Epidemic respiratory infections, as previously observed with influenza [10, 11] and SARS [23] , result in high mortality, sometimes before the pathogen is identified, and often without effective post-exposure treatment options. Seasonal influenza, though the case-mortality rate is lower, still kills ten of thousands of Americans annually [4, 5] and clinicians are faced with the problems of ineffective vaccine strategies and declining effectiveness of neuraminidase inhibitors [24, 25] . In such situations, rapidly dispersed, broad protection would be highly advantageous.