Viral challenge experiments were performed 3 weeks after the last immunization. All challenged animals were exposed under CO 2 anesthesia to an intranasal viral inoculum of 100 LD 50 of A/ Vietnam/1203/2004 virus. Body weight and survival were monitored for 21 days. The viral challenge experiments were conducted at the Centers for Disease Control and Prevention (Atlanta, GA) as previously described [17] .