To determine the efficacy of these alternative DNA immunizations, mice were challenged with the HPAI A/Vietnam/1203/ 2004 virus. Mice were challenged intranasally with 100 LD 50 of A/Vietnam/1203/2004 virus, providing a stringent evaluation of protective efficacy. All animals in the control and NP alone groups died within 6 days after viral challenge, whereas animals immunized with HA alone, HA+NP and HA+NP+M2 showed survival rates of 100, 90 and 70%, respectively ( Figure 3A ); these survival rates were not statistically significantly different. As expected, the HA alone group showed the least amount of body weight loss, while groups HA+NP and HA+NP+M2 showed similar weight loss patterns ( Figure 3B ). In contrast, the control and NP groups that showed no immune protection demonstrated severe weight loss ( Figure 3B ). This finding suggests that NP DNA immunization does not confer protection against H5N1 viral challenge at the doses used here.