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How Many Strains Of The Flu Are There?

Are Flu Vaccines Always Effective?

As the temperatures start to drop, cases of the flu become more and more common, causing a sore throat and chills in those infected. While we commonly think of the flu as being caused by just one virus, there are actually many different variations of the virus. So, how many strains of the flu are there?

What Is the Flu?

The flu, sometimes referred to as influenza, is a highly contagious illness that primarily affects the respiratory system. It is caused by the influenza virus and is highly volatile, mutating each year through antigenic shift or antigenic drift.

While anyone can catch the flu, the disease is especially dangerous for children under the age of five, immunocompromised individuals, people with underlying medical conditions, and the elderly.

What Are the Symptoms of the Flu?

While the flu is primarily a respiratory illness, it can cause symptoms that affect the entire body. The primary symptoms of the illness are a cough, a runny or stuffy nose, and nasal congestion, but it is also common for people to have a fever, body aches, headache, and fatigue.

Some people may also experience symptoms that include nausea, diarrhea, and vomiting, although these symptoms are more common in children than in adults.

How Many Strains of the Flu Are There?

While we commonly refer to “the flu” as if it is just caused by one virus, the flu can actually be caused by different strains of the influenza virus. There are four main types of influenza viruses, referred to as influenza A viruses, influenza B viruses, influenza C, and D.

Influenza A and Seasonal Flu

Influenza A is one of the two types of the virus that are responsible for the seasonal epidemics that emerge in the United States nearly every winter. However, of these two types of the virus, only influenza A viruses are associated with global pandemics of the flu.

Flu pandemics occur when a new strain of the influenza A virus emerges that the majority of the population has not been exposed to. When the virus is highly infectious and spreads rapidly among people, a pandemic can occur.

There are different subtypes of influenza A that are classified based on specific proteins on the virus’ surface: hemagglutinin (H) and neuraminidase (N). There are different subtypes within the H and N designations, allowing for a possible 198 different combinations of influenza A.

However, only 131 subtypes have been detected in humans to date, including H1N1 and H5N1, both of which have an N1 mutation.

Two of the most common subtypes of influenza A areA(H1N1), better known as the avian flu, which first emerged in 2009, and A(H3N2), which was first discovered in 1968.

The A(H1N1) virus that is currently circulating today has mutated slightly, undergoing small genetic changes that allow it to continue to infect people, even when the person has been vaccinated or has already been sick with the virus.

However, the A(H3N2) virus is the most rapidly changing of the influenza A viruses, with many variations circulating simultaneously. This tendency to undergo genetic change is part of what makes influenza A so dangerous.

Influenza B

Like influenza A, influenza B viruses circulate among humans and contribute to the millions of cases of the flu in the United States each year. However, influenza B is not associated with global pandemics. Unlike influenza A, which is classified into subtypes, influenza B viruses are classified into two different lineages: B/Yamagata and B/Victoria.

Compared to influenza A viruses, which are known to mutate rapidly, influenza B viruses change more slowly over time. Currently, there are a number of different B/Yamagata and B/Victoria viruses in the United States and around the globe. Because these viruses change more slowly and are not spread as rapidly, they are not associated with pandemics.

Influenza C

Influenza C viruses are the last type of influenza virus that affects humans. Compared to influenza A and influenza B, influenza C viruses are generally associated with only mild respiratory symptoms. Many people infected with influenza C have no symptoms at all. As a result, this type of influenza is considered much less dangerous than influenza A or B and does not contribute to epidemics or pandemics.

Influenza D

Influenza D is a type of influenza that is not known to infect or cause illness in humans. Instead, this type of the virus most commonly infects cattle and other forms of livestock.

How Do Flu Viruses Get Their Names?

If you’ve watched or read the news during a particularly bad flu season, you’re likely to have heard the newscasters refer to that year’s strain of flu by a specific name, such as H1N1.

In fact, theWorld Health Organization established a global naming convention for influenza viruses in order to ensure that countries are able to communicate effectively about the viruses each year. The naming convention provides a great deal of information about the specific type of flu going around in a given year.

The naming convention for the flu uses the following steps:

  1. List the antigenic type first (influenza A, B, C, or D)
  2. If the flu has originated from an animal, list the name of the animal (swine flu, avian flu, etc). Flu viruses that originate in humans do not list the origin.
  3. Next, list the name of the location where the flu is believed to have originated, such as a city or country.
  4. List the strain number.
  5. List the year the virus was isolated.
  6. If the virus is influenza A, list the combination of hemagglutinin and neuraminidase as a description in parentheses.

An example of a full name for seasonal influenza originating in humans would be A(H3N2), A/Denver/12/2021.

In some cases, viruses that are associated with significant spread, such as a pandemic, are given a distinctive name that allows them to be more easily identified compared to other related viruses.

How Do Scientists Choose Which Strains of the Flu To Include in the Vaccine?

With so many strains of the flu virus, you may be wondering how it is possible to get vaccinated against a disease that is constantly changing. In fact, the changing nature of the influenza viruses is the reason why a new flu vaccine is released each year.

Typically, the annual flu vaccine contains three to four strains of the flu that are anticipated to be especially widespread and dangerous in the upcoming flu season; this information is published by the World Health Organization and changes each year depending on the spread of various strains.

The vaccine typically includes one strain of influenza A(H1N1), one strain of influenza A(H3N2), and one to two strains of influenza B. Flu vaccines do not include strains of influenza C or D, as these types of the virus are not considered dangerous for humans.

Production of the flu vaccine in the northern hemisphere typically begins in February, about eight months prior to the start of the flu season. Scientists use the best available information to carefully select which strains of the flu they anticipate to be highly contagious and dangerous during the upcoming year.

However, the types of flu viruses may mutate over the course of the months in which the vaccine is developed, which can impact how effective the vaccine is in preventing the flu.

How Effective Is the Flu Shot?

Development of the flu vaccine takes many months, so scientists use the best available information that they have at the time to determine which strains of the virus to combat with the vaccine. However, this information is not perfect and the virus can change over time.

As a result, the Center for Disease Control and Prevention (CDC) estimates that the flu vaccine is anywhere from40 to 60 percent effective, depending on the season. The vaccine is most effective in preventing illnesses caused by influenza A(H1N1) and influenza B viruses and is less effective at preventing infection by the influenza A(H3N2) viruses.

Should You Get a Flu Vaccine?

Although the flu vaccine is not 100 percent effective, there are a number of benefits associated with getting the vaccine, particularly for those who are considered at risk of complications from the flu, such as young children, immunocompromised individuals, pregnant women, the elderly, and people with chronic health conditions.

The flu vaccine can help prevent you from getting sick with the flu in the first place, and if you do get sick, you may experience more mild symptoms and recover more quickly. The flu shot is especially important during the coronavirus pandemic to prevent complications.


There are four types of the flu, including influenza A, B, C, and D. Of these, only influenza A, B, and C affect humans. Influenza A is responsible for many epidemics and global pandemics, while influenza B typically does not occur on a global scale. Both types of the virus have the potential to cause serious, life-threatening illness.

The best way to minimize your risk of catching the flu is to get a flu vaccine, which is developed each year based on predictions of the most contagious and dangerous strains for the upcoming flu season.


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