• respiratory syncytial virus;
  • cohort studies;
  • infant;
  • palivizumab;
  • prophylaxis



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.


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.


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.


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. Copyright © 2011 John Wiley & Sons, Ltd.