Limited humoral and cellular responses to Q fever vaccination in older adults with risk factors for chronic Q fever

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OBJECTIVES: In the Netherlands, people at risk for chronic Q fever were vaccinated against Coxiella burnetii with the inactivated whole cell vaccine Q-vax(R). We aimed to measure the immune responses to C. burnetii six and twelve months after vaccination in this relevant population. METHODS: In 260 vaccinees, antibody responses were assessed by immunofluorescence assay (IFA), complement fixation test and ELISA. The cellular immune responses were assessed by measuring C. burnetii-specific interferon (IFN)-gamma production in blood. Serological results of 200 individuals with past Q fever were used for comparison. RESULTS: At six months, 46% of vaccinees showed low IFA antibody titres and 67% had a positive IFN-gamma assay; At twelve months, both were 60%. In contrast, individuals with a past Q fever were seropositive in 99.5% at six and twelve months, with relatively higher IFA titres. Interestingly, vaccinees with positive IFN-gamma assay pre-vaccination, showed a higher seroconversion rate than IFN-gamma negative vaccinees: 74% vs. 41% (p < 0.001). CONCLUSIONS: The immune response after Q-vax(R) vaccination is lower and restricted to a smaller proportion than found after past Q fever and than previously described after vaccination, suggesting decreased vaccine immunogenicity in this high-risk population. A positive IFN-gamma assay before vaccination in seronegative vaccinees likely points to pre-existing immunity resulting in boosting by vaccination.