Respiratory syncytial virus in early infancy. Circulating antibody and the severity of infection.

Forty-one children less than 6 months of age from whom respiratory syncytial virus (RSV) was isolated were divided into groups of varying severity of illness. No relationship was demonstrated between severity of illness and the complement fixation titer to RSV in cord sera, acute sera, or convalescent sera. An increase in IgM level of greater or equal to 15 mg/100 ml in association with the RSV infection was seen more frequently in infants who had higher IgM levels in their acute sera than in infants who had lower levels.