Bacterial vaccines of various sorts were widely used for both preventive and therapeutic purposes during the great influenza pandemic of 1918-19. Some were derived exclusively from the Pfeiffer's bacillus, the presumed cause of influenza, while others contained one or more other organisms found in the lungs of victims. Although initially most reports of the use of these vaccines claimed that they prevented influenza or pneumonia, the results were inconsistent and sometimes contradictory. During the course of the debates over the efficacy of these vaccines, it became clear that the medical profession had no consensus on what constituted a proper vaccine trial. Even among those who asserted that clinical impression was not enough, there was no agreement on how a trial ought to be conducted. The American Public Health Association, through its Working Program on Influenza, sought to establish standards for the profession. The standards the APHA set in December 1918 guided American vaccine trials for a quarter century.