A live attenuated bovine parainfluenza virus type 3 (PIV3), harboring the fusion (F) and hemagglutininneuraminidase (HN) genes of human PIV3, was used as a virus vector to express surface glycoproteins derived from two human pathogens, human metapneumovirus (hMPV) and respiratory syncytial virus (RSV). RSV and hMPV are both paramyxoviruses that cause respiratory disease in young children, the elderly, and immunocompromised individuals. RSV has been known for decades to cause acute lower respiratory tract infections in young children, which often result in hospitalization, while hMPV has only been recently identified as a novel human respiratory pathogen. In this study, the ability of bovine/human PIV3 to express three different foreign transmembrane surface glycoproteins and to induce a protective immune response was evaluated. The RNAdependent RNA polymerase of paramyxoviruses binds to a single site at the 3 end of the viral RNA genome to initiate transcription of viral genes. The genome position of the viral gene determines its level of gene expression. The promoter-proximal gene is transcribed with the highest frequency, and each downstream gene is transcribed less often due to attenuation of transcription at each gene junction. This feature of paramyxoviruses was exploited using the PIV3 vector by inserting the foreign viral genes at the 3 terminus, at position 1 or 2, of the viral RNA genome. These locations were expected to yield high levels of foreign viral protein expression stimulating a protective immune response. The immunogenicity and protection results obtained with a hamster model showed that bovine/human PIV3 can be employed to generate bivalent PIV3/RSV or PIV3/hMPV vaccine candidates that will be further evaluated for safety and efficacy in primates.Despite control of many infectious diseases in the industrialized world, acute viral respiratory tract infections remain a leading cause of illness and reason for hospitalization. Two paramyxoviruses, respiratory syncytial virus (RSV) and human parainfluenza virus type 3 (hPIV3), are the causative agents of acute respiratory diseases of infancy and early childhood, resulting in 20 to 25% of pneumonia and 45 to 50% of bronchiolitis in hospitalized children (8). In addition, a recently identified human metapneumovirus (hMPV) appears to be associated with lower respiratory tract infections in children (25). Preliminary epidemiological reports have estimated an hMPV disease incidence rate of 7 to 10% in young children (5,11,16,18,25). The symptoms of hMPV infections are similar to those caused by RSV and hPIV3, and hospitalizations of young children with acute lower respiratory tract infections are necessary in some cases (17). Recently, Greensill et al. reported the detection of hMPV in bronchoalveolar lavage fluids from 21 of 30 infants (70%) ventilated for RSV bronchiolitis (7).RSV remains one of the most common respiratory pathogens afflicting infants, the elderly, and immunocompromised individuals. Hospitalization and immunoglobulin treatment are of...