MN assays was performed as described previously [51] . In brief, influenza virus containing 100 TCID 50 was incubated with equal volume of two-fold dilutions of the specific heat-inactivated serum overnight at 37uC in a 5% CO2 humidified atmosphere for 1 hr. After the incubation, 100 ml virus-serum samples were added to a 96-well plate containing Madin Darby Canine Kidney (MDCK) cell monolayer and incubated for 5 days at 37uC and 5% CO2. The microneutralization titer was defined as the highest dilution of serum that neutralized 100 TCID 50 of virus in MDCK cell [52, 53] cultures (as detected by the absence of cytopathic effects). The MN assays were conducted in two different labs: 1) National Microbiology Laboratory, Public Health Agency of Canada, and 2) Beijing Institute of Microbiology and Epidemiology, according to the viruses available.