Influenza pandemics (outbreaks that affect a large proportion of the world due to a novel virus) are unpredictable but recurring events that can have health, economic and social consequences worldwide. An influenza pandemic occurs when key factors converge: an avian or zoonotic influenza virus emerges with the ability to cause sustained humanto-human transmission, and the human population has little to no immunity against the virus. With the growth of global trade and travel, a localized epidemic can transform into a pandemic rapidly, with little time to prepare a public health response.
Ongoing circulation of some avian influenza subtypes in poultry, such as A(H5) or A(H7N9) viruses, are of public health concern as these viruses commonly cause severe disease in humans and the viruses have the potential to mutate to become more transmissible between humans. To date, although human-to-human transmission of these viruses is thought to have occurred in some rare instances when there had been very close and prolonged contact between a very sick patient and caregivers such as family members, there has been no sustained humanto-human transmission. If these viruses adapt or acquire certain genes from human viruses, they could trigger a pandemic.
Whether currently-circulating avian and other zoonotic influenza viruses will result in a future pandemic is unknown. However, the diversity of avian and other zoonotic influenza viruses that have caused human infections necessitates ongoing surveillance in both animal and human populations, detailed investigation of every human infection and risk-based pandemic planning.