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Monoclonal antibody that imanunoprecipitates this protein neutralizes the virus in vitro, prevents formation of syncytia, and may have some protective effect when administered passively to small animals subsequently inoculated with RSV. The other surface glycoprotein has a molecular weight of 84,000–90,000 and has no known function. Monoclonal antibody to this larger glycoprotein may be neutralizing in the presence of complement. Neither of the glycoproteins has been purified.

Although major portions of DNA complementary to the RSV genome have been cloned (Collins and Wertz, 1983; Venkatesan et al., 1983), it is not entirely clear which of the cloned fragments correspond to the messages for the two surface glycoproteins. Undoubtedly, this information will emerge in the very near future.

Host Immune Response

RSV infection and disease occur in the very young in the presence of maternal IgG, but there is some evidence that infants with high levels of serum antibody are less often infected or severely ill than infants with low levels (Glezen et al., 1981; Parrot et al., 1973). There is also evidence that partial immunity may be conferred by natural infection: adults who have been inoculated with tissue culture grown virus have shown subsequent resistance to reinfection by the same route (Mills et al., 1971).

Most studies suggest, however, that RSV infection recurs at yearly or biennial intervals under natural conditions (Beem, 1967; Henderson et al., 1979). Reinfections are frequently less severe than first infections, but this appears to be a function of increasing age more than immunity (Henderson et al., 1979). Reinfections in the same RSV epidemic probably are rare, however. It seems likely that secretory immunity is more important in protection against reinfection than systemic, although this point cannot be made with certainty.

Disease Burden Estimates

The descriptions and estimates supplied below are based largely on information in Chanock et al. (1982), Denny and Clyde (1983), and Tyeryar (1983), and on advice from Glezen based on his unpublished and published observations (personal communication, 1983; Glezen, 1983; Glezen et al., 1983).

Conditions produced by RSV range from asymptomatic infections to severe life-threatening lower respiratory tract illnesses, such as bronchiolitis and pneumonia. The peak of severe illness is under six months of age, and by two years of age nearly 100 percent of children have been infected. Reinfection occurs with somewhat decreasing severity in older children (Chanock et al., 1982).

For this disease comparison, all clinically significant illness has been assumed to occur in infants and children less than five years of age. The following attack rates, assumed on the basis of advice from

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