Vaccine manufacturers, which share a responsibility for detecting adverse events associated with vaccination, described their procedures as follows. Adverse event reports are reviewed by a team representing safety surveillance, clinical research, regulatory affairs, and quality control. The review team examines events individually and with an eye to repeated reports. The review committee may use a threshold level of, for example, 10 reports during a vaccine's lifetime as a signal that follow-up is needed. Manufacturers also monitor the pattern of adverse event reports for particular lots or batches of vaccine over time. Any atypical patterns are carefully reviewed. The entire surveillance process is carried out in close collaboration with and regular reporting to FDA, and thereby VAERS.
In addition to conducting periodic in-house investigations of vaccines for which a company and VAERS have received large numbers of adverse event reports, local investigations are sometimes necessary. An evaluation of the large numbers of adverse events reported from immunization clinics in South Africa, for example, revealed that the vaccines were improperly administered and that some of the reactions were labeled inaccurately.
Manufacturers noted that postmarketing reporting is voluntary and consequently represents incomplete reporting of the true numbers of adverse events that are actually occurring in the population. Furthemore, these reports are coded in the terminology used by the reporter, and there are no standard case definitions for various adverse events. If a health care provider reports a case of encephalitis, for example, it is recorded as a case of encephalitis.
Because of these limitations and because the standard premarketing clinical trials are not sufficiently large to detect rare adverse events, manufacturers also conduct postmarketing surveillance studies. These studies either search for a particular adverse event in a specific population or scrutinize a large number of vaccines for general safety issues. As an example, an epidemiologic analysis of selected reported neurologic adverse events associated with the administration of the hepatitis B vaccine was presented. The manufacturer reported that, although a causal association could not be entirely ruled out, it found no evidence from the analysis of postmarketing reporting data to suggest that Guillain-Barré syndrome, myelitis, transverse myelitis, multiple sclerosis, optic neuritis, or peripheral neuropathies were causally associated with the administration of the hepatitis B vaccine.