Vaccines against
parainfluenza (PIV) and respiratory syncytial viruses (RSV) that are currently being developed include both live and
subunit vaccines. Candidate live PIV
vaccines that have been found to be attenuated and efficacious in rodents or primate models are (1) cold-adapted, temperature-sensitive mutants of PIV-type 3 that have been serially passaged at low temperature (20 degrees C) in simian kidney tissue culture; (2)
protease-activation mutants (PIV-1-Sendai), which have mutations that decrease the cleavability of their F
glycoprotein by host cell
protease; (3) an animal virus, bovine PIV-3 virus, which is antigenically related to the human PIV-3 virus, and (4)
vaccinia recombinant viruses bearing RSV or PIV-3
glycoproteins. Subunit RSV and PIV-3 viruses are being produced and evaluated as immunogens. A major concern with these
vaccines is the possibility of disease potentiation following
virus infection as occurred previously with
formalin-inactivated
measles and
RSV vaccines. Studies indicate that PIV-3 and RSV
glycoprotein vaccines are immunogenic and efficacious in animals but insufficient data exist to estimate their capacity to potentiate disease. However, since a cotton rat model is available to detect potentiated disease resulting from
infection of cotton rats previously immunized with
formalin-inactivated
RSV vaccine, it is now possible to systematically evaluate new
vaccines in experimental animals for disease potentiation before studies are initiated in humans. It is likely within the next several years that one or more of these PIV or
RSV vaccines will be tested in humans for safety and immunogenicity.