The progression of maternal RSV antibodies in the offspring.

The concentrations of maternal anti-RSV IgG antibodies were followed in 49 healthy newborns over the first six months of life. At birth, 41 mothers (83%) tested positive for anti-RSV IgG and all of their babies carried maternal anti-RSV IgG. Anti-RSV IgG positivity dropped to 73% at 1 month, 6% at 3 months, and 2% at 6 months. Between 3 and 6 months, 8% did acquire RSV infection, half of them as acute bronchiolitis and half as non-specific respiratory infection. All of the patients who acquired clinical RSV disease had an antibody concentration of <20 RU/ml which may be the cut off value for protection.