What do the letters and numbers in “H1N1” mean?

Scientists identify different flu strains by using two special molecules found on the outside of the virus particle. Hemagglutinin (H) allows the virus to bind to and enter a specific cell. This is usually what your immune system targets, so changes to this molecule allow the virus to avoid detection. Neuraminidase (N) allows the newly created viruses to exit a cell and spread the infection. There are 16 different hemagglutinin subtypes and 9 different neuraminidase subtypes, and they’re what the numbers after the letters refer to.

There are only three known influenza A subtypes (H1, H2, and H3) circulating among humans. (The H5N1 avian influenza virus in the news over the last few years has crossed the barrier to infect humans, but human-to-human spread of the virus has been limited, inefficient, and unsustained. See our feature on avian flu here.)

Influenza viruses can change in two different ways.

One is called "antigenic drift." These are small changes in the virus that happen continually over time. Antigenic drift produces new virus strains that may not be recognized by the body's immune system. This process works as follows: a person infected with a particular flu virus strain develops antibody against that virus. As newer virus strains appear, the antibodies against the older strains no longer recognize the “newer” virus, and reinfection can occur. This is one of the main reasons why people can get the flu more than once. In most years, one or two of the three virus strains in the influenza vaccine are updated to keep up with the changes in the circulating flu viruses. So people who want to be protected from flu need to get a flu shot every year.

The other type of change is called "antigenic shift." Antigenic shift is an abrupt, major change in the influenza A viruses, resulting in new hemagglutinin and/or new hemagglutinin and neuraminidase proteins in influenza viruses that infect humans. It happens when two or more viruses infect a single host and their genes get mixed up. (This is what scientists think happened with the 2009 H1N1 flu strain.) Shift results in a new influenza A subtype. When shift happens, most people have little or no protection against the new virus. While influenza viruses are changing by antigenic drift all the time, antigenic shift happens only occasionally.