Sound Immunization Recommendations and Policy
Public health recommendations for vaccine programs and practices represent a dynamic balancing of risks and benefits. Vaccine safety monitoring is necessary to accurately weigh this balance and adjust vaccination policy. This was done in the United States with smallpox and oral polio vaccines as these diseases neared global eradication. Complications associated with each vaccine exceeded the risks of the diseases, leading to discontinuation of routine smallpox vaccination in the United States (prior to global eradication) and a shift to a safer inactivated polio vaccine. Sound immunization policies and recommendations affecting the health of the nation depend upon the ongoing monitoring of vaccines and continuous assessment of immunization benefits and risks.
Adverse Events Following Immunization and Assessment of Causality
Adverse events following immunization can be classified by frequency (common, rare), extent (local, systemic), severity (hospitalization, disability, death), causality, and preventability (intrinsic to vaccine, faulty production, faulty administration). Adverse events following immunizations may be coincidental events or the vaccine may have increased the risk of the adverse event. Some adverse events following immunization may be due to the vaccine preparation itself and the individual response of the vaccinee, and would not have occurred without vaccination. Examples of such events are vaccine-associated paralytic poliomyelitis after oral polio vaccine, or vaccine-strain measles viral infection in an immunodeficient recipient.
Other health events may be precipitated by an immunization, such as a vaccine-associated fever precipitating a febrile seizure. Vaccine administration errors may lead to adverse events as well, for example, when administration of a vaccine too high in an adult’s arm causes deltoid bursitis. However, many adverse events following immunization are coincidental; they are temporally related to immunization, but occurring by chance without a causal relationship.
To assess causality of an adverse event following immunization, much information is generally needed. A good reference for causality determination is available at www. ncbi.nlm.nih.gov/pubmed/22507656. An adverse health event can be causally attributed to vaccine more readily if: 1) the health problem occurs during a plausible time period following vaccination; 2) the adverse event corresponds to those previously associated with a particular vaccine; 3) the event conforms to a specific clinical syndrome whose association with vaccination has strong biologic plausibility (e.g., anaphylaxis) or occurs following the natural disease; 4) a laboratory result confirms the association (e.g., isolation of vaccine strain varicella virus from skin lesions of a patient with rash); 5) the event recurs on re-administration of the vaccine (“positive rechallenge”); 6) a controlled clinical trial or epidemiologic study shows greater risk of a specific adverse event among vaccinated vs. unvaccinated (control) groups; or 7) a finding linking an adverse event to vaccine has been confirmed by other studies.
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