Log2-transformed neutralizing antibody titers pre- and 28 days post-vaccination are shown for female (black) and male (white) HCWs to H1N1 in the 2017-18 season (A), H1N1 in the 2018-19 season (B), H3N2 in the 2017-18 season (C), and H3N2 in the 2018-19 season (D). Dashed lines indicate the median titer for each group and dotted lines indicated the 25th and 75th percentiles. Differences between female and male HCWs at each time point for each strain and between pre- and post-vaccination titers were calculated using two-tailed t-tests. Asterisks indicate the significance level with * = p-value <0.05, ** = p-value <0.01, *** = p-value <0.001 and **** = p-value <0.0001.