RSV is a common pathogen, infecting nearly all children by 6 years of age (1). Although most infections are mild, the ubiquitous nature of the virus means that the total number of serious cases is still large. In the United States, RSV is thought to be responsible for up to 50% of admissions for bronchiolitis and 25% of admissions for pneumonia among pediatric patients (2). Serious RSV infections tend to occur in infants younger than 6 months; children with underlying pulmonary or cardiac disease; and children with chronic or transient immunodeficiency (3-5). However, the most serious illness and the highest mortality rates are found in posttransplant patients (both adult and pediatric) and in those undergoing chemotherapy for leukemia; mortality rates in this population exceed 50% even in those treated with ribavirin (6). Thus, the morbidity and mortality from RSV infections, together with the lack of an effective vaccine, have led to the use of respiratory syncytial virus immunoglobulin (RSVIG) for immunoprophylaxis in high-risk populations. Randomized trials in various pediatric populations at risk have shown reductions in hospitalizations, hospital days, and intensive care unit days among the groups receiving the highest dose of RSVIG (7).
However, the strategy of immunoprophylaxis with RSVIG-enhanced immune globulin has flaws. The high volume of immunoglobulin that must be administered carries risks, as well as the need for prolonged infusion in a monitored setting. The high cost of the infusions is also prohibitive. For these reasons, a variety of monoclonal antibodies to RSV have been developed that can be administered in small volumes by intramuscular injection. Of these, palivizumab (Medlmmune, Gaithersburg, MD) has been shown to have efficacy in the prevention of RSV-associated disease in at-risk children. Palivizumab is a humanized immunoglobulin G-1 with affinity for the F protein of RSV and has good activity against clinical isolates of both type A and type B RSV. In in vitro neutralization assays, palivizumab is 20 times more potent than RSVIG (8).
A clinical trial conducted over the 1996-1997 RSV season showed efficacy of this antibody in RSV disease in high-risk pediatric populations. At 139 centers in the United States, United Kingdom, and Canada, 1502 children with either prematurity or bronchopulmonary dysplasia were enrolled and randomized to receive five injections of either palivizumab at 15 mg/kg or placebo. Palivizumab prophylaxis resulted in a 55% reduction in hospitalization attributable to RSV infection. The benefit was greatest in children with prematurity alone as a risk factor, compared with children who had bronchopulmonary dysplasia. The incidence of adverse events was similar in the treatment and placebo groups (9). Not surprisingly, the monoclonal antibody to RSV did not seem to have the same protective effect with regard to otitis media and respiratory infections other than RSV that the pooled RSVIG provides, since the latter probably contains antibodies to multiple pathogens. Current guidelines for pediatric prophylaxis with palivizumab or RSVIG are in Table 1 (10). Prophylaxis should be administered from the beginning to the end of RSV season.
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