Humoral immunity induced by a live Francisella tularensis vaccine. Complement fixing antibodies determined by an enzyme-linked immunosorbent assay (CF-ELISA).

A complement-fixing enzyme-linked immunosorbent assay (CF-ELISA) with bacterial sonicate as the antigen, human AB serum as the source of complement and alkaline-phosphatase-labelled anti-(human C3c) as the conjugate was developed for detecting antibodies to Francisella tularensis. Humoral responses after inoculation with a live attenuated tularemia vaccine were studied in 13 subjects by this method. Twelve responded with CF antibodies, which in most cases appeared 4 weeks after vaccination, the response time varying from 1 week to 2 months. The highest individual CF titres, of 140-9990, were reached 1-4 months after vaccination (average 2.4 months). The CF titres tended to decrease with time, and at 10 months two of the 12 subjects with an earlier CF positive response were negative again. CF antibodies were nevertheless still detectable in all three subjects studied 1.5 years after vaccination.