The avian influenza A virus (AIV) is naturally prevalent in aquatic birds, infecting different avian species and transmitting from birds to humans. Both AIVs, the H5N1 and H7N9 viruses, have the potential to infect humans, causing an acute influenza disease syndrome in humans, and are a possible pandemic threat. AIV H5N1 is highly pathogenic, whereas AIV H7N9 has comparatively low pathogenicity. A clear insight into the disease pathogenesis is significant to understand the host's immunological response, which in turn facilitates the design of the control and prevention strategies. In this review, we aim to provide comprehensive details on the pathogenesis and clinical features of the disease. Moreover, the innate and adaptive immunological responses to AIV and the recent studies conducted on the CD8+ T cell immunity against AIVs are detailed upon. Further, the current status and advancement in the development of AIV vaccines, along with the challenges, are also discussed. The information provided will be helpful in combating the transmission of AIV from birds to humans and, thus, preventing severe outbreaks leading to pandemics worldwide.
Keywords: adaptive immunity; avian influenza virus; diagnosis; innate immunity; vaccine development.