L., C. foreign viral genes at the 3 terminus, at position 1 or 2 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 CPI 4203 viral respiratory tract infections remain a leading cause of illness and reason for hospitalization. Two paramyxoviruses, respiratory syncytial virus (RSV) and human parainfluenza TAN1 virus type 3 (hPIV3), are the causative brokers of acute respiratory diseases of infancy and early childhood, resulting in 20 to CPI 4203 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 often necessary to alleviate serious RSV infections. For decades, approaches to generate an RSV vaccine employing virus subunits or inactivated virus vaccines have failed either due to lack of immunogenicity or the potential for causing enhanced pulmonary disease upon reinfection with naturally occurring wild-type RSV. The advent of reverse genetics for negative-strand CPI 4203 RNA viruses resulted in a plethora of genetically designed RSV vaccine candidates that harbor mutations in essential RSV genes or deletions of nonessential RSV genes CPI 4203 in an effort to attenuate virus replication without compromising immunogenicity (3). The results from these studies clearly demonstrated that an RSV vaccine candidate must be properly fine tuned to balance immunogenicity and attenuation for administration to 1- to 2-month-old infants. If the RSV candidate vaccine is usually insufficiently attenuated, the children will show signs of disease. If the vaccine virus does not replicate enough, the level of immunogenicity elicited in the immunized children will be low and ineffective (4). Two subgroups of RSV (A and B) circulate at any given time. The F proteins of the two RSV subgroups are highly conserved (98% amino acid identity), unlike the RSV attachment glycoproteins (G), which display only 53% identity at the amino acid level. hPIV3 is the causative agent of croup, an acute lower respiratory disease most often observed in young children (6, 8, 29). Two live attenuated hPIV3 vaccine candidates have been evaluated so far in human clinical trials. A live attenuated bovine PIV3, a Jennerian-type vaccine, was administered to children between 2 and 6 months of age in human clinical trials and was shown to be safe and immunogenic (12, 14). A cold-passaged hPIV3 strain that displayed a temperature-sensitive phenotype (cpts-hPIV3) was also shown to be immunogenic in human clinical trials (13, 23). hMPV causes clinical symptoms similar to those of RSV, ranging from moderate respiratory problems to severe cough, CPI 4203 bronchiolitis, and pneumonia (2, 25). Two subgroups of hMPV (clades A and B) were identified based on sequence comparison of the F and G genes derived from a number of different clinical isolates. The F genes of clades A and B are highly conserved and display 95% identity at the amino acid level. In contrast, the hMPV G proteins are variable and.