Controlled field trial of the effectiveness of cholera and cholera El Tor vaccines in Calcutta.

To assess the effectiveness of cholera vaccines, 2 controlled field trials were made in Calcutta-an endemic area-during 1964 and 1965. Three Indian vaccines of which 1 was grown on casein hydrolysate and 2 on agar, a freeze-dried vaccine from the Walter Reed Army Institute of Research (WRAIR), Washington, D.C., and an El Tor vaccine from the Philippines were used, with typhoid-paratyphoid (TAB) vaccine as a control. The 210 112 volunteers were vaccinated subcutaneously with a single dose of one of the vaccines.In the 1964 trial, the number of bacteriologically confirmed cases was not enough to show statistically significant differences in incidence between the 5 vaccine groups and the control group. However, the WRAIR freeze-dried vaccine protected about 40% of the vaccinees for 6 months after vaccination, although the efficacy was higher (57%) during the first 3 months than during the subsequent 3 months (28%). Agar-grown vaccine, produced by the Central Research Institute, Kasauli, was 37% efficacious.In the 1965 trial, owing to the small number of cases in the study area, the Kasauli vaccine was the only one to show statistically significant protection (40%).