Evidence Convincingly Supports a Causal Relationship
The committee concludes that the evidence convincingly supports a causal relationship between some vaccines and some adverse events.
As a live vaccine, the varicella zoster vaccine is linked to four specific adverse events, all due to infection from the vaccine virus strain:
- Disseminated varicella infection (widespread chickenpox rash shortly after vaccination)
- Disseminated varicella infection with subsequent infection resulting in pneumonia, meningitis, or hepatitis in individuals with demonstrated immunodeficiencies
- Vaccine strain viral reactivation (appearance of chickenpox rash months to years after vaccination)
- Vaccine strain viral reactivation with subsequent infection resulting in meningitis or encephalitis (inflammation of the brain)
The MMR vaccine is linked to a disease called measles inclusion body encephalitis, which in very rare cases can affect people whose immune systems are compromised and usually occurs within a year of acute measles infection or vaccination. The MMR vaccine also is linked to febrile seizures, which are a type of seizure that occurs in infants and young children in association with fever. Febrile seizures are generally benign and hold no long-term consequences.
Six types of vaccines—MMR, varicella zoster, influenza, hepatitis B, meningococcal, and tetanuscontaining vaccines—are linked to anaphylaxis.
The committee also found convincing evidence of a causal relationship between injection of vaccine, independent of the antigen involved, and two types of adverse events, including syncope, or fainting, and deltoid bursitis, or frozen shoulder, characterized by shoulder pain and loss of motion.
Evidence Favors Acceptance of a Causal Relationship
The evidence favors acceptance of four vaccine–adverse event relationships. In these cases, the evidence is strong and generally suggestive, but not firm enough to be described as convincing. These relationships include:
- HPV vaccine and anaphylaxis;
- MMR vaccine and transient arthralgia (temporary joint pain) in female adults;
- MMR vaccine and transient arthralgia in children; and
- certain trivalent inactivated influenza vaccines used in Canada in some recent years and a mild and temporary oculorespiratory syndrome, which is characterized by conjunctivitis, facial swelling, and upper respiratory symptoms, including coughing and wheezing.
Evidence Favors Rejection of a Causal Relationship
The evidence favors rejection of five vaccine–adverse event relationships:
- MMR vaccine and autism
- MMR vaccine and type 1 diabetes
- DTaP (tetanus) vaccine and type 1 diabetes
- Inactivated influenza vaccine and Bell’s palsy (weakness of the facial nerve)
- Inactivated influenza vaccine and exacerbation of asthma or reactive airway disease episodes in children and adults
Evidence Inadequate to Accept or Reject a Causal Relationship
For the vast majority, (135 vaccine-adverse event pairs), the evidence is inadequate to accept or reject a causal relationship. In many cases, the adverse event being examined is an extremely rare condition, making it hard to study. In these cases, there was not adequate evidence to determine if the vaccine was or was not causally associated.
As some of the conclusions suggest, individuals with certain characteristics are more likely to suffer certain adverse effects from particular immunizations. Individuals who have serious immunodeficiencies are clearly at increased risk for specific adverse reactions to live viral vaccines, such as MMR and varicella vaccines. Thus, the committee was able at times to reach more limited conclusions for subgroups of the population.
In applying consistent standards across all the evidence, the committee found that some conclusions were easy to reach: the evidence was clear and consistent or, in the extreme, completely absent. Others required substantial discussion and debate.
The committee was not charged with making recommendations, and it did not pinpoint any particular areas for continued research. Much research already occurs to determine the safety of vaccines for the populations for whom they are recommended. However, there is much to learn about the human immune system, autoimmunity, and the effects of genetic variation, all of which may influence how people respond to vaccines.
Vaccines offer the promise of protection against a variety of infectious diseases. Despite much media attention and strong opinions from many quarters, vaccines remain one of the greatest tools in the public health arsenal. Certainly, some vaccines result in adverse effects that must be acknowledged. But the latest evidence shows that few adverse effects are caused by the vaccines reviewed in this report.