Background: Respiratory Syncytial Virus (RSV) causes considerable paediatric morbidity. Prevention is limited to passive immunisation with monthly doses of Palivizumab for high risk infants fulfilling certain eligibility criteria.
Aim: To determine if monthly Palivizumab prophylaxis on the relative incidence of RSV in infants graduating from Neonatal Intensive Care Units (NICU) using a self-controlled case series approach.
Methods: Palivizumab dispensing, routine respiratory viral testing, perinatal and death data were probabilistically linked for a cohort of NICU births, 2002-2013. To account for non-independence of recurring events, cases were restricted to their first RSV detection before age 2 years. The exposure (benefit) period for each Palivizumab dose was 0-28 days from the dispensing date. RSV relative incidence in benefit periods versus control periods was estimated using conditional Poisson regression.
Results: The case-only dataset consisted of 1505 RSV positive cases with 37 (2.5%) having at least 1 Palivizumab dose (62% had ≤3 doses). RSV incidence was 69% reduced with 1 dose (IRR 0.31, 95% CI: 0.11, 0.87) and 43% reduced with 2 doses (IRR 0.57; 95% CI: 0.13,2.41). No beneficial effect was seen with ≥3 doses.
Conclusions: This is the first time a self-controlled case series has been used to determine beneficial effects of a time-dependent exposure on a disease outcome. Controlling for individual-level confounding factors is an advantage of this method and results indicate a beneficial effect of 1-2 Palivizumab doses. Subsequent analyses will explore the impact of Palivizumab eligibility throughout the RSV season and the effect within the first 12 months.