A Rift Valley fever vaccine trial: 2. Serological response to booster doses with a comparison of intradermal versus subcutaneous injection.

A booster dose of a formalin-inactivated, cell culture-propagated Rift Valley fever vaccine (TSI-GSD-200) was administered without significant reactogenicity to 60 volunteers 18 months after they received a three dose primary series. Blood was drawn for serological testing prior to boosting and 10, 49, and 180 days thereafter. Neutralizing antibody response was prompt, being maximum on day 10 and considerably higher than the peak titres obtained on day 42 after the initial series. A marked decline in antibody titres was evident by day 180, as seen for the primary series, but residual titres were substantially higher. The initial series was given in doses of 1.0, 0.5, 0.3, or 0.1 ml and the immune status achieved with these injections was a major determinant of the response to boostering. Secondary responses were highly dependent in magnitude and duration on the prior immune status of the recipient. Evaluation of dose and route of recall inoculation showed 1.0 ml given subcutaneously (SC) to be equivalent to 0.1 ml administered intradermally (ID), though inference should be guarded due to the small sample size. Although 0.1 ml SC dose was inferior to either, it was modestly effective. Intradermal administration of vaccine may be useful for extending the small reserve of vaccine available for use in emergency.