Natural infection of infants with respiratory syncytial virus subgroups A and B: a study of frequency, disease severity, and viral load.

Heterogeneity in respiratory syncytial virus (RSV) disease severity likely is due to a combination of host and viral factors. Infection with RSV subgroup A is thought to produce more severe disease than RSV-B. Higher RSV loads correlate with greater disease severity in hospitalized infants. Whether subgroup-specific variations in disease severity result from differences in RSV load has not been studied. A total of 102 RSV-hospitalized infants <2 y of age were studied. Nasal washes were collected in a standardized manner and were cultured in <3 h in parallel with an RSV quantitative standard in a HEp-2 plaque assay. RSV-A (72%) was more frequent than RSV-B. Disease severity risk factors were similar between subgroups. RSV loads were similar between A and B subgroups (4.77 versus 4.68 log PFU/mL). Measures of disease severity were also similar between subgroups.