The Q fever skin test is used to measure cell-mediated immunity to Coxiella burnetii in pre-vaccination screening to exclude individuals with pre-existing immunity. We investigated whether this in-vivo test influences subsequent measurements of immune response.

We assessed the humoral and cellular immune responses before, and 6 and 12 months after skin testing in 63 individuals who were not vaccinated because of either a positive skin test or positive serology in screening. IgG anti-C. burnetii antibodies were measured using immune-fluorescence assay (IFA). The cellular immune response was assessed by measuring in-vitro C. burnetii-specific interferon (IFN)-γ production in blood.

Of the 35 subjects with a positive skin test and negative serology, 15/35 (43%) showed seroconversion at 6 months, and 7/32 (22%) seropositivity at 12 months. The mean ± SE specific IFN-γ production in this group increased from 185 ± 88 pg/mL (at baseline) to 422 ± 141 pg/mL at 6 months (P = 0.009) and 223 ± 91 pg/mL at 12 months (P = 0.17). Of the 28 subjects with positive serology (and unknown skin test results), 21/28 (75%) showed an increase in IgG anti-phase I titres at 6 months, and 11/25 (44%) at 12 months. The mean ± SE specific IFN-γ production was significantly increased at 6 months, but not at 12 months.

Q fever skin testing causes higher antibody titres and higher in-vitro IFN-γ to C. burnetii, and therefore affects subsequent Q fever diagnostics.