DDX41 and influenza: To understand the reactivity of different HA domains to serum Abs, purified rH1 and rHA1 of pH1N1 virus A/California/07/2009 (CA/07) as well as rHA2 of pH1N1 virus A/Michigan/45/2015 (MI/45) were tested for their binding to 20 paired human sera before and after vaccination with the 2010–2011 inactivated egg-grown trivalent influenza vaccines (TIVs) containing the H1 component of pH1N1 A/California/07/2009-PR8-like virus.