Naming the 2015-2016 Influenza Viruses: H3N2 and H1N1

The 2015-2016 flu season has already begun, and the CDC has already started to update its weekly flu maps to show state-wide statistics. Soon, we will start to hear terms such as epidemic, pandemic and outbreak in the news. Here is a  video we made that can give you a quick review of the differences between an epidemics, outbreak and pandemic:

So when we hear of a “flu epidemic’, what does that really mean? As you probably know, “flu’ is short for “influenza”, a communicable disease caused the influenza viruses. It is important to recognize that there are several different forms of influenza. Influenza A and Influenza B are the two most prevalent forms that infect humans. Influenza viruses are typically named according to their type (A or B), host, geographical origin, year of isolation -etc. For example, this year’s trivalent flu vaccine contains protection against the following three strains:

A/California/7/2009 (H1N1)-like virus,

 A/Switzerland/9715293/2013 (H3N2)-like virus

B/Phuket/3073/2013-like (Yamagata lineage) virus

The tetravalent version of the vaccine adds protection against the B/Brisbane/60/2008-like (Victoria lineage) virus.

Remember that the purpose of a virus is to hijack a host cell and make lots of copies of itself (see our Viral Life Cycle animation for more information). The genome of a virus may be either DNA or RNA (it is RNA in influenza) and it contains the instructions for making new viruses. The protein capsid encloses the genome. In influenza viruses, the capsid is surrounded by a membrane-like structure called an envelope. The spikes in this diagram are glycoproteins that assist the virus in identifying and entering its host cell.


Now, lets focus on the H and N aspects of an influenza virus. Notice how in the diagram of an influence virus below that there are several different forms of glycoprotein spikes  on the surface of the virus.

Human Influenza Virus

For influenza A viruses, there are two important forms of these spikes – H spikes and N spikes.  H spikes are associated with hemagglutinin, a type of glycoprotein that assists the virus in identifying the receptors on the host cell.  The N spikes are an enzyme called neuraminidase. N spikes help break down the mucous material surrounding host cells in the respiratory tract and initiate penetration of the virus into the host cell.

These spikes are not the same in all influenza A viruses. There are over 17 different known forms of H spikes, and 10 different variations in N spikes. These variations are what gives the influenza A viruses their distinct name. In humans, H1N1 and H3N2 are the two forms of influenza A the contribute to the normal seasonal flu outbreak. So, for example, the H1N1 virus has hemagglutinin form 1 and neuraminidase form 1.

Here are some additional examples of influenza A names in other animals:


Notice that H5N1 (avian flu) has a number of hosts, while others (H13N9 – whales) are fairly host specific. Interestingly, birds are hosts for all variants of influenza A. These types of relationships help scientists identify not only reservoirs for the virus, but from where future outbreaks may originate.

How does the influenza virus evolve over time? There are basically two mechanisms, both of which are based on the virus altering the types of H and N proteins on its surface over time to avoid a response by the host’s immune system. The term that is associated with this process is antigenic 

  • Antigenic drift – small changes in the proteins on the surface of the virus that cause the host antibodies to be less effective in identifying and fighting the virus.
  • Antigenic shift – a larger change in the H and N proteins, often producing new combinations that organisms have not been exposed to over time.  Sometimes this happens when a host is infected with two unrelated influenza viruses. In infected cells, these may cause the cellular machinery to form a new form of virus. These are typically referred to as emerging diseases, since they often cause problems with rapid rates of movement within and between populations and

Recently, scientists have been able to develop a processes by which a vaccine may not be based on the H and N spikes, but rather on the short stems that connect these spikes to the virus. This is still in the experimental stage, but if successful, it may change the way that vaccines are developed and the frequency that individuals must receive vaccines.

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