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The phoP/phoQ virulence regulatory genes of Salmonella typhi Ty2 were deleted, and the resultant strain (Ty800) was tested as a live attenuated typhoid fever vaccine in human volunteers. Groups of 2 or 3 subjects received single oral doses of 10(7), 10(8), 10(9), or 10(10) cfu. Two volunteers who received the largest dose had self-limited side effects; no bacteremias were detected. Ten of 11 subjects had evidence of intestinal immune responses to the vaccine as measured by increases in S. typhi lipopolysaccharide-specific IgA-secreting cells in peripheral blood samples. Humoral immune responses were measured and compared with those of control vaccinees who received 4 oral doses of S. typhi Ty21a. In the most sensitive assays, 9 of 11 volunteers and 5 of 8 Ty21a control vaccinees had evidence of IgG directed against S. typhi antigens. Ty800 is safe, and single oral doses are highly immunogenic in humans.
Hohmann et al. (Sat,) studied this question.