If you are wondering what the different letters and numbers used to name influenza viruses mean, such as H1N1, H3N2, H5N1 and influenza A, B, C or D, you are probably not alone. The letters and numbers used for influenza viruses have significance. Depending on what strain of influenza virus is circulating in a given year, the letters and numbers may indicate the risk of spreading beyond the usual prevalence of infection and expected severity of illness.
There has been a lot of attention paid recently in the news about H5N1 bird flu, but it is also important not to lose sight of the ever-present risk of seasonal influenza. We are seeing a gradual uptick in influenza infections across the country, especially in the south and west, as is typical this time each year. As families and friends gather in crowded, close quarters for the holidays, one thing they may not be thinking about is the risk of transmission of influenza and other respiratory viruses. In addition, because people travel far and wide this time of year, they are unknowingly facilitating the spread of viruses across great distances.
The name of a typical influenza virus has multiple parts. The first part indicates which type of virus it is, either A, B, C, or D. Influenza A viruses generally cause more public health challenges than B or C, and D viruses typically infect cattle or other animals. In addition to the viral type, the name includes the location where the virus was first isolated, the year of isolation, and the strain number.
To round out the name, the viruses are given an H and N number to indicate a subtype. A full name might then look like this: A/Sydney/05/97 (H3N2), meaning a type of influenza A virus, strain 05, discovered in Sydney, Australia in 1997, and a subtype with H3 and N2 antigens.
What The H Stands For
H stands for hemagglutinin, which is a type of protein found on the surface of influenza viruses. The term “hemagglutinin” derives from two parts of the word: one relates to blood and the other to “agglutinate.” Therefore, hemagglutinins have the ability to cause red blood cells to clump together.
The important thing is that the hemagglutinin on the viral surface is the part that binds to our cells and helps facilitate cellular entry. In short, they are responsible for cell infections. Without this component or without the proper receptors on the cell surface, infection won’t occur. Once inside our cells, the viruses can crank out hundreds to thousands of progeny, thus causing an active infection. The number after the letter indicates which specific hemagglutinin an influenza virus possesses. Currently there are 18 identified influenza hemagglutinins.
What The N Stands For
The N stands for neuraminidase, another protein on the surface of influenza viruses that is key to influenza’s ability to spread from one cell to another. After influenza enters a cell, it makes copies of its genetic material and creates new influenza viruses. Those viral progeny will exit the cell, but they aren’t able to gain release from the cell surface without neuraminidase, which facilitates release, allowing them to find and enter a new cell and start the infection cycle all over. There are 11 distinct types of neuraminidase.
The Importance Of H And N
Our bodies don’t take infection lying down. We mount a response against the viral hemagglutinin and neuraminidase. This helps to prevent someone from getting infected again by the same virus; however, there is a catch. One of the major challenges with countering influenza viruses is that when they reproduce, they make errors that can lead to subtle changes in their genetic material, which is RNA. Millions of infections occurring around the world each year provides fertile ground for lots of errors. This then leads to a phenomenon called “antigenic drift.” Even if the same H and N numbered virus, such as H1N1, circulates in a subsequent flu season, the virus will be slightly different each year. The end result is that you can get infected again – you may not be as ill as the year before, because your immune system mounted an effective response to the virus or to a vaccine, but the virus might be different enough to still infect you.
To make things more complicated, every now and then, we get a complete change in the circulating influenza viruses. For example, one year we might have H1N1 viruses predominantly in the population, but the following year, something like H2N2 pops up. We call that an “antigenic shift.” This is a more concerning situation, because a sizable portion of the population may not have had recent exposure to the new viruses, and are therefore susceptible to infection, and potentially severe infection. This is what leads to major outbreaks and pandemics, such as those that occurred in 1918, 1957, 1968, and 2009.
Influenza adds another challenge in that different groups and subgroups of viruses can be in circulation among the population at the same time. If someone is infected with a subgroup that is similar to a virus they’ve already been infected with or vaccinated against, their immune system may recognize it and limit illness. On the other hand, infection or vaccination against one virus may not effectively provide cross protection against a different virus.
Why You Need A Flu Vaccine Every Year
The phenomena of antigenic drift and antigenic shift are why public health authorities recommend getting an influenza vaccine each year. Every year, leveraging a global system of viral surveillance, the World Health Organization holds expert meetings to assess the appropriate viruses to vaccinate against based on several factors: which viruses are circulating, which are causing illness, and which vaccines might lead to cross protection. WHO committees make recommendations, and each country then makes their own decision. In the United States, the FDA makes the final decision on what to include in U.S. vaccines.
What You Can Do To Protect Yourself
Viruses transmit on respiratory droplets, so to minimize the risk of getting influenza try to avoid people who are sick and cover your cough or sneeze and ask those around you to do the same. Viruses can also transmit through contact with surfaces touched by others who are infected, so wash your hands regularly, and especially after contact with communal surfaces, such as door handles, toilet handles, elevator buttons, etc. Finally, don’t forget to get your annual influenza vaccine.
Predicting which viruses will circulate in a given year is not an exact science. Just like stock market predictions, they aren’t always correct. Sometimes the authorities get it right and the vaccine protects well. Other times, not so much. As Yogi Berra famously quipped, “It’s tough to make predictions, especially about the future.”
