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Descriptive epidemiology of adverse events after immunization: reports to the Vaccine Adverse Event Reporting System
1Division of Biostatistics and Epidemiology, Food and Drug Administration, Rockville, Maryland 20852, USA.
Objective:
To provide an overview of the data, function, and performance of the Vaccine Adverse Event Reporting System.
Design:
Descriptive and correlational analyses.
Setting:
United States, 1991 through 1994.
Subjects:
Reports to the Vaccine Adverse Event Reporting System, a passive national surveillance system, that represents temporal (but not necessarily causal) relationships between vaccinations and adverse events.
Main Outcome Measures:
Demographic variables, birth weight, vaccine type, severity of adverse event after immunization.
Results:
A total of 38,787 adverse events was reported during the study period without a clearly increasing or decreasing trend in the annual number of total reports or deaths. Of the deaths with known age, 72.4% were reported in the first year of life, and 63.7% of these were male. The peak age for death reports was 1 to 3 months, with a gradual decline through age 9 months, after which death was relatively rare. Adverse events with onset of symptoms the day of vaccination accounted for 45.5% of total reports; 20.4% had onset of symptoms the following day. Onset within 2 weeks after vaccination was noted for 92.5% of all reports. Simultaneous administration of multiple vaccines was noted in 75.7% of reports for immunizations at ages younger than 20 years. In contrast, among those 20 years or older, only 6.0% of reports named multiple vaccines. Wide geographic variations were noted in adverse event reporting rates for children younger than 2 years, and the states with the lowest reporting rates of less serious events included the most populous states.
Conclusions:
The peak age of deaths at ages 1 to 3 months could be expected on the basis of prior studies showing that sudden infant death syndrome deaths peak at that age, that most deaths in the Vaccine Adverse Event Reporting System are attributed to sudden infant death syndrome, and that sudden infant death syndrome has not been associated with vaccination. The large number of reports and national coverage of the Vaccine Adverse Events Reporting System make it useful for monitoring the safety of vaccine lots and for accumulating case series to detect or better understand adverse events that may occur too rarely to be assessed in clinical trials or in the larger studies that are sometimes carried out by manufacturers after vaccine licensure (phase IV studies).