Comparison Studies

Noting that VAERS primarily receives reports of adverse events that occur shortly after vaccination, several speakers suggested developing procedures to detect longer-term or more subtle adverse events that might be associated with vaccination, such as learning disabilities or chronic fatigue syndrome. It was suggested that retrospective studies, as well as prospective studies that use nonvaccinated individuals as controls over a period of 10 or 20 years, be conducted for this purpose. A consumer representative suggested that there be more government funding for studies designed to detect long-term adverse effects of vaccines or to identify individuals at high risk for adverse effects. It was also suggested that children given vaccines in clinical trials be followed for 5 to 10 years to help assess long-term adverse events that do not become apparent until children reach later stages of development. Other participants commented that such studies would be extremely difficult to design, especially if a vaccine comes into general use so that it is given to large numbers of children.

Several speakers suggested that more studies be done using LLDB or other methods to compare vaccinated children with unvaccinated children. There is concern, however, that children who are not vaccinated are different in important ways from those who are vaccinated and therefore are not a suitable control population. To avoid the problem of controls who differ from cases in unmeasured ways, a CDC representative described a method that uses cases as their own controls (Farrington et al., 1996). He illustrated this methodology with a hypothetical example, in which the adverse events and vaccinations that an infant experienced during each month are monitored, and then applied this methodology to seizures after vaccination with the measles-mumps-rubella vaccine reported in CDC's LLDB study. By comparing the rates of occurrence of adverse events during periods immediately following vaccination and at all other times, the risk of adverse events with short latencies can be determined.

To better separate the adverse effects of a vaccine from background adverse events that occur in the age group vaccinated, a vaccine researcher suggested that different vaccine schedules be used for different groups of children in a random manner. This would not be unethical, the speaker noted, if the schedules were devised such that some children received a vaccine only a month or two later than other children.

To improve assessment of the frequency of occurrence of rare adverse events, a vaccine manufacturer's representative suggested that more effort be made to improve general surveillance of specific disorders that could potentially be associated with vaccination, such as Guillain-Barré syndrome or transverse myelitis. The likelihood of these disorders occurring as a result of vaccination



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