In 1918, an influenza pandemic that killed tens of millions of people swept the globe. This wasn’t the first time influenza made an impact on humankind; the virus has been circulating for centuries. Indeed, according to scientists at the National Institute of Allergy and Infectious Diseases, the first known influenza pandemic occurred in 1510.
But the 1918 pandemic was different. It was far more lethal than most pandemics, killing mostly young, healthy adults, and – since it was the first such pandemic in modern times – it provided scientists with a benchmark for studying influenza. In the ensuing years, virologists learned a great deal about the structure and behavior of influenza viruses.
Function Follows Structure
According to “The Merck Manual of Diagnosis and Therapy,” influenza viruses are classified as type A, B, or C, depending on their protein structures. Types A and B are responsible for most human infections, and type A accounts for the majority of severe pandemics.
Two specific viral proteins allow influenza to infect human cells. The first, a protein called hemagglutinin (HA or H), allows the virus to attach to and enter a cell, where it hijacks the cell’s genetic machinery in order to produce multiple copies of itself. Once these viral “offspring” have been replicated, another protein called neuraminidase (NA or N) allows the duplicated copies to escape from the cell. The new viruses then disperse to infect other cells.
Viral Mutation Keeps Influenza One Step Ahead
Influenza viruses constantly undergo changes in their genetic structure which alter the appearance of the H and N proteins. These changes can occur as the result of relatively minor mutations in the proteins’ structure, or they can accompany major genetic shifts. In this way, influenza maintains the means to partially evade the immune systems of the animals it infects, such as birds, pigs, ferrets, and humans.
Since influenza viruses were first recognized as a cause of illness in animals, sixteen different H proteins and nine different N proteins have been identified. Influenza viruses are labeled according to which H and N proteins they contain, giving us H1N1 (the first influenza virus to be studied), H3N2, H5N1, H9N2, and so on. Each unique H/N combination is called a serotype.
How Pandemics Get Started
When new H or N proteins appear or when new combinations of H and N proteins occur, people may lack significant immunity to the new serotype. Pandemics are more likely to occur when such novel serotypes emerge.
New serotypes aren’t the only triggers for pandemics. Serotypes that have been around for a while can alter their underlying genetic structure without exchanging their H or N proteins. These viruses, which represent new strains of old serotypes, are sometimes just different enough to present a problem for our immune systems, too. In 2009, a highly infectious strain of H1N1 triggered a worldwide pandemic which, fortunately, was far milder than most experts initially feared it would be.
Staying Ahead of the Numbers Game
The World Health Organization and the U.S. Centers for Disease Control and Prevention keep a close eye on the evolution of influenza viruses. When a novel strain or a new serotype emerges, these organizations mobilize vaccine and antiviral drug production, alert local and regional healthcare networks, and establish quarantine procedures when necessary.
Any citizen can follow the dynamics of influenza outbreaks – and adopt protective measures as needed – by visiting the CDC and WHO websites.
1. David M. Morens, Jeffery K. Taubenberger, Gregory K. Folkers, Anthony S. Fauci. Pandemic Influenza’s 500th Anniversary.Clinical Infectious Diseases, 2010
2. The Merck Manual of Diagnosis and Therapy, 18th Edition: Influenza. Mark H. Beers, M.D., Editor-in-Chief. 2006