Influenza: Decoding its Strains & Why Flu Vaccine Changes Yearly?

Posted on : August 25, 2023 by Clinic One on Flu Vaccines

What is Influenza?

Influenza is an acute respiratory illness caused by various types and sub-types of Influenza viruses. These viruses belong to the RNA family of viruses and can cause seasonal outbreaks of Flu. It can be easily transmitted from one person to another by the droplets released during talking, sneezing, or coughing.

What are the different strains of Influenza (Flu)?

Before describing the strain, its types and sub-types are important to be known. There are 4 types of Influenza viruses, and they are as follows:

  1. Influenza Type A
  2. Influenza Type B
  3. Influenza Type C
  4. Influenza Type D

Influenza Virus (Type A) is the most common type of virus known to cause the most severe outbreaks of influenza flu.  This virus was responsible for causing the Flu pandemic in 2009.

This type of virus is further classified into two sub-types:

  1. Hemagglutinin (HA)
  2. Neuraminidase (NA)

The strains causing Influenza Type A include H1N1, H5N1, and H3N2. Among these strains, H3N2 is found to cause more severe illness.

Influenza Virus (Type B) is another common type that causes localized outbreaks of the flu. Influenza type B is more common among children, while Influenza Type A is more common among adults.

Type B viruses can further be classified into 2 sub-types:

  1. Yamagata
  2. Victoria

Influenza Virus (Type C) is another type that is less common than types A and B and is known to be less severe and milder than Influenza type A and B. However, people with chronic respiratory illnesses such as asthma, chronic bronchitis, and pneumonia can be severely affected by this type of Influenza.

Influenza Virus (Type D) hasn’t yet been confirmed to infect human beings and is known to infect only some animals, such as Pigs and cattle (Influenza: Definition, Symptoms, Treatment, and Prevention | Britannica, 2023).

How can we protect ourselves from the Flu?

There are various preventive measures we can take for the prevention of influenza, such as wearing face masks during an outbreak, washing hands with soap and water or hand sanitizer after shaking hands with other people.

Caretakers of infected people should practice very good hygiene practices. However, the best way to protect ourselves from the Flu is to get a Flu shot or vaccination every year.

How is the Flu vaccine made?

Influenza flu is a seasonal acute respiratory infection that can be caused by 4 different types of influenza virus namely; A, B, C, and D. Influenza vaccines are designed to protect people from all of these types of viruses. Thus, the Influenza virus is also known as the quadrivalent vaccine.

These vaccines may either be egg-based, recombinant, or inactivated and the additional components within the vaccine may vary on the basis of the strain causing the flu (Influenza Vaccine Options: 2022-2023 Season – NFID, n.d.).

Why do we need to get vaccinated every year for the same infection?

Now that we have come to know about different types and sub-types of Influenza Flu along with its strain, it is not possible to predict which type or strain of Influenza will cause the flu outbreaks.

Every existing virus and microorganism has the capacity to multiply itself creating resistance to the existing vaccines. Flu viruses are among them. They are able to mutate and new strains of viruses with different characteristics are likely to emerge every year (Why the Flu Vaccine Is Reformulated Every Year, n.d.).

The re-emergence of the same virus can also build resistance to the existing vaccinations over time resulting in little to no effectiveness of the vaccine. Thus, there is a need for vaccines to change according to the emerging strains or variants of the flu virus.

The components of the vaccines need to be reformulated considering various different factors that might have contributed to changes in the nature of the virus.

In which season does an influenza outbreak occur?

It is necessary to know that its outbreak is seasonal and can re-emerge during any season throughout the year. However, the Winter seasons are more prone to the flu outbreak.

Can we get the flu even after the vaccination?

Receiving a Flu shot doesn’t mean you will not catch the flu. However, the illness’s severity can be minimal and negligible. Sometimes, you may not even realize you are sick.

There are some rare cases when the vaccine might not be effective, for example;

1. If the vaccine hasn’t been reformulated according to the nature and characteristics of the strain of virus.

2. If the flu-causing strain has been mutated and has become resistant to the vaccination received.

Other than these cases, a vaccine is always the best preventive option one can undertake to stay protected from the flu.


In a nutshell, an influenza virus is a seasonal flu caused by a group of 4 different types of viruses. Vaccination is one of the best ways to get rid of seasonal Flu. The vaccines for the virus are not always the same and require reformulation every year on the basis of changes in the strain.

Key Points:

  1. An infected person can spread the flu before the occurrence of any symptoms.
  2. Flu severity differs from person to person.
  3. The H3N2 strain of the influenza virus is known to be severe.
  4. Taking a flu shot every year is the best way to stay protected.
  5. Viruses can mutate and create resistance to the existing vaccines.
  6. One Flu vaccine can protect people from 4 different types of Influenza.
  7. A flu vaccine is required every year to stay protected from emerging new strains.


Influenza | Definition, Symptoms, Treatment, & Prevention | Britannica. (2023, August 16).

Influenza Vaccine Options: 2022-2023 Season—NFID. (n.d.). Retrieved August 24, 2023, from

Why the Flu Vaccine Is Reformulated Every Year: East Meadow Medical P.C.: Primary Care Physicians. (n.d.). Retrieved August 24, 2023, from

Related Articles:

इन्फ्लुएन्जा (Influenza) बारे जान्नै पर्ने महत्वपुर्ण कुराहरू

इन्फ्लुएन्जा (Influenza) सामान्यतया फ्लू (Flu) भनेर चिनिन्छ । इन्फ्लुएन्जा (फ्लू) विभिन्न प्रकारका भाइरस र त्यसका उप-प्रकारका कारणले हुने तीव्र श्वासप्रश्वाससम्बन्धी […]


The Importance of Annual Flu Vaccination: Myths vs. Facts

As flu season approaches, the topic of flu shots returns to the forefront stage. Every year, myths and facts regarding […]


Flu Vaccine/Influenza Vaccine 2023/2024 Batch Available at Clinic One

In a continuously battling world for health concerns, one villain returns year after year. That is the flu, or influenza […]



In the realm of infectious diseases, two prominent adversaries have been making headlines: Seasonal Flu and COVID-19 Flu. While they […]


Flu season preparedness: Understanding flu & tips for staying healthy during flu season

Influenza commonly referred to as flu is an infectious viral disease caused by the Influenza virus that infects the nose, […]


Leave a Reply

Your email address will not be published. Required fields are marked *

Contact Us

Select Language