The objectives of this study were to investigate the prevalence of respiratory syncytial virus (
RSV) infections in Bulgaria, to characterize the genetic diversity of the RSV strains, and to perform amino acid sequence analysis of the RSV
G protein. Clinical, epidemiological data and nasopharyngeal swabs were prospectively collected from children aged less than 5 years presenting with acute
respiratory infections from October 2016 to September 2018. Real-time polymerase chain reaction for 12 respiratory viruses, and sequencing, phylogenetic, and
amino acid analyses of the RSV G gene/
protein were performed. Of the 875 children examined, 645 (73.7%) were positive for at least one viral respiratory pathogen. RSV was the most commonly detected virus (26.2%), followed by rhinoviruses (15%),
influenza A (H3N2) (9.7%), adenoviruses (9%), bocaviruses (7.2%), human metapneumovirus (6.1%),
parainfluenza viruses 1/2/3 (5.8%),
influenza type B (5.5%), and A(H1N1)pdm09 (3.4%). The detection rate for RSV varied across two winter seasons (36.7% vs 20.3%). RSV-B cases outnumbered those of the RSV-A throughout the study period. RSV was the most common virus detected in patients with
bronchiolitis (45.1%) and
pneumonia (24%). Phylogenetic analysis indicated that all the sequenced RSV-A strains belonged to the ON1 genotype and the RSV-B strains were classified as BA9 genotype. Amino acid substitutions at 15 and 22 positions of the HVR-2 were identified compared with the ON1 and BA prototype strains, respectively. This study revealed the leading role of RSV as a causative agent of serious respiratory illnesses in early childhood, year-on-year fluctuations in RSV incidence, the dominance of RSV-B, and relatively low genetic diversity in the circulating RSV strains.