Coincidental events (AEFIs) can result from underlying or emerging conditions of the vaccine as well as external exposures that can cause harm independent of immunization.
Coincidental events occur after a vaccination has been given but are not caused by the vaccine or its administration.
Vaccinations are normally scheduled in infancy and early childhood, when illnesses are common and congenital or early neurological conditions become apparent. Coincidental events are inevitable when vaccinating children in these age groups, especially during a mass campaign. Applying the normal incidence of disease and death in these age groups along with the coverage and timing of immunizations allows estimation of the expected numbers of coincidental events after immunization.
Coincidental events include but are not limited to:
- Underlying or emerging conditions in the vaccine:
- Manifestation or complication of a congenital or inherited underlying disease condition or birth injury.
- Manifestation or complication of an underlying acquired disease condition that may or may not have been diagnosed prior to immunization.
- Psychogenicillness.
- Conditions caused by exposure to something other than vaccine:
- Infection due to agents such as bacteria, viruses, fungi or parasites.
- Adverse reaction due to recent or concomitant medication or use of illicit substances.
- Allergic and other hypersensitivity reactions due to exposure to allergens other than those present in the vaccine.
- Injury due to exposure to environmental toxins.
- Injury due to trauma including surgery.
Careful interpretation of vaccine safety signals was crucial to detect real reactions to vaccine and to ensure that coincidental events were not caused by vaccination and did not affect public confidence in the vaccine. Identifying real safety concerns with new vaccines means we have to untangle actual safety signals from background medical events, which are those that would happen without vaccination.
Examples:
- Meningitis that occurs within days of MMR vaccination that upon investigation is shown to be caused by Streptococcus pneumoniae
- Flu-like symptoms due to a rhinovirus infection after influenza immunisation
Case studies:
- Interruption of a 2006 seasonal influenza campaign in Israel, where four deaths occurred within 24 hours of immunization. The clustering of fatalities and close timing of vaccination resulted in global news coverage, public trepidation and compromised the inoculation campaign. In actuality, the four patients who died all were in a group already at high risk for sudden death from age and underlying medical conditions. Their deaths were consistent with a cardiac cause of death, and the number of deaths was lower than would be expected normally for such a high risk population. Further analysis of the fatalities in Israel showed death normally occurs in this high risk group at a rate greater than one per 1,000 individuals in the same time period. Based on this, the researchers said 20 coincidental deaths among that group could be expected to occur by chance within 24 hours of an immunization.
- Swine flu vaccination program: The vaccine in that campaign was associated with an increased number of Guillian-Barre Syndrome cases, in which the body’s immune system mistakenly attacks part of the nervous system. Guillian-Barre normally affects about one out of every 100,000 people a year. Based on this Guillian-Barre background rate, if 100 million people in the United States are inoculated in a pandemic flu vaccination campaign, the researchers said one would expect 215 new cases of the disease within six weeks of vaccine. These cases would be expected to occur whether or not the vaccine had been given.
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