Rift Valley fever virus vaccine trial: study of neutralizing antibody response in humans.

The serological response to immunization with a formalin inactivated Rift Valley fever (RVF) vaccine was studied in 963 Swedish UN soldiers serving in the Sinai peninsula. Antibody titres were determined with a plaque reduction neutralization test (PRNT). Attempts were made to give all soldiers three injections (1 ml s.c. days 0, 7-10 and 28-30) but 128 soldiers received only two injections. In a group of 51 fully vaccinated individuals, repeated blood samples were collected. Fifty of the vaccinees seroconverted. Serum collected six weeks after the first vaccination revealed the highest antibody titres. The geometric mean titre then decreased rapidly during the following two weeks. Six months after vaccination sera were collected from 433 vaccinees who had received three injections and 379 had antibodies detectable by PRNT (88% PRNT greater than or equal to 10). The corresponding figures one and two years after vaccination were 223 seropositives out of 255 (91% PRNT greater than or equal to 10) and 91 out of 123 (74% PRNT greater than or equal to 10), respectively. Multiple stepwise regression showed that three injections gave a better antibody response than two injections. This analysis also showed that the magnitude of the antibody response was reduced with increasing age. Slight, local and general side effects were reported in 6% of the vaccinees and these reactions occurred in individuals with relatively higher antibody response.