The effectiveness of vaccines for seasonal influenza (A/H3N2) is highly variable and has been less than 10% in some years (teal bars). The H3N2 vaccine strain is shown for seasons when it was changed from the previous season. Black stars indicate seasons where the vaccine strain mismatched circulating H3N2 viruses (https://www.cdc.gov/flu/season/past-flu-seasons.htm); yellow stars indicate seasons in which H3N2 vaccine strains acquired mutations during passage in eggs. Research advances are listed at the top of the figures and are color coded as follows: surveillance in red; experimental approaches in blue; sequencing approaches in purple; computational approaches in green. Advances in understanding the structure of hemagglutinin (Knossow et al., 1984; Wiley and Skehel, 1987) and predicting the evolution of H3 (Bush et al., 1999) occurred before the period shown in the figure. Point estimates of vaccine effectiveness are taken from the following references: Skowronski et al., 2005 (04/05); Skowronski et al., 2007 (05/06); Skowronski et al., 2009 (06/07); Belongia et al., 2011 (07/08); Skowronski et al., 2010 (08/09); Treanor et al., 2012 (10/11); Ohmit et al., 2014 (11/12); McLean et al., 2015 (12/13);; Gaglani et al., 2016 (13/14); Flannery et al., 2016 (14/15); Jackson et al., 2017 (15/16); Flannery et al., 2019 (16/17); Rolfes et al., 2019 (17/18); Flannery et al., 2020 (18/19); estimates were not available during the 2009/10 A/H1N1 pandemic.