
  • Varicella: moderate association with purpura in children aged 11 to 17.


It’s important to note that all of these AE’s are extremely rare, and there is little or no commonality across vaccines. The AE’s are so rare, in fact, that in most cases it’s impossible to investigate whether there are specific characteristics shared by the patients who experienced the AE’s (and the researchers found few studies that attempted to do so). IT would be helpful to know if there were particular risk factors that health professionals and parents should be cognizant of.

The bottom line is still the same: vaccinating your child is extremely safe. A news report covering this study pointed out that the risk of an AE from a vaccination is lower than the risk of a traffic accident, so the greatest risk in vaccinating your child is in driving her to and from the doctor’s office.


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