How much Pittsburgh will learn from the experience with vaccines will depend on the numerous analyses of data which were acquired during the epidemic. _Data on the Prophylactic Value of Mixed Vaccines_ Proof of the prophylactic value of mixed vaccines for epidemic influenza depends entirely upon the results of its practical application to human subjects in times when the disease is prevalent. Animal determinations are out of the question, because it has not been possible to produce the epidemic form of influenza experimentally. If all people were equally susceptible and were equally exposed, it would be a simple matter to compare the number of vaccinated persons who developed the disease with the number of unvaccinated persons who contracted it; but since many thousands were vaccinated and some of them contracted the disease in spite of it, and a greater number of persons who were not vaccinated entirely escaped, the analysis is extremely difficult. The time element is a big factor. In instances where vaccination was completed in a community before the epidemic appeared there, the figures are worth more than those in which vaccination was undertaken after the epidemic had become established. This is true, because the most susceptible persons in a community developed the disease as soon as they were exposed, the less susceptible ones were not attacked until later, and the insusceptible ones escaped altogether. Whenever vaccination is begun during an epidemic, the persons vaccinated for prophylactic purposes are necessarily chosen from those who have not yet developed an attack. The later in the epidemic that vaccination is begun, the greater will be the number of persons selected for vaccination from among those more or less naturally immune. Then, if the total number of cases among the vaccinated is compared with the total number of cases among the unvaccinated, the apparent value of the vaccine is increased; but the estimation is not a fair one, because the vaccinated group is unavoidably selected from among relatively immune persons, while the controls include all of the very susceptible people who were suffering from the disease at the time vaccination was begun. Where vaccination is begun after the epidemic is advanced, the only figures worth while are those obtained by a day-by-day or a week-by-week comparison between the number of cases developing among controls and the number of cases appearing among those vaccinated, and by beginning that comparison at a time subsequent to the day on which the prophylactic inoculations were completed. Aside from the interpretation of the results there is possibly a more serious reason for objecting to the beginning of vaccination during an epidemic. This lies in the danger of producing a temporary negative phase in the patient, which makes him somewhat more susceptible to natural infection for a few hours immediately following each administration.