A speaker raised two related questions: (1) Can the effects of individual vaccines be discerned even though vaccines are usually given in combination; and (2) Does the simultaneous administration of vaccines pose more risk than the administration of vaccine separately? A representative of a manufacturer stated that it is possible to answer these questions by designing a randomized trial in which, for example, one group is given three vaccines simultaneously, another group receives each of those vaccines at separate times, and other groups receive two of the three vaccines simultaneously. One could then assess whether there are any statistically significant differences in the risk of adverse events when these groups are compared. Such studies would be large, cumbersome, and expensive to run, he noted, but theoretically, they can be done.

One participant expressed concern that large prelicensure trials that take several years to conduct and analyze would place too much of a financial burden on vaccine manufacturers and could delay the availability of new vaccines. An epidemiologist commented that large-scale clinical trials, although potentially valuable, would not obviate the continued need for systems such as VAERS to detect the adverse effects of the vaccines. Even if a vaccine is shown to be safe in a large clinical trial, the potential for lot and batch variability and for variability in the use of vaccines require continual surveillance for adverse events.

Several speakers suggested conducting more focused, powerful, larger, and better-designed postlicensure clinical studies to detect serious , rare adverse events of vaccines that are not possible to detect in a clinical trial. FDA continues to work with manufacturers in this regard but recognizes that such studies, without randomized control groups, will be far less valid than the randomized trials that can be done prelicensure.

A vaccine manufacturer's representative suggested that each manufacturer should contribute money to a pool that could be used to establish a state-of-the-art, large, linked database system rather than doing its own small-scale postmarketing studies to detect adverse effects from vaccines. This would create a standard system for the detection of rare adverse events, that might go undetected in the smaller studies conducted by individual companies. Manufacturers could pay a maintenance fee for receiving data from such a large linked database system. To improve further the detection of rare adverse events, a vaccine manufacturer's representative suggested establishing an international network of databases of adverse events following vaccination. Such a network would require strict standardization of the information entered into it, however.