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Conclusion

Avian influenza virus contributed to 3 pandemics in the 20th century, through both antigenic shift and direct adaptation. Although waterfowl and shorebirds are the likely reservoir, because all 16 HA and 9 NA types are found in these species, influenza viruses can change species. Influenza has likely been endemic in humans, swine, horses, and sea mammals since the identification of influenza viruses. More recently, equine influenza virus jumped into dogs and may now be established in the species, and HPAI has infected domesticated and large cats.^[2]

Although the current problematic H5N1 viruses have infected hundreds of humans and H7N7, H7N2, and H9N2 viruses have infected humans on rare occasions, none of these viruses have taken the next step of efficiently transmitting from human to human. Without that critical step, these viruses may be a problem, but they will only infect individuals in contact with infected birds and/or large amounts of virus. Science has described some of the changes that may be required for an avian virus to establish itself in humans, but it cannot determine when it will happen, and nature can certainly provide yet undescribed means for adaptation to humans.

History tells us that the world is due for another influenza pandemic, but no one scientist, epidemiologist, public health official, government, or otherwise can definitively state when it will occur, where it will emerge, or what virus will cause the pandemic. Moreover, pre-existing immunity will also influence the outcome of the emergence of a new human influenza virus. The antigenic shift that generated the H3N2 pandemic only altered the HA and not the NA. This contributed to a relatively mild pandemic with approximately 34,000 deaths in the United States, the same level of mortality currently caused by seasonal influenza each year in the Unites States.

Similarly, when H1N1 influenza reappeared in the 1977 "Russian influenza" epidemic, only persons younger than 23 years were generally affected because older persons maintained immunity from exposure to H1N1 influenza viruses circulating prior to 1957. Therefore, we cannot predict whether the next pandemic will be like the mild 1968 Hong Kong influenza or more similar to the devastating 1918 pandemic, which may have killed more than 50 million people worldwide. Only the future holds the answer; however, pandemic preparations being made today may help mitigate the morbidity, mortality, and disorder caused by even an unknown pandemic.

This activity is supported by an independent educational grant from GlaxoSmithKline.