Bias: 31.9% [95% Confidence Interval ?=?+17.1% to +46.7%]. infection have lower H5N1 antibody titres compared to those with severe illness and that in many asymptomatically infected patients the antibody titer decreased to levels below the threshold of positivity within one year. These data are essential for the design and interpretation of sero-epidemiological studies. Introduction Since 1997, the highly pathogenic avian influenza A (H5N1) virus has spread among poultry and possibly also in wild birds in Asia, Middle-East, Europe and Africa and caused over 470 cases of reported human diseases with more than 280 deaths [1]. The virus evolves as it continues to circulate endemically in poultry in many countries. Continuing occurrences of human infection provides opportunities to H5N1 viruses to adapt to efficient human-to-human transmission. Furthermore, the novel 2009 pandemic H1N1 virus has repeatedly been detected in pigs in many countries including southern China (Peiris C personal communication) and the triple-reassortant gene constellation possessed by this virus has shown a propensity to acquire novel viral haemagglutinin 3-AP via reassortment [2]. H5N1 virus has occasionally been documented in pigs [3]. Thus, the presence of the pandemic virus in pigs may provide an increased risk 3-AP of reassortment between avian H5N1 viruses with the pandemic H1N1 virus. This may allow additional opportunities for H5N1 virus adaptation to human-to-human transmission posing potentially new threats to public health. Hence, it is important to conduct sero-epidemiological studies to monitor the extent of asymptomatic or clinically mild H5N1 illness among humans. Such studies will also help define the risk factors for human infections [4]C[10]. Serological methods are essential for the detection of asymptomatic infections and may be helpful to retrospectively confirm suspected cases of H5N1 disease [11]. A significant limitation for the interpretation of serological data, especially for sero-epidemiological studies, is the lack of information within the kinetics of the anti-H5 neutralizing antibody response and particularly that of asymptomatic infections. In this study, we analyzed the characteristics of the antibody response in individuals from Vietnam and Cambodia infected by clade 1 H5N1 computer virus who experienced a spectrum of illness ranging from fatal or severe disease to moderate illness or asymptomatic illness. Cambodia shares are porous border for humans and poultry with South Vietnam and during the period under study, the H5N1 viruses isolated from southern Vietnam and Cambodia were phylogenetically closely related [12]. Materials and Methods Serum samples 3-AP Human being sera were collected at the Hospital for Tropical Disease (HTD) Ho Chi Minh City, Vietnam, from individuals with severe H5N1 computer virus infection confirmed by RT-PCR [13]C[15]. Timing of serum collection from hospitalized individuals with H5N1 disease (N?=?11) between 2003 and 2005 in southern Vietnam are summarised in table 1. Sera from Cambodia (N?=?1370) were from people living within 1 km radius of the households of three H5N1 individuals. None of them of the individuals were epidemiologically linked [9], [10]. All of them reported having experienced direct contact with ill/dead poultry a few days to weeks before sign onset [9], [10], [16]. First blood samples were collected among village participants 1C2 months after the date of the patients’deaths. We repeated blood collection for seropositive individuals 9C11 months later on. These studies were authorized by the Cambodian National Ethics Committee, the Ethics and Scientific Committee of HTD and the Oxford Tropical Study Ethics Committee (OXTREC). A written educated consent form was from all the participants involved in the studies. Since the asymptomatic KLF1 seropositive individuals were more likely to have individually acquired.