Fifty infants younger than six months, hospitalized for infection with respiratory syncytial virus (RSV), were studied by examination of serial samples of nasal secretion. Secretory neutralizing activity was measured by plaque reduction and secretory antibody by indirect fluorescence using conjugated antiserum to human IgA, IgG, or IgM. Secretory neutralizing activity during infection rose or fell fourfold with approximately equal frequency (20% and 26%, respectively). In contrast, levels of IgA antibody to RSV in secretions rose fourfold in 56%--65% of the infants and fell in none. The frequency of such rises in titer of antibody was directly related to age. In individual secretions the correlation between neutralizing activity and IgA antibody to RSV was poor: neutralizing activity was often found in the absence of detectable antibody, and IgA antibody to RSV was often nonneutralizing. Nevertheless, the development of IgA antibody to RSV correlated in time with the disappearance of virus from the respiratory tract. The timing of this secretory response is consistent with the hypothesis that antibody contributes significantly to cure of infection.

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An enzyme-linked immunosorbent assay (ELISA) was developed for detection of respiratory syncytial virus (RSV) in nasal secretions from infants with respiratory disease. RSV was detected in 23 of 29 secretions positive for RSV by tissue culture and in one of 36 samples negative for RSV by tissue culture. The ELISA was a simple rapid, and surprisingly sensitive test for identification of RSV infection in infants.

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