Gaglani M, Vasudevan A, Raiyani C, et al. - Researchers examined vaccine effectiveness of quadrivalent vs trivalent inactivated vaccines (IIV4 vs IIV3) against illness due to influenza B during the transition when IIV4 use increased rapidly. Analysis of 25,019 of 42,600 outpatients (aged ≥ 6 months) enrolled within 7 days of illness-onset during six seasons was done from 2011-2012 by the US Influenza Vaccine Effectiveness (Flu VE) Network. Influenza virus type and B-lineage were tested in upper respiratory specimens. The trivalent vaccines comprised the B lineage predominantly circulating in four of six seasons. During four influenza seasons, wide use of both IIV4 and IIV3 was observed. Outcomes revealed no increased protection against any influenza B illness in association with the uptake of quadrivalent inactivated influenza vaccines, despite these being more effective against the added B virus lineage. Findings emphasize the necessity for assessing public health impact and analyzing cost-benefit globally.