A new systematic review has found that while some childhood vaccines are associated with serious adverse effects, these events are extremely rare and, surprise surprise, do not include autism, food allergies or cancer. The study has been published in Pediatrics.
Despite the achievements of vaccines over the last century and the overwhelming amount of evidence for their safety, many remain hesitant and there is a growing trend of parental vaccine refusal. Unfortunately, this has led to a resurgence of vaccine preventable diseases such as measles and whooping cough in certain countries.
In order to address this issue, researchers compiled and analyzed a large number of studies in what is called a systematic review to assess the safety of childhood immunizations. Systematic reviews are a way to summarize large amounts of research evidence in an impartial manner. The process involves first identifying all relevant published and unpublished evidence, selecting studies for inclusion based on quality and other criteria, and finally synthesizing and interpreting the findings in a balanced manner.
This particular study builds on a 2011 report carried out by the Institute of Medicine but includes several other vaccines such as hepatitis A, Haemophilus influenza type b (Hib), polio and rotavirus. While the 2011 study also identified some adverse effects associated with certain vaccines, it found “few health problems that are caused by or clearly associated with vaccines.”
Over 3,000 relevant studies were identified by the team but only 67 of these were selected for inclusion into the review, all of which had control groups. The other studies were excluded for various reasons such as poor study design, individual case reports which lacked comparison groups or insufficient assessment.
The researchers identified associations with adverse events (AEs) for several vaccines but some, such as hepatitis B, were not associated with any AEs. The Hib (meningitis and pneumonia) vaccine was found to be associated with local discomfort such as redness at the site of injection, but was not associated with any serious AEs or hospitalization. They found moderate associations between the Hepatitis A vaccine, the MMR and the chicken pox vaccine with purpura, a skin rash caused by blood vessel leakage.
There was some evidence to suggest that two vaccines against rotavirus, RotaTeq and Rotarix, were associated with a condition called intussusception where part of the intestine slides into an adjacent part. The incidence was rare, however, with around 1.5 cases per 100,000 doses of RotaTeq and 5 cases per 100,000 of Rotarix, and risk factors were not investigated in any of the studies.
They found that both the MMR vaccine and the influenza vaccine TIV were associated with febrile seizures (seizures associated with fever), but the MMR was not associated with autism. Furthermore, the polio vaccine was not associated with sensitivity to food allergens and none of the vaccines were associated with either leukemia or death.
“We found that serious adverse events that are linked to vaccines are really rare, and that when they do occur they are often not necessarily severe,” co-author Courtney Gidengil told AFP. “We think this adds to the body of evidence that the benefits really do seem to clearly outweigh the low risk of serious side effects from vaccines.”
Gidengil hopes that by acknowledging that there are rare but actual side effects, the study will increase some trust in both the vaccine process and the healthcare system.