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

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Abstract
The effect of vaccination schedule on the immune response of M. 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 104 plaque-forming units of yolk sac-grown R. rickettsii were assessed. Seroconversion was noted in all monkeys after the 1st dose of vaccine. A 2nd dose administered 8 or 15 days after the primary infection, or a 3rd given 7 or 30 days after the 2nd, produced no long-term effect on antibody titer. Only monkeys given 2 doses of vaccine at a 30-day interval showed an increase in antibody titer during the period before challenge. Vaccination with 1, 2 or 3 doses of CEC vaccine prevented development of rash and rickettsemia after challenge. The 2-dose schedules appeared to induce the highest degree of resistance to challenge, as indicated by unaltered hematological parameters and body temperature in monkeys. The 1- and 3-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. Administration of 2 doses of CEC vaccine at 15- or 30-day intervals is the immunization schedule of choice.