Human influenza virus A/WSN/33 (H1N1) London was obtained from the strain collection of the Institute of Medical Microbiology, University Greifswald, Germany, and propagated in embryonated hen eggs for 72 h. The infected allantoic fluids were harvested, the hemagglutination (HA) titer and virus infectivity were determined on MDCK cells and the virus stock was stored at À70 C.