Risk Assessment: Tools, Techniques, and Their Applications

20.9 Influenza Example

Influenza is caused by an orthomyxovirus that belongs to three genera (6). Influenza A virus is the causative agent for the vast majority of the epidemics. The H and N antigens are present in influenza A virus and routinely undergo “antigenic shift.” This periodic alteration in the antigens present in the flu virus does not allow persistent immunity to develop in human beings. Thus, until 1957, the most common subtype was the H1N1 subtype. After that, it shifted to H2N2. This shift resulted in an epidemic in the United States, killing more than 70,000 people. It typically presents in the early winter months. The Spanish flu epidemic of 1918 resulted in the death of more than 650,000 people. The spread was exacerbated by the wartime zeal and the many rallies set to increase support for the war effort.

The virus is spread by aerosolized droplets. A person is contagious until 2–3 days after symptoms have appeared. Often symptoms appear quickly and the disease progresses rapidly. Influenza can lead to a secondary bacterial pneumonia, which can be devastating.