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Parainfluenza type 3 virus (PIV-3) infections cause lower respiratory tract illness in children throughout the world. A licensed PIV-3 vaccine is not yet available.A live attenuated cold-adapted (ca) and temperature-sensitive (ts) PIV-3 vaccine, designated cp-45, was evaluated sequentially in open label studies in 20 adults and in placebo-controlled, double blind studies in 24 PIV-3-seropositive children, 52 PIV-3-seronegative infants and children and 49 infants 1 to 2 months old. A single dose of this intranasal vaccine was evaluated in adults [106 plaque-forming units (pfu)] and seropositive children, and 104 and 105 pfu were evaluated in seronegative children. In the infant study, two 104 pfu doses of vaccine were administered at 1- or 3-month intervals. Safety, infectivity, immunogenicity and phenotypic stability of the vaccine were evaluated in all cohorts.The cp-45 vaccine was well-tolerated in all age groups and infected 94% of vaccinated seronegative children and 94% of vaccinated infants. Although immunization with the first dose of cp-45 diminished the replication of a second dose in all infants, those immunized after 3 months shed vaccine virus more frequently than those immunized after 1 month (62%vs. 24%, respectively). Antibody responses to PIV-3 were readily detected in seronegative children with a variety of assays; however, the IgA response to the viral hemagglutinin-neuraminidase was the best measure of immunogenicity in young infants. Of 109 vaccine virus specimens recovered from nasal washes, 98 were ts and 11 were temperature-sensitive intermediate (tsi) viruses, with pinpoint plaques visible at 40°C. tsi viruses appeared transiently at the time of peak viral replication, represented a very small proportion of the total virus shed and were not associated with changes in clinical status. ca revertants were not detected.The cp-45 vaccine is appropriately attenuated and immunogenic in infants as young as 1 month of age. Further development of this vaccine is warranted.