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A safe and effective parainfluenza type 3 (PIV-3) virus vaccine is needed to prevent serious PIV-3-associated illness in infants younger than 6 months of age. In previous studies a live bovine PIV-3 (BPIV-3) vaccine, which was developed to prevent human PIV-3 (HPIV-3) disease, was shown to be safe, infectious, immunogenic and phenotypically stable in 6- to 36-month-old infants and children.The safety, infectivity and immunogenicity of a single dose of the BPIV-3 vaccine was evaluated in a randomized, placebo-controlled, double blinded trial in 19 infants 2 to 5.9 months of age and in 11 additional 6- to 36-month-old subjects.The BPIV-3 vaccine was well-tolerated in both age groups and infected 92% of those younger than 6 months and 89% of those older than 6 months of age. Serum hemagglutination-inhibition (HAI) antibody responses to HPIV-3 and to BPIV-3, respectively, were detected in 42 and 67% of the younger infants, compared with 70 and 85% of the older subjects. In the younger infants we analyzed the rate of antibody response by titer of maternally acquired antibodies; low titer was defined as a preimmunization serum HAI titer <1:8 and high titer was defined as a preimmunization serum HAI titer ≥1:8. Young infants with a low titer of maternally acquired antibodies were significantly more likely to respond to the BPIV-3 vaccine than those with a high titer (89% vs. none for serum HAI response to BPIV-3; P = 0.02, Fisher's exact test).This study demonstrated that the BPIV-3 vaccine was safe and infectious in infants younger than 6 months of age and was also immunogenic in the majority of these young infants. Additional studies are needed to determine whether two or more doses will enhance the immunogenicity of the BPIV-3 vaccine in young infants and to assess its safety and immunogenicity when given simultaneously with routine childhood immunizations.