Influenza vaccines effectiveness 2013-14 through 2015-16, a test-negative study in children.

Background: Trivalent inactivated and live attenuated influenza vaccines (IIV3 and LAIV3) have been reformulated with an extra B strain (IIV4 and LAIV4). They were licensed based on immunogenicity and their effectiveness (VE) still must be empirically tested.

Methods: Children 1-17years tested for influenza during 2013-16 were included and their immunization status verified. They were considered vaccinated if received ≥1 dose of an influenza vaccine ≥10days before evaluated for a respiratory episode. Age-groups were classified as 1-4years or 5-17years. VE was estimated by comparing vaccination status of influenza-positive versus influenza-negative cases.

Results: 6779 children were enrolled in the three seasons. Overall, 27.2% received an influenza vaccine (87.1% IIV3 or IIV4 and 12.9% LAIV4), and 15.6% tested positive for influenza (77.9% A). IIV3 was predominantly used in 2013-14 and IIV4 in 2014-15 and 2015-16. IIV3 and IIV4 had comparable VE over the three seasons (60%, 57% and 53%) and performed similarly against influenza A and B and both age-groups. LAIV4 performed poorly for influenza A (15%, 37% and 48%) but better for influenza B (100%, 56% and 100%), especially among children 5-17years of age with VE=100% (95%CI: 55, 100).

Conclusions: Influenza vaccination showed modest but consistent effectiveness over the years. The switch from IIV3 to IIV4 did not affect VE. LAIV4 did not perform as well as IIVs, yet it improved over the years and was particularly good protecting older children against influenza B. These results emphasize the regional nature of influenza and the need for local surveillance.