To the Editor. A humanized monoclonal antibody designed for the prevention of serious disease attributable to respiratory syncytial virus (RSV) has recently been approved by the Food and Drug Administration (FDA). Palivizumab, manufactured by MedImmune, Inc (Gaithersburg, MD) and marketed as Synagis, is indicated for pediatric patients who are at high risk for severe infection with RSV. Because it is highly concentrated, it can be administered as a single monthly intramuscular injection, a significant advantage over RSV immune globulin, which requires intravenous access and is logistically challenging to administer. The results of the IMpact-RSV study,1 which provided the basis for FDA approval, included the finding that use of palivizumab was associated with a 55% reduction in the need for hospitalization attributable to RSV, compared with placebo, among high-risk children. The thoughtful commentary by Dr Storch2 included an analysis of costs associated with preventing infection in high-risk children using either RSV immune globulin or palivizumab. It states that 16 patients will have to be treated to prevent one RSV-related hospitalization. Reference is made to another analysis3that calculated that the number needed to treat (NNT) in the PREVENT trial of RSV immune globulin4 was 18. An important point that was not considered in either of these analyses was that there may have been a substantial placebo effect in both the Impact-RSV and PREVENT studies. The NIAID trial5 of RSV immune globulin demonstrated a 20.2% rate of hospitalization among control children who were at high risk, but these children were not given placebo or treated as such in the trial. Previous studies6,,7 have suggested even higher rates of hospitalization for high-risk children who were not involved in clinical trials. In the PREVENT study, 13.5% of placebo recipients were hospitalized because of RSV infection and …