The influenza H1N1pdm virus, A/Kentucky/180/2010, (KY/ 180; GenBank CY99332 and CY99333) was isolated from the nasal swab of a severe hospitalized case (hospitalized in March 2010) provided by the Severe Influenza Pneumonia Surveillance project, an ongoing clinical study of hospitalized patients with influenza pneumonia in Kentucky (courtesy of Dr. Julio Ramirez). The virus was isolated and passaged in the allantoic cavity of tenday-old embryonated hens' eggs at 37uC. The allantoic fluid was harvested 72 h after inoculation, pooled and stored in stored in aliquots at 280C until use. The infectious virus titer of the resulting seed stock was determined by TCID 50 (50% tissue culture infectious dose) and the titer calculated by Reed and Muench [46] and confirmed by plaque assay on MDCK cells. Passage E2 was used for the studies reported herein.