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What the 1918 influenza pandemic can teach us about the coronavirus outbreak

Pandemics, or global epidemics, of influenza
have occurred for hundreds of years. There were three pandemics during the 20th Century; they occurred in 1918, 1957, and 1968. The 1918 pandemic took the largest toll, causing an estimated 50 million deaths worldwide. The first pandemic of the 21st Century began in 2009. But how does a pandemic occur? Influenza A viruses vary slightly from
year to year, a phenomenon called antigenic drift. These small variations are the reason why the
vaccine must be updated every flu season. Influenza A viruses also can undergo dramatic
genetic changes, called antigenic shift, that may cause a new influenza
strain to which we have little to no immunity. If this strain spreads easily among people, it may cause a pandemic. The immune system targets two proteins on
the surface of influenza A viruses: hemagglutinin and neuraminidase.
Hemagglutinin has 16 confirmed subtypes of proteins. Neuraminidase has nine. From these protein combinations, scientists
name influenza viruses. For example, an H5N1 virus contains
hemagglutinin subtype 5 and neuraminidase subtype 1. Antigenic shift can occur when two
different influenza A viruses circulating in the environment infect and
multiply inside the same cell. While inside the cell, these parent viruses
can exchange, or reassort, genetic material and create a new virus. Through reassortment of the 8 gene segments
of each virus, there are theoretically 256 possible virus combinations. Pandemic viruses also can emerge in other ways. Among these is adaptation. Scientists believe that the 2009 H1N1
virus was the product of two influenza A viruses that were
circulating in pigs. The viruses are thought to have reassorted
and adapted to create a new virus capable of infecting people. If an emerging virus transmits efficiently
from person to person, it would be a challenge for scientists to
produce an effective vaccine fast enough to limit its spread. That is particularly true with the amount
of international travel that occurs today. In a matter of hours, one person infected with
a new form of influenza can spread the virus to people in
another part of the world. The National Institute of Allergy and Infectious
Diseases and other research institutions around the globe are working to find
better ways to prevent, diagnose, and treat pandemic influenza. These efforts include testing new drug
and vaccine concepts, studying flu viruses in animals and humans and documenting factors that could lead to
the emergence of strains with pandemic potential. The aims of these projects are to help
health officials quickly detect an emerging influenza A virus and implement
measures that bring the outbreak under control.