Seasonal influenza vaccine effectiveness in pre- and full-term children aged 6-23 months over multiple seasons

Abstract

Introduction: This study aimed to determine the effectiveness of seasonal influenza vaccine in pre- and full-term children aged 6-23 months.

Methods: We examined a cohort of 683,354 young children (7.7% preterm) over five influenza seasons (2004-2005 to 2008-2009) in Ontario, Canada. Vaccine effectiveness was estimated using influenza-coded ambulatory visits during virologically-confirmed influenza season periods as the outcome and multivariable Cox proportional hazards modeling.

Results: Full vaccination was associated with a 19% reduction in influenza-coded ambulatory visits (HR=0.81; 95% CI, 0.68-0.97) in all children, and an 18% reduction in full-term children (HR=0.82; 95% CI, 0.68-0.99). We did not find significant vaccine effectiveness for preterm children. No benefit was found for partial vaccination.

Conclusions: In children younger than two years, only full influenza vaccination is associated with reduced influenza-coded ambulatory visits. Since the effectiveness of influenza vaccination in preterm children remains uncertain, further study of this highly vulnerable population is warranted.