Impact of the Withdrawal of Palivizumab Immunoprophylaxis on the Incidence of Respiratory Syncytial Virus (RSV) Hospitalizations Among Infants Born at 33 to 35 Weeks' Gestational Age in the Province of Quebec, Canada: The RSV-Quebec Study

Background. Infants born at 33-35 completed weeks' gestational age (wGA) aged <6 months at the start of or born during respiratory syncytial virus (RSV) season and classified as moderate/high risk of severe RSV disease were included in a palivizumab RSV prophylaxis program in the province of Quebec, Canada, until 2014-2015. We assessed the impact of withdrawal of this indication on lower respiratory tract infection (LRTI)/RSV hospitalizations (H) in this population. Methods. We conducted a 4-year, retrospective, cohort study in 25 Quebec hospitals (2 seasons with and 2 without palivizumab prophylaxis for moderate- to high-risk infants). Our primary outcome was LRTI/RSV-H incidence. We compared LRTI/RSV-H incidence before (2013-2015; seasons 1 + 2 [S1/2]) and after (2015-2017; S3/4) the change in indication. Results. We identified 6457 33-35 wGA births. LRTI/RSV-H occurred in 105/3353 infants (3.13%) in S1/2 and 130/3104 (4.19%) in S3/4. Among LRTI/RSV-H, 86.4% were laboratory-confirmed RSV-H. Adjusting for sex, wGA, and birth month, S3/4 was significantly associated with increased LRTI/RSV-H incidence (adjusted odds ratio [aOR], 1.36; 95% confidence interval [CI], 1.04-1.76) but not with laboratory-confirmed RSV-H (aOR, 1.19; 95% CI, 0.90-1.58). Mean duration of LRTI/RSV-H was 5.6 days; 22.6% required intensive care unit admission. Comparing S3/4 with S1/2, infant percentage with LRTI/RSV-H classified as moderate/high risk increased from 27.8% to 41.9% (P = .11). Conclusions. In a province-wide study, we observed a significant increase in LRTI/RSV-H incidence among infants born at 33-35 wGA in the 2 years after withdrawal of RSV prophylaxis.