Influenza viruses: what’s in a name?

If you’re 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’re probably not alone. The letters and numbers used for flu viruses have meaning. Depending on which strain of influenza virus is circulating in a given year, the letters and numbers may indicate the risk of spread beyond the usual prevalence of infection and expected severity of the disease.

A lot of attention has been paid to the H5N1 bird flu in the news lately, but recent statements from former President Bill Clinton hospitalization for flu emphasizes the importance of not losing sight of the ever-present risk of seasonal flu. We are seeing a gradual recovery influenza infections throughout the country, especially in the south and west, as is customary every year at this time. As families and friends gather in crowded, close quarters during the holidays, they may not be thinking about the risk of transmitting flu and other respiratory viruses. Furthermore, because people travel far and wide at this time of year, they unknowingly facilitate the spread of viruses over great distances.

The name of a typical flu virus consists of several parts. The first part indicates which type of virus it is: A, B, C, or D. Influenza A viruses generally cause more public health problems than B or C, and D viruses typically infect livestock or other animals. In addition to the virus type, the name also includes the location where the virus was first isolated, the year of isolation and the strain number.

To complete the name, the viruses are given an H and N number to indicate a subtype. A full name could 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, a type of protein found on the surface of influenza viruses. The term ‘hemagglutinin’ is derived from two parts of the word: one relating to blood and the other to ‘agglutinate’. Therefore, hemagglutinins have the ability to cause red blood cells to clump together.

Most importantly, the hemagglutinin on the viral surface is the part that binds to our cells and helps facilitate access to the cells. In short, they are responsible for cell infections. Without this component or without the correct receptors on the cell surface, infection will not occur. Once inside our cells, the viruses can produce hundreds to thousands of offspring, creating an active infection. The number after the letter indicates which specific hemagglutinin an influenza virus possesses. Currently there are 18 identified influenza cases hemagglutinins.

What the N stands for

The N stands for neuraminidase, another protein on the surface of influenza viruses that is essential for influenza’s ability to spread from one cell to another. After flu enters a cell, it makes copies of the genetic material and creates new flu viruses. Those viral progeny will leave the cell, but they cannot obtain 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 again. There are 11 different types of neuraminidase.

The importance of H and N

Our body does not absorb infections while lying down. We set up a response against the viral hemagglutinin and neuraminidase. This helps prevent someone from becoming reinfected with the same virus; however, there is a catch. One of the biggest challenges in combating influenza viruses is that they make mistakes during reproduction that can lead to subtle changes in their genetic material, namely RNA. Millions of infections happening every year around the world provide fertile ground for many mistakes. 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 become infected again – you may not be as sick as the year before because your immune system has responded effectively to the virus or to a vaccine, but the virus may be different enough to still infect you.

To complicate matters, every now and then we get a complete change in the circulating flu viruses. For example, one year we may have predominantly H1N1 viruses in the population, but the next year something like H2N2 shows up. We call that an ‘antigenic shift’. This is a more worrying situation because a significant portion of the population may not have been recently exposed to the new viruses and is therefore susceptible to infections and potentially serious infections. This leads to major outbreaks and pandemics, such as those in 1918, 1957, 1968 and 2009.

Influenza adds another challenge, because different groups and subgroups of viruses can be circulating in the population at the same time. If someone is infected with a subgroup that resembles a virus with which they have already been infected or against which they have already been vaccinated, the immune system can recognize this and limit the disease. On the other hand, infection or vaccination against one virus may not effectively provide cross-protection against another virus.

Why you need a flu vaccine every year

The phenomena of antigenic drift and antigenic shift are why public health authorities recommend getting a flu vaccine every year. A global system of viral surveillance is used every year World Health Organisation holds expert meetings to assess which viruses to vaccinate against, based on several factors: which viruses circulate, which cause disease, and which vaccines can lead to cross-protection. WHO committees make recommendations, and each country then makes its 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 are transmitted through respiratory droplets and so on minimize the risk If you get the flu, 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 be transmitted through contact with surfaces touched by others who are infected, so wash your hands frequently, and especially after contact with common surfaces, such as door handles, toilet knobs, elevator buttons, etc. Finally, don’t forget to get your annual flu vaccine.

Predicting which viruses will circulate in a given year is not an exact science. Like stock market predictions, they are not always correct. Sometimes the authorities are right and the vaccine protects well. Other times not so much. As Yogi Berra once said, “It’s hard to make predictions, especially about the future.”