Attenuated strain of a prokaryotic microorganism, being an attenuated strain of Salmonella suitable for use in a live vaccine for administration to a human or animal, in which the microorganism is transformed with nucleic acid encoding Human papilloma virus 16 L1 major capsid protein. The protein assembles in the microorganism to form virus-like particles (VLPs) that are capable of illiciting an immune response sufficient to produce neutralizing antibodies in serum and genital secretions specific for Human papilloma 16 infection when the microorganism is administered to the human or animal body.