Evaluation in volunteers of a candidate live oral attenuated Salmonella typhi vector vaccine

Candidate vector vaccine strain CVD 906 (aroC- and aroD- derivative of virulent Salmonella typhi strain ISP1820) was evaluated in phase 1 clinical trials. The first nine volunteers ingested a single dose of 5 x 10(7) CVD 906 bacilli. At this dose CVD 906 stimulates remarkable systemic and mucosal immune responses, inasmuch as 89% of volunteers developed marked serum antibody levels to S. typhi antigens and high numbers of antigen-specific gut-derived antibody-secreting cells. Four (44%) volunteers developed asymptomatic vaccinemia 4-10 d after immunization and all volunteers excreted CVD 906 on at least one occasion. However, two volunteers developed febrile adverse reactions, one on the day of vaccination and the other on day 4. Of 11 volunteers who ingested a single dose of 5 x 10(3) CVD 906 bacilli, none displayed side effects but 27% developed significant serum responses to S. typhi LPS. In vitro, CVD 906 replicates for only nine generations in pooled human serum, indicating that CVD 906 growth is limited in this physiologically relevant medium. In phorbol myristate acetate-induced U937 human macrophage-like cells, CVD 906 replicates intracellularly to a lesser extent than parent strain ISP1820. Although, strain CVD 906 is attenuated and highly immunogenic, the occasional febrile reactions at high doses indicate that further attenuation of this strain is necessary.