Sensitivity test against human and avian strains. A. The H5N1 primers were tested for sensitivity against the currently recommended WHO H5 primer set using three H5N1 strains, one human strain from stored archive RNA, and two freshly extracted avian strains. Human 3028 (A/Vietnam/3028/2004, lanes 3 to 11); Avian 933 (lA/Chicken/Vietnam/933/2004, lanes 15 to 23), and Avian 949B (A/Chicken/Vietnam/949B/2004, lanes 24 to 32) are shown. Serial dilution was performed on the RNA samples from 10-1 to 10-8 as indicated. The 219 bp fragment amplified using the WHO H5 primer set was also performed in parallel as indicated. B. Sensitive detection of H5N1 avian influenza A virus. Five H5N1 samples from Vietnam that have been confirmed by viral isolation and RT-PCR were tested in parallel with an existing H5 primer set (lanes 67 to 71) and the H5N1 primer set (lanes 73 to 77) described in this report. Only three out of five samples were detected by the H5 primer set (lanes 67 to 69), while the new H5N1 primer set detected all five samples (lanes 73 to 77).