Effect of vaccination schedule on immune response of Macaca mulatta to cell culture-grown Rocky Mountain spotted fever vaccine.

The effect of vaccination schedule on the immune response of Macaca mulatta to formalin-inactivated chicken embryo cell culture (CEC)-grown Rickettsia rickettsii vaccine was studied. Schedules consisted of inoculation on day 1 only, on days 1 and 15, on days 1 and 30, on days 1, 8, and 15, or on days 1, 15, and 45. Humoral antibody measured by microagglutination and indirect immunofluorescence and resistance to challenge with 10(4) plaque-forming units of yolk sac-grown R. rickettsii were assessed. Seroconversion was noted in all monkeys after the first dose of vaccine. A second dose administered 8 or 15 days after the primary infection, or a third given 7 or 30 days after the second, produced no long-term effect on antibody titer. Only monkeys given two doses of vaccine at a 30-day interval showed an increase in antibody titer during the period before challenge. Vaccination with one, two, or three doses of CEC vaccine prevented development of rash and rickettsemia after challenge. The two-dose schedules appeared to induce the highest degree of resistance to challenge, as indicated by unaltered hematological parameters and body temperature in monkeys. The one- and three-dose schedules were somewhat less effective, in that some challenged monkeys within each group displayed febrile and leukocyte responses associated with Rocky Mountain spotted fever infection. Our data suggest that administration of two doses of CEC vaccine at 15- or 30-day intervals is the immunization schedule of choice.