The possibility of developing a living oral cholera vaccine with naturally avirulent El Tor cultures isolated in a Middle East country or from water sources in Calcutta in the absence of cholera El Tor in these areas has been further studied.

In an extended series of experiments the markedly low pathogenicity of the proposed vaccine strains in laboratory animals has been confirmed. The vaccine strains have been shown to get established and multiply regularly in the ligated intestinal loops of adult rabbits and in the intestine of infant rabbits without producing pathogenic reactions.

The apathogenic character of the vaccine strains has been found to be stable. When propagated serially in these two laboratory models of experimental cholera the vaccine strains show no enhancement of pathogenicity.

Intra-intestinal administration of the live vaccine has been shown to protect adult rabbits fully in the intestinal loop test. Immunized animals were also protected against challenge with V. cholerae strains, though to a somewhat lesser extent. Protective immunity could not, however, be demonstrated in infant rabbits probably because of the immunity-forming mechanism being still rudimentary.

Immunization with live vaccine was found to inhibit the growth of homologous strains in the ligated intestinal loop of the adult rabbit. The growth rate of pathogenic V. eltor and V. cholerae strains was also seen to be markedly reduced throughout the 24 hr. period of observation, but more markedly during the first 4–8 hr. However, the total count of pathogenic V. eltor and V. cholerae strains at 24 hr. after inoculation in ligated ileal loops of immunized rabbits reached levels that invariably caused inflammation and accumulation of fluid in normal rabbits' loops. The absence of gut-inflammatory reaction in immunized rabbits under these conditions has been discussed in relation to the nature of the immunity produced by administration of live vaccine.