Immunizations are a cornerstone of the nation’s efforts to protect people from a host of infectious diseases. Though generally very rare or minor, there are side effects, or “adverse effects,” associated with some vaccines. Importantly, some adverse events following a vaccine may be due to coincidence and are not caused by the vaccine. To make this distinction, researchers use evidence to determine if adverse events following vaccination are causally linked to a specific vaccine; if so, these events are referred to as adverse effects. The Health Resources and Services Administration asked the IOM to review a list of adverse events associated with eight vaccines—varicella zoster, influenza (except 2009 H1N1), hepatitis B, HPV, MMR, hepatitis A, meningococcal, and those that contain tetanus—and evaluate the scientific evidence about the event–vaccine relationship. The IOM committee appointed to this task was not asked to assess the benefits or effectiveness of vaccines but only the risk of specific adverse events.
Using epidemiologic and mechanistic evidence, the committee developed 158 causality conclusions and assigned each relationship between a vaccine and an adverse health problem to one of four categories of causation:
- Evidence convincingly supports a causal relationship
- Evidence favors acceptance of a causal relationship
- Evidence favors rejection of a causal relationship
- Evidence is inadequate to accept or reject a causal relationship
The committee finds that evidence convincingly supports a causal relationship between some vaccines and some adverse events—such as MMR, varicella zoster, influenza, hepatitis B, meningococcal, and tetanus-containing vaccines linked to anaphylaxis. Additionally, evidence favors rejection of five vaccine-adverse event relationships, including MMR vaccine and autism and inactivated influenza vaccine and asthma episodes. However, for the majority of cases (135 vaccine-adverse event pairs), the evidence was inadequate to accept or reject a causal relationship. Overall, the committee concludes that few health problems are caused by or clearly associated with vaccines.