Studies of immunity in typhoid fever. Protection induced by killed oral antigens or by primary infection.

Adult male volunteers were orally vaccinated with two "killed" antityphoid preparations. The recommended doses of both vaccines resulted in serum antibody development in only a few of the subjects. When the dose of the monovalent preparation (Taboral) was doubled, serological responses occurred more frequently, with a rise in O agglutinins in nearly one-fifth of the subjects, in H agglutinins in approximately one-fourth, and in Vi antibodies in nearly half.When vaccinated volunteers were fed virulent typhoid organisms, disease occurred less frequently among those men vaccinated with Taboral at twice the recommended dose (38%) than among those not vaccinated (54%). This preparation did not confer protection at the recommended dose.Volunteers who had previously recovered from an induced typhoid infection received a further challenge with virulent organisms. These persons developed typhoid fever less frequently (23%) than individuals without prior typhoid exposure (30%).