Vaccine prophylaxis of Q fever. A follow-up study of the efficacy of Q-Vax (CSL) 1985-1990.

OBJECTIVES: To examine the efficacy of various batches of a formalin-inactivated whole cell Coxiella burnetti vaccine (Henzerling strain, Phase 1 [Q-Vax, CSL]) in the prevention of Q fever among abattoir workers. DESIGN AND SETTING: The study was a retrospective cohort survey of all employees at three South Australian abattoirs to determine the incidence of Q fever among vaccinated and unvaccinated employees during the period 1985 to 1990. RESULTS: There were two cases of Q fever among 2555 vaccinated employees of the three abattoirs, compared with 55 cases among 1365 unvaccinated employees. The two Q fever cases in vaccinated employees were within a few days of vaccination, before immunity had developed, and represented a coincidence of natural infection and vaccination. Protective efficacy was 100%, even with a batch of Q-Vax containing 20 micrograms/dose rather than the standard dose of 30 micrograms/dose. CONCLUSIONS: Vaccination was effective for at least five years, although it was uncertain whether this was due to the vaccine per se or to a combination of vaccine immunity reinforced by periodic natural exposure.