Protection Against Meningococcal Challenge Afforded Mice After Immunization with Typhoid-Paratyphoid Vaccine USP.

Mice immunized with typhoid-paratyphoid vaccine USP were found to be protected when challenged with serological groups A, B, and C of Neisseria meningitidis. Mice immunized with this vaccine both 14 and 7 days prior to challenge were protected against mean lethal doses (LD(50)) of 5,220 and 1,151 of groups A and B, respectively, and in two separate experiments they were protected against 3,568 and 268 LD(50) of group C. Mice immunized four or more times on successive days prior to challenge were also protected. They demonstrated tolerance and survived a much larger LD(50) challenge than mice immunized by any other immunization schedule. Mice immunized once or twice anytime within 7 days prior to challenge were not protected and thus failed to exhibit nonspecific resistance. Serum antibody probably was responsible for the protection afforded mice immunized 14 and 7 days prior to challenge. However, this was not demonstrated conclusively.