Based on mass spectroscopy of purified viral particles of human respiratory syncytial virus (RSV) A2, the ratio of P protein to M2-1 protein is ~1:1 (Radhakrishnan et al. 2010). This is expected based on the knowledge that this is the ratio in the viral RNA-dependent RNA polymerase (RdRP) complex, where both proteins are present as tetramers. The ratio of the L protein, the catalytic subunit of RdRP, to P protein and M2-1 protein is ~1:3 (Radhakrishnan et al. 2010), close to the expected ratio of 1:4, as the L protein participates as a monomer in the RdRP complex. Slight divergence from the expected ratio may be due to the large size of the L protein (Radhakrishnan et al. 2010). The ratio of P protein and M2-1 protein to N protein is lower than but close to ~1:2 (Radhakrishnan et al. 2010).

Based on the curatorial estimation that viral genomic RNA is packaged around 209 oligomeric rings of N protein, of which 129 are N decamers (10 N subunits) and 80 are N hendecamers (11 N subunits), and taking into consideration the experimentally determined length of RSV genomic RNA (Bose et al. 2015), number of genomic RNA nucleotides bound by each N subunit (Tawar et al. 2009), and the ratio of N decamers to N hendecamers in RSV nucleocapsids (Tran et al. 2007), the total number of N proteins in a viral nucleocapsid is expected to be 2170. If each N oligomeric ring binds one RdRP complex, this would result in the total number of 832 P proteins, 832 M2-1 proteins, and 209 L proteins in a viral particle. This is close to the ratios observed by mass spectrometry analysis.

Virion assembly occurs in cytoplasmic inclusion bodies localized in the vicinity of the plasma membrane region where virus filaments, which represent budding virions, form. Each infected cell usually contains several viral inclusion bodies of irregular shape and varying sizes (Radhakrishnan et al. 2010).