Brief literature review for the WHO global influenza research agenda – highly pathogenic avian influenza H5N1 risk in humans

Highly pathogenic avian influenza A H5N 1 viruses remain a significant health threat to humans given the continued rare occurrence of human cases with a high case fatality rate. This brief literature review summarizes available evidence of risk factors for H 5N1 infection in humans and updates a recent systematic review published in early 2011. Several epidemiologic studies have been published to evaluate the risk factors for H 5N1 infection in humans, including contact with poultry and poultry products and non‐poultry‐related contact such as from H 5N1‐contaminated water. While most H 5N1 cases are attributed to exposure to sick poultry, it is unclear how many may be due to human‐to‐human transmission. The collective results of published literature suggest that transmission risk of H 5N1 from poultry to humans may be highest among individuals who may have been in contact with the highest potential concentrations of virus shed by poultry. This suggests that there may be a threshold of virus concentration needed for effective transmission and that circulating H 5N1 strains have not yet mutated to transmit readily from either poultry to human or from human to human. However, the mode of potential transmission can be quite varied throughout different countries and by study with exposures ranging from visiting a wet market, preparing infected poultry for consumption, to swimming or bathing in ponds frequented by poultry. Several important data gaps remain in the understanding of the epidemiology of H 5N1 in humans and limit our ability to interpret the results of the available H 5N1 seroepidemiologic studies.