Since 1957 at 14 national hospitals field studies influenza vaccination were carried out 52 times. The student-nurses of the national hospitals were innoculated with Y₅ strain (Q-phase) or A₂ influenza vaccine, which was made from Adachi strain (P-phase of A₂ influenza virus), and hemagglutination inhibition titer (HI titer) before and after innoculation were measured to investigate their preventive effects. For the antibody measurement, at any times, Adachi strain was used as antigen.<br>Results<br>1) HI titer to Adachi virus (A2/Adachi/1957) to be able to avoid suffering from A2 influenza.<br>a) Individual Case<br>Judging from the results of the experiments which we will mention below, we should be able to conclude that in case of each person, if his HI titer is ≥1: 128, he will protected from contacting influenza. However, if it is ≤1.64 some may be protected and others will not.<br>(1) When Asian influenza oceured in 1957, at the 1st Tokyo National Hospital among 65 out of 82 student-nurses there was a mass outbreak of influenza.<br>In regard to the distribution of HI titer to Adachi virus before occurence of thed isease, it was indicated that 7 students had been ≥1: 128 and 75≤1: 64. 7 students with titer ≥1: 128 could prevent the influenza infection while 65 out of 74 students with titer ≤1: 64 could not prevent the infection and only 10 students did not contact the disease.<br>(2) In the 1st Tokyo National Hospital for a period of 9 months 114 students cought cold 184 times. Among the 84 times were A2 influenza. And their HI titers before the contact were <1: 16 77 times, 1: 16 twice, 1: 32-3 times and 1: 64 twice. These results indicated that the HI titer of each patient was only ≤1: 64.<br>(3) At the Ureshino National Hospital Nurse School there was an outbreak of influenza and 29 students suffered from A2 influenza. The distribution of the HI titer before contact indicated that 28 students had <1: 16 and 1 had titer 1: 16.<br>(4) In the Tochigi National Hospital, the 1st Tokyo National Hospital and the Iwakuni National Hospital there were 9 students who suffered from influenza more than two times. The distribution of HI titer before their reinfection was ≤1: 64 one was 1: <64, three 1: 32 and three 1: 64.<br>b) Case a Group<br>Even after every one of a group is infected with influenza, we cannot say that each one of them had a titer as high as ≥1: 128.<br>In both of the 1st National Tokyo Hospital and the Ureshino National Hospital after their group outbreak the distribution of H antibody was summarized as ≥1: 128 for 43.5% of the group while ≥1: 16 for 15.3% of the group.<br>From these results we are able to conclude that some groups shall able to avoid mass infection by A2 influenza, if the group is under the conditions: a) more than 45% of the members will have HI titer ≥1: 128, b) more than 15% of the members will have HI titer 1: 16, namely more than 85% of the members will have the antibody.<br>2) Standards available for judging the effect of A2 influenza vaccination after innoculation, if we do not have influenza prevalence at every proper season, we will not be able to investigate its protective effect clinically. For this purpose, before and after the innoculation of a group, we studied the distribution of HI titer, then we compared it with the following standard. This indicated, if someone of the group is found to have higher antibody than the standard, he is recognized to have the protective effect. The standard is as follows:<br>a) The member with HI titer ≥1: 16 should be more than 85% of the group.<br>b) The member with HI titer ≥1: 128 should be more than 45% of the group.<br>3) HI titer before and after the vaccination.<br>Vaccination can make one's HI titer higher only in the case that the one had more or less antibody, but the vaccination cannot produce one's titer higher because the one had no antibody at all.