Effectiveness of palivizumab prophylaxis in infants and children in Florida.

PURPOSE: Palivizumab effectiveness data on respiratory syncytial virus (RSV) infections are limited to trial settings and vary considerably between selected high-risk populations. This study aimed to evaluate effectiveness in a community-based sample.

METHODS: We conducted a cohort study of children with ≥ 3 months Florida Medicaid fee-for-service eligibility between 1998 and 2004 who also had matching birth certificates. Children entered the cohort at the beginning of the RSV season, after a minimum of 60 days in ambulatory care, and were followed until the earliest of the following: season end, second birthday, loss of eligibility, hospitalization, or death. Study endpoint was the first RSV-related hospitalization. To evaluate the presence of confounding, a second endpoint, hospitalizations for pneumonia or bronchiolitis secondary to specified bacterial or viral pathogens other than RSV, was used. Palivizumab exposure defined as first use (day 1-30 of first dose), subsequent use (days 1-30 of each subsequent dose), and former use (days 31-60 after any dose if delays or no readministration occurred) was compared with non-use with a Cox regression model, adjusting for confounders.

RESULTS: Hazard ratios (HRs) for RSV hospitalizations were 0.89 (95%CI, 0.71-1.12), 0.56 (95%CI, 0.46-0.69), and 0.71 (95%CI, 0.51-0.97) for first, subsequent, and former use, respectively. HRs for hospitalization because of non-RSV infections were 1.31 (95%CI, 1.04-1.65), 1.03 (95%CI, 0.86-1.23), and 1.05 (95%CI, 0.78-1.41), indicating residual confounding for first but not for subsequent and former use.

CONCLUSION: In this community-based study, palivizumab was associated with a reduction in severe RSV infections of a magnitude comparable to the lower clinical trial efficacy estimates. Protection appears to extend beyond the currently recommended monthly dosing schedule.