Influenza Viruses: What’s in a Name?

If you’re wondering what the different letters and numbers used to name flu viruses like H1N1, H3N2, H5N1 and influenza A, B, C, or D mean, you’re probably not alone. The letters and numbers used for flu viruses are significant. Depending on the type of flu virus 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 illness.

H5N1 avian flu has received a lot of attention in the news recently, but it’s also important not to forget the ever-present threat of seasonal flu. We are seeing a gradual increase in flu infections across the country, especially in the South and West, as is typical this time of year. As families and friends gather in crowded, close quarters for the holidays, one thing they may not think about is the risk of transmitting the flu and other respiratory viruses. Additionally, because people travel far and wide this time of year, they are unknowingly facilitating the spread of viruses over great distances.

The name of a typical flu virus has several 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 usually infect livestock 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 off the name, viruses are given an H and N number to indicate a subtype. A full name might look like this: A/Sydney/05/97 (H3N2), meaning a type of influenza A virus, type 05, discovered in Sydney, Australia in 1997, and a subtype with antigens H3 and N2.

What does H represent?

The H stands for hemagglutinin, which is a type of protein found on the surface of flu viruses. The term “hemagglutinin” is derived from two parts of the word: one related to blood and the other to “agglutination”. Therefore, hemagglutinins have the ability to cause red blood cells to clump together.

Importantly, 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 appropriate receptors on the cell surface, infection will not occur. Once inside our cells, viruses can produce hundreds to thousands of progeny, thus causing an active infection. The number after the letter indicates which specific hemagglutinin an influenza virus possesses. Currently, 18 influenza hemagglutinins have been identified.

What does N stand for?

N stands for neuraminidase, another protein on the surface of influenza viruses that is key to the flu’s ability to spread from one cell to another. Once the flu enters a cell, it makes copies of its own genetic material and creates new flu viruses. Those viral progeny will exit the cell, but they are unable to be released from the cell surface without neuraminidase, which facilitates release, allowing them to find and enter a new cell and begin the cycle of infection from the whole. There are 11 distinct types of neuraminidase.

Significance of H and N

Our bodies do not take infection lying down. We have a response against viral hemagglutinin and neuraminidase. This helps prevent someone from getting infected by the same virus again; however, there is a catch. One of the main challenges with fighting flu viruses is that when they reproduce, they make mistakes that can lead to subtle changes in their genetic material, which is RNA. The millions of infections that occur worldwide each year 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 get infected again – you may not be as sick as you were a year ago because your immune system has mounted an effective response to the virus or to a vaccine, but the virus may be different enough to make you still infected.

To make things more complicated, every once in a while, we have a complete change in the flu viruses circulating. For example, one year we may have H1N1 viruses mostly in the population, but the next year, something like H2N2 appears. We call it an “antigenic shift.” This is a more worrisome situation because a significant portion of the population may not have been recently exposed to new viruses, and are therefore susceptible to infections and potentially severe infections. 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 circulate simultaneously among the population. If someone is infected with a subtype that is similar to a virus they have already been infected with or vaccinated against, their immune system can recognize it 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 shot every year

The phenomena of antigenic shift and antigenic drift are the reason why public health authorities recommend getting a flu shot every year. Each year, using a global viral surveillance system, the World Health Organization holds expert meetings to assess which viruses are appropriate for vaccination based on several factors: which viruses are circulating, which are causing disease, and which vaccines can lead to in cross defense. WHO committees make recommendations and each country then makes its own decision. In the United States, the FDA makes the final decision about what to include in American vaccines.

What you can do to protect yourself

Viruses are transmitted through respiratory droplets, so to minimize the risk of catching 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 regularly, and especially after touching shared surfaces, such as doorknobs, toilet handles, elevator buttons, etc. Finally, don’t forget to get your annual flu shot.

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