Administration of time-expired yellow fever vaccine: public health response and results of a serological investigation.

The discovery that a local travel clinic had administered 101 doses of time-expired yellow fever vaccine over a six month period prompted an immediate investigation in order to advise vaccinees about to travel to areas where yellow fever is endemic. No data were available to provide adequate reassurance about the potential efficacy of time-expired vaccine, so a rapid serological investigation was conducted, which provided evidence that the yellow fever vaccine had remained potent beyond its expiry date.