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Influenza: The H’s and N’s

A tremendous amount of media attention has been placed on avian flu and pandemic situations. Hyped media attention and public confusion on the topic underscores the need for emergency responders to have the basic information and understanding of key concepts regarding types of influenza, terminology, and other details. Responders not armed with a basic understanding may lack the ability to gain situational awareness placing themselves, the public, and potentially their families at risk.

The term influenza is not synonymous with Avian Flu or pandemic. Influenza can be categorized in a variety of ways, but in general, influenza or flu is caused by a family of viruses known as Orthomyxoviridae and can be broken out into three types; type A, B, and C. There are numerous illnesses that can be responsible for the classic flu symptoms like body aches, chills and fever. Symptoms can range from mild to severe and include serious complications such as bacterial infections and pneumonia. Type A influenza crosses species and is the most hearty, or virulent, of the three strains. Highly Pathogenic Avian Influenza, or HPAI, is a type A flu virus. Type B flu virus targets only humans, is common and less severe than type A while type C, which impacts humans and swine, is rare and my have only mild symptoms or none at all.

Type A flu viruses receive much of our attention and is home to H5N1 or avian flu. The H’s and N’s represent designation of proteins of the virus and are important to the classification of Type A flu. Understanding the role of the H’s and N’s will also aid in understanding why vaccine development can be difficult. Designations such as H5N1 are used to further classify one type A virus from another. The “H” stands for hemagglutinin antigen (sometimes HA is used rather than just H). There are fifteen different hemagglutinin antigen (H/HA) proteins. The H proteins give the virus the ability to attach to the host cell. The “N” represents neuraminidase antigen (again, sometimes documented as NA rather than N). The neuraminidase protein allows the virus to be released from the cell and spread infection. There are nine neuraminidase antigen (N/NA) proteins. The numerous combinations of H’s and N’s allow the type A virus to infect such a large number of species. Remember, there are 15 different H proteins and 9 N proteins...that means there are 135 combinations of protein variations for type A influenza virus. Here’s the catch; a vaccine designed to work for one combination of proteins will not work for another and since type A (and B) influenza changes slightly from one flu season to another, creation of a vaccine for that particular season is tricky business. Keep in mind that the virus wants to survive and to do so will have to change; either slightly or drastically. A slight change in the virus is known as antigenic drift while drastic changes are called antigenic shifts. It is the antigenic shift that can cause a virus to change enough to cause severe disease or pandemic.

References: Pandemic Planning and Preparedness


  1. Aaron Marks4:39 PM

    Glad to see that you're still going strong here Rick.. but I have a question for you. With all of the other pathogens out there in the world, why is everybody so focused on H5N1? We've got XDR TB that has already shown up in the US on several occasions (not just the well-publicized traveling lawyer), Dengue Fever has been confirmed on the Texas/Mexico border, there are continuous outbreaks of River Fever and other hemorrhagic lovelys in Africa, and last of all the occurences of WNV in the US are skyrocketing - don't you think that by focusing on one specific pathogen we are weakening our overall ability to respond to an epi/pandemic? (similar to the terrorism-centric focus that our media and Federal government seem to love) The more research that I do - both focused and just keeping up with world events - the more that I think we're going to see issues with TB or something else, and that the H5N1 mutation to easy human-to-human transfer is not the biggest bully on the block anymore. What is your take?

  2. Aaron - good to hear from you!
    I do agree that a narrow focus is not how to manage the emerging disease situation. We (public) are at the whim of the media when it comes to topic. We (in preparedness) have to be above that...hence, the broad base of material in MJ.
    Avian flu has captured the media and, therefore, the public. Next week it could be something know that. And whatever the next media focus is; WNV, SARS rerun, or whatever, we'll have to be ready to meet the infomration and preparedness needs of those who "never heard of that before"...
    Thanks for writing in. Check out the Podcast at and the new web page at
    Don't forget: I could always use a guest author it you'd like to chime in!


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