Survey sites, study population and antigens used in seroprevalence survey among close contacts exposed to HPAI H5N1 case-patients, China, 2005–2008.

#A/CK/HN/21/05(H5N1) and A/CK/LN/23/05(H5N1) isolated from poultry epidemiologically linked to case-patient 1 and case-patient 2 respectively, were used as antigens in the serological assays for close contact exposed to the matched H5N1 case; these two patients were confirmed by serology only.*For serological testing of exposed contacts without isolation of H5N1 viruses from either human cases or poultry epidemiologically linked to cases, a representative H5N1 virus strain [clade 2.3.4, A/Anhui/1/2005(H5N1)] was used as the antigen.