The recombinant lentiviral vectors expressing a luciferase reporter gene were produced as previously described [22, 29, 31, 41] . This assay has been developed as a safer, highly sensitive alternative to HAI and MN assays that can be applied in a high-throughput format for influenza vaccine evaluation [42] [43] [44] . Briefly, HA-pseudotyped lentiviral vectors encoding luciferase were first titrated by serial dilution. The concentration of virus giving 25% maximum activity was then incubated with the indicated amounts of mouse anti-serum before being added to 293A cells. Plates were washed and replaced with fresh media 14-16 hours later. Luciferase activity was measured after 48 hours as previously described [45] using mammalian cell lysis buffer and luciferase assay reagent (Promega, Madison, WI) according to the manufacturer's protocol.