Effectiveness of palivizumab immunoprophylaxis to prevent respiratory syncytial virus hospitalizations in healthy full-term <6-month-old infants from the circumpolar region of Nunavik, Quebec, Canada.

In Quebec, Canada, eligibility for palivizumab (PVZ) immunoprophylaxis was expanded in fall 2016 to include healthy-full-term (HFT) infants residing in the circumpolar region of Nunavik and aged <3 months at the start of the RSV season or born during the season. This study assessed the effectiveness of PVZ to prevent RSV hospitalizations in these infants during the 3 seasons following its implementation. Medical and laboratory records of <1-year-old infants (375 average annual birth cohort) admitted to regional and tertiary hospitals with respiratory infection during 6 years were reviewed. Individual pharmacy data and birth registries were used to estimate adherence to PVZ and direct PVZ effectiveness in 0-5-month-old HFT infants by comparing the incidence of RSV hospitalizations 1) in protected and unprotected infants, and 2) during PVZ-protected and unprotected days. Over six seasons, the RSV hospitalization rate was 50.2/1000 (72.6/1000 adjusted for underdetection) in <1-year-old infants. PVZ was administered to 73% (469) of eligible HFT infants; 37% (237) received all recommended doses. Overall for the three RSV seasons the incidence of RSV hospitalization in PVZ-protected infants was similar to PVZ-unprotected infants, resulting in PVZ direct effectiveness of -6.7% (95% CI -174.8%, 85.6%). The incidence of RSV hospitalization during PVZ-protected and during PVZ-unprotected days was also similar, resulting in PVZ direct effectiveness of -3.8% (CI -167.6%, 64.9%). Over three RSV seasons, there was no evidence that PVZ reduced RSV hospitalizations in HFT Nunavik infants. In addition, the sub-optimal adherence to the recommended PVZ administration schedule suggests feasibility and acceptability issues.