Thermal Stability of an Oral Killed‐Cholera‐Whole‐Cell Vaccine Containing Recombinant B‐Subunit of Cholera Toxin

An oral killed cholera vaccine containing 1 x 10(11) cells of Vibrio cholerae O1 (heat- or formalin-killed) representing the Ogawa and Inaba biotypes and containing 1 mg of B-subunit of cholera toxin (CTB) produced by recombinant DNA technology (the WC/rCTB vaccine) was subjected to temperatures of 4 C, 30 C or 42 C for up to 6 months time. Lipopolysaccharide antigen (LPS) and CTB content of the vaccine samples determined at various times remained unchanged during the study except for the CTB component which decreased by about 50% after 6 months of storage at 42 C. Immunogenicity determined by immunization of rabbits with the vaccine in Freund's complete adjuvant and measuring anti-LPS and anti-CTB antibody titers in the serum by an ELISA was also found to be unaltered. Lyophilization of the vaccine and storage at room temperature for 7 days also did not have any adverse effect on antigen content or immunogenicity as tested above. There was up to one log reduction in serum antibody titers after immunization without using any adjuvant or using Freund's incomplete adjuvant, and up to two logs following oral immunization. Immunization by oral feeding of the vaccine followed by RITARD challenge with a virulent V. cholerae O1 strain showed evidence of protection against severe or lethal diarrhea. The results suggest that the vaccine retains its antigen content and ability to induce antibodies unchanged when maintained at elevated temperatures for relatively long periods of time.