Common types of influenza viruses and the ability to prevent disease with vaccines

The article was professionally consulted with Senior Doctor, Dr. Vu Van Tam - Infectious Internal Medicine - Department of Medical Examination & Internal Medicine - Vinmec Ha Long International General Hospital.

Influenza is a respiratory infection caused by the influenza virus. However, not all flus are created equal and are caused by the same virus. Current flu types that can cause seasonal epidemics are influenza A and B viruses.

The flu is a contagious respiratory illness caused by a virus that infects the nose, throat, and sometimes the lungs. It can cause mild to severe illness, and can even lead to death.
Flu symptoms can be fever, chills, cough, sore throat, runny or stuffy nose, headache, fatigue... Some people may experience vomiting and diarrhea.
Influenza is spread mainly by droplets produced when an infected person coughs, sneezes or talks. These droplets can land in the mouths or noses of healthy people nearby. The less frequent way of transmission, is that a person with the flu touches surfaces or objects and then an otherwise healthy person touches those areas and then touches their mouth, nose, or eyes.

There are four types of influenza viruses: influenza A, B, C, and D viruses. The most common influenza viruses are human influenza A and B viruses that cause seasonal epidemics (known as seasonal flu).
Learn about the different types of flu: Influenza A, B, C and others
Influenza A virus is the only influenza virus known to cause pandemic influenza, i.e. a global epidemic of influenza. A pandemic can occur when a new and very different type of influenza A virus emerges, both infecting people and being able to spread between people.
Influenza C virus infections usually cause mild illness and are not thought to cause epidemics of influenza in humans. Influenza D virus mainly affects livestock and is not known to infect or cause disease in humans.
2.1. Influenza A viruses Influenza A viruses are divided into subtypes based on two viral surface proteins: hemagglutinin (H) and neuraminidase (N). There are 18 different hemagglutinin (H) subtypes and 11 different neuraminidase (N) subtypes (H1 to H18 and N1 to N11). While there are about 198 different combinations of influenza A subtypes, only 131 have been detected in the wild. Current influenza A viruses that circulate frequently in humans include: A (H1N1) and A (H3N2).
A subtype of influenza A can only be subdivided into different genetic clades, such as:
Influenza virus subtype: A (H1N1) → group: 6B.1 → subgroup: 6B.1A Viral subtype: 6B.1A : A (H3N2) → group: 3C.2a; 3C.3a → subgroup: 3C.2a1; 3C.2a2; 3C.2a3; 3C.2a4 The groups and subheadings may be alternately referred to as subgroups and subgroups, respectively. An influenza cluster is a subsequent subgroup of influenza viruses (outside of subtypes or by genera) based on the similarity of their HA gene sequences. Groups and subgroups are shown in the phylogenetic tree as a group of viruses that often have the same genetic change (i.e., nucleotide or amino acid changes) and have a single stub group represented as tree node. Dividing viruses into groups and subtypes allows influenza experts to track the proportion of viruses from different strains in circulation.
Currently, circulating influenza A (H1N1) viruses are related to the 2009 pandemic influenza virus that emerged in the spring. This influenza virus has the scientific name pdm09(A) virus and is commonly known as the 2009 H1N1 virus. It has continued to circulate since then. These H1N1 viruses have undergone relatively small genetic changes and alter their antigenic properties (i.e. the properties of the virus that affect immunity) over time.
Of all the influenza viruses that frequently circulate and cause disease in humans, influenza A (H3N2) viruses tend to change more rapidly both genetically and antigenically. Influenza A (H3N2) virus has formed many distinct, genetically different strains in recent years, and it continues to circulate.

2.2. Influenza B virus Influenza B virus is not divided into subtypes, but instead it is classified into B/Yamagata and B/Victoria lineages. Similar to influenza A viruses, influenza B viruses can then be classified into groups and subtypes. Influenza B viruses often change more slowly in genetic and antigenic characteristics than influenza A viruses, especially influenza A (H3N2) viruses.
Influenza surveillance data from recent years indicate influenza B virus co-infection from both strains in the United States and worldwide. However, the prevalence of influenza B virus from each circulating strain can vary by geographic location.

The Centers for Disease Control and Prevention (CDC) follows an internationally accepted naming convention for influenza viruses. And this formula was accepted by the World Health Organization (WHO) in 1979 and published in February 1980. The way to name each ingredient is as follows
Antigen type (eg A, B, C, D) Origin of the host (eg pig, horse, chicken...). For viruses of human origin there are no symbols, but designations of which origin are given. For example: Influenza A (H1N1) in ducks is A/duck/Alberta/35/76 or Influenza A (H3N2) in humans is A/Perth/16/2019
Geographical origin (eg Denver, Taiwan.. .) Numeric sequence (eg 7, 15...) Year of circulation For influenza A virus, description of hemagglutinin and neuraminidase antigens is provided in parentheses. For example influenza A virus (H1N1), influenza A virus (H5N1) . As for the 2009 pandemic influenza A virus, it was assigned a separate name: A(H1N1)pdm09 to distinguish it from the seasonal influenza A (H1N1) virus circulating before the pandemic.
When humans are infected with influenza viruses that normally circulate in pigs, these viruses are called variant viruses and are designated by a letter. An example is the A(H3N2) virus.

Influenza A (H1N1), A (H3N2), and one or two influenza B viruses (depending on the vaccine) are included in a seasonal flu vaccine. Getting a flu shot can protect against flu viruses like the ones used to make the vaccine.
However, the seasonal influenza virus vaccine does not protect against influenza B virus and influenza C virus. In addition, the flu vaccine will not protect against infection and illness caused by other viruses that can also cause flu-like symptoms. Because there are many viruses other than the flu that can lead to flu-like illness that spreads during flu season.
Learn about the different flu vaccines

Limit contact with infected people Stay home when sick Cover your nose and mouth when sneezing or coughing Wash your hands often Disinfect surfaces that may contain flu viruses Get a flu vaccine.

Reference source: cdc.gov