Influenza (Flu) Viruses: Types, Symptoms, Naming and Vaccine

Influenza or flu is caused by influenza viruses. It is a contagious respiratory illness and may lead to mild to severe illness. Let us read in detail about Influenza (flu), types of influenza viruses and symptoms.
Created On: Mar 24, 2020 18:35 IST
Modified On: Mar 24, 2020 18:38 IST
Influenza (flu) Viruses
Influenza (flu) Viruses

Influenza viruses can circulate in all parts of the world. Serious flu infection can result in hospitalisation or even death. People who basically are at risk due to serious flu infection include older people, young children, and people with certain health complications.

Influenza Viruses: Types

There are four types of influenza viruses namely A, B, C and D. Influenza A and B viruses circulate, causes acute respiratory infection and the seasonal epidemic of disease. Or we can say that Human influenza A and B viruses cause seasonal epidemics of disease.

Influenza A viruses: These are the only influenza viruses that cause flu pandemics that is global epidemics of flu disease.

Here the question arises what is a pandemic?

When a new and very different influenza A virus emerges and both infects people. Also, has the ability to spread efficiently among people. Then it is said to occur pandemic.

Influenza A viruses according to the combinations of the hemagglutinin (HA) and the neuraminidase (NA), they are further classified into subtypes. HA and NA are the proteins on the surface of the virus. Influenza viruses that circulated in humans are subtype A(H1N1) and A(H3N2). A(H1N1) is also written as A(H1N1)pdm09 as it caused a pandemic in 2009 and replaced the seasonal influenza virus A(H1N1) that was circulated prior to 2009. Let us tell you that only influenza type A viruses are known to have caused a pandemic. 

Note: There are 18 different hemagglutinin subtypes and 11 different neuraminidase subtypes. Potentially there are 198 different influenzas A subtype combinations, only 131 subtypes have been detected in nature.

Influenza B viruses: They are not classified into subtypes and can be broken down into lineages. Influenza B type virus belongs to either B/Yamagata or B/Victoria lineage.

Influenza C viruses: This type of virus is detected less frequently and usually causes mild infections and does not present public health importance. Or we can say that they do not cause human flu epidemic.

Influenza D viruses: They primarily affect cattle and does not infect or cause illness in people.

Influenza Viruses


The above graph clearly shows that only two types of influenza viruses namely A and B cause most human illness and are responsible for the flu season each year.

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Influenza Viruses: Symptoms

As we know that seasonal influenza is characterised by sudden onset of fever, cough (mainly dry), headache, muscle and joint pain, feeling of unwell, sore throat and a runny nose. It is also said that cough can be severe and can last for 2 or more weeks. In fact, most of the people recover from fever and other symptoms within a week without the need of medical attention. But for the people at high risk, influenza can be dangerous and can cause severe illness or death.

Let us tell you that illness range from mild to severe and even death. According to WHO, in the whole world, these annual epidemics are estimated to result in about 3 to 5 million cases of severe illness, and about 290000 to 650000 respiratory deaths.

Most of the deaths in industrialised countries are associated with influenza and occur among people age 65 or older. No doubt epidemics may result in high levels or worker/school absenteeism and productivity losses.

Seasonal influenza spreads easily with the rapid transmission in crowded areas like schools, nursing home, and markets. As we know that when an infected person coughs or sneezes the droplets that contain virus are spread in the air and can even spread up to one meter, and infect persons who are close and also to those who breathe these droplets in. A virus can also spread by hands contaminated with influenza viruses. Therefore it is recommended to prevent transmission; people should cover their mouth and nose with a tissue when coughing, and wash their hands regularly.

In a temperate climate, seasonal epidemic mainly occurs during winter and in tropical regions, it can occur throughout the year.

The incubation period is the time from infection to illness that is about 2 days but ranges from one to four days.

Influenza Viruses: Naming

There is an internationally accepted naming convention for influenza viruses. In 1979, this convention was accepted by WHO and published it in February 1980 in the Bulletin of the World Health Organisation, 58(4):585-591 (1980).

Following components are used:

Naming of the virus


The antigenic type (like A, B, C, D).

- The host of origin (like swine, chicken, etc.). There is no host of origin designation for human-origin viruses. Examples:

(Duck example): avian influenza A(H1N1), A/duck.Alberta/35/76

- Geographical origin (Like Denver, Taiwan, etc.)

- Strain number (like 7,15, etc.)

- Year of collection (like 57,2009. etc.)

- For influenza A viruses, the hemagglutinin and neuraminidase antigen description are given in parentheses.

- There was a different name assigned to the 2009 pandemic virus that is A(H1N1)pdm09 to differentiate it from seasonal influenza viruses A(H1N1), etc.

- A variant virus is when influenza virus infects humans that normally circulate in swine (pigs). They are designated with a letter 'v'. Like A(H3N2)V virus.

Influenza Viruses: Vaccines

Few influenza viruses are associated with seasons influenza vaccines like one influenza A(H1N1), one influenza A(H3N2) and one or two influenza B viruses. A flu vaccine can protect against flu viruses that is it is like viruses used to make a vaccine. Seasonal flu vaccines don't protect against influenza C or D viruses. Therefore, we can say that flu vaccines will not protect against infection and illness that is caused due to other viruses and causes influenza-like symptoms. There are several other viruses besides influenza that can result in influenza-like illness and can spread during flu season.

Therefore, now you may have come to know about flu or influenza viruses, their types, symptoms, etc.

Source: who,

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