A preliminary report on the immunological responses after oral vaccine (Ty 21a).

Humoral and cell-mediated immune responses, specific for lipopolysaccharide (LPS), were evaluated before and after oral immunization with the Ty 21a strain of Salmonella typhi in a group of healthy volunteers. No rise in seric and foecal antibody titres, detected by the ELISA technique, was seen after vaccination. On the contrary, we were able to demonstrate the development of specific cell-mediated immunity, as assayed by the leukocyte migration inhibition test, 21 days after vaccination. This conversion was still present 40 days after the last dose of vaccine. We believe that the Ty 21a strain of S. typhi, recently proposed for immunization against typhoid fever and based on the use of live micro-organisms, is effective due to its ability to induce specific cell-mediated immunity.