Cell culture and virus production IVA-Uruguay (H3N2, #07/360, NIBSC, South Mimms, UK) was produced in either adherently growing MDCK (No. 84121903) or Vero (No. 88020401) cells purchased from ECACC (Salisbury, UK) . For cell growth GMEM (Invitrogen, #22100-093, Darmstadt, Germany) was supplemented with 5.5 g/L glucose (Roth, #X997.3, Karlsruhe, Germany), 2 g/L peptone (IDG, #MC33, Lancashire, UK), 10 % FCS (Invitrogen, #10270-106) and 4 mg/mL NaHCO 3 (Roth, #6885.3). Infections were performed in the same medium without addition of FCS but supplemented with trypsin (Invitrogen, #27250-018) at a final concentration of 5 U/mL. Virus was quantified by a hemagglutination assay according to Kalbfuss et al. [1] and is expressed in HAU (log HA/100 µL).