According to Sonja J. Olsen, PhD, deputy chief, Epidemiology and Prevention Branch, Centers for Disease Control and Prevention (CDC), Atlanta, Georgia, seasonal flu is associated with a significant burden of disease in humans each year. In the United States alone, 9.2 million to 35.6 million cases of flu occur each year, causing 12,000 to 56,000 deaths.
“Understanding seasonal flu is necessary to understanding pandemics,” she said.
Of the 4 types of influenza virus, only types A and B are major human pathogens.
Influenza A viruses are especially problematic, she noted, because they can infect a variety of host species, including waterfowl (their natural reservoir), poultry, humans, and pigs. These viruses can also cross the species barrier, causing illness in another species. The viruses have 8 segmented genes and constantly undergo mutation, Dr. Olsen added. And if influenza A viruses from 2 different species infect the same animal or person, the segmented genome allows the 2 different viruses to mix and create a novel influenza A virus.
“A novel flu virus is a subtype of an influenza virus that is different from influenza viruses currently circulating among humans,” she explained. “Pandemics can occur when a novel influenza virus emerges in the human population.”
Pandemic influenza virus strains also easily infect humans and spread from person-to-person in an efficient and sustained way, said Dr. Olsen. In the event of a flu pandemic arising, the population will have no immunity to the novel virus, and no vaccine will be available to protect against it, she added.
She highlighted avian influenza A H7N9 as having the current greatest potential to cause a pandemic. This virus is circulating in poultry in China and sporadically infecting humans. It has caused 5 annual epidemics of human infections since 2013, said Dr. Olsen, and is associated with severe illness: 90% of infected persons have developed pneumonia, 70% have required intensive care unit admission, and 40% have died.
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