Types of Influenza Virus and Their Characterization

Introduction

Influenza viruses (flu viruses) are of different types, of which only two types (type A and B) are mainly responsible for human infections. These viruses usually affect the respiratory tract causing fever and other respiratory symptoms like cough, cold, sore throat, etc. There are flu vaccines available for protection. However, chances for reinfection after vaccination or previous exposure to the virus do exist. This occurs as the virus continuously undergoes genetic evolution and changes in its antigenic characteristics. Therefore, knowing the genetic and antigenic characterization of the flu virus is important for proper vaccine development, control of infection, and prevention.

What Are the Different Types of Influenza Viruses?

1. The influenza virus belongs to the viral family Orthomyxoviridae. Influenza viruses are of four types - type A, B, C, and D.

2. Influenza viruses type A and B cause seasonal epidemics (flu season). Influenza type A is the only type that can cause a flu pandemic.

3. Influenza virus type C causes mild illness and does not cause a human epidemic.

4. Influenza virus type D mainly affects cattle.

5. Influenza virus type A is subdivided based on viral surface proteins. There are two types of surface proteins namely, hemagglutinin (HA/H) and neuraminidase (N/NA). HA is again divided into 18 subtypes and NA into 11 subtypes. More than 130 influenza A subtypes combinations have been identified. When two influenza viruses infest a host at the same time, swapping of genetic information can take place. This process of swapping genetic segments is called Reassortment.

6. Influenza virus type A subtypes are further divided into genetic clades and sub-clades. Clades and sub-clades are also called groups and subgroups. Influenza type A subtypes that commonly circulate among people are A (H1N1) and A (H3N2). Influenza type B is further divided into two lineages - B/Yamagata and B/Victoria.

7. Influenza type B is further divided into clades and subclades. Influenza B virus changes more slowly (based on genetic and antigenic properties) when compared to the Influenza A virus.

8. Virus Nomenclature (naming).

9. The Centres for Disease Control and Prevention (CDC) takes into account the following guidelines in naming the virus:

• The type of antigen (A, B, C, and D).

• Geographical origin (for example Taiwan).

• Strain number.

• The host of origin. If the host of origin is human, no designation is given.

• The year of collection.

• Variant viruses are designated by the letter ‘v’.

• The HA/H and NA/N description of type A is mentioned in parenthesis (for example influenza A(H5N1) virus. Eg. A/Sydney/05/97(H3N2). Here A stands for the viral type, Sydney is the geographical origin, 05 is the strain number, 97 represents the year of collection, and (H3N2) represents the subtype.

What Does the Term ‘Drift’ and ‘Shift’ Mean?

The influenza virus is under constant change. There are two ways by which this change happens:

1. Antigenic Drift: Mutations that take place in the genes of the influenza virus leads to changes in viral surface protein (HA and NA). The viral surface proteins (HA and NA) are antigens. Antigens are proteins capable of producing antibodies by evoking the body’s immune response. These small changes in genetic material over time can result in viruses that are closely related to one another. These small changes can accumulate with time and result in viruses that are antigenically different. Due to this, an immunity previously acquired against the virus will be ineffective later, as the virus has become antigenically different.

2. Antigenic Shift: Antigenic shift is a major abrupt change that takes place in the influenza A virus. The antigenic shift can result in a new subtype that is infecting the population for the first time.

What Is Antigenic Characterization?

Antigens are proteins that can trigger an immune response in the host and result in the production of antibodies. The influenza virus A has two surface antigens HA and NA. The immune response produced by the viral antigen is called its antigenic properties. Analyzing the virus’s antigenic properties to assess its relation with another virus is called antigenic characterization. Antigenic characterization helps in determining the flu vaccine's ability to produce an immune response against the circulating viral infection. It also helps in determining the viral composition for upcoming flu vaccines.

Hemagglutination Inhibition Assay (HI Test)-

HI test is the test used for antigenic characterization of the flu virus. HA (Hemagglutinin) proteins are surface proteins found in influenza viruses. This protein binds to RBCs (red blood cells) and forms a lattice structure through a process called hemagglutination. HI test measures how well antibodies can bind to HA proteins. The binding of antibodies to HA protein prevents hemagglutination (that is binding of RBCs and HA proteins). This process is called hemagglutination inhibition.

HI test consists of three components:

• RBC - Taken from animals (guinea pig or turkey).

• Influenza Virus - Obtained from infected humans (usually from respiratory samples).

• Antibodies - Obtained by infecting an immunologically naive animal.

All three components are mixed in a microtiter plate for the test. In cases where the circulating influenza virus is antigenically different from the vaccine, the antibody produced as a result of the vaccine will not bind with the new virus. When the HI test is done in such cases, hemagglutination occurs. Antigenic characterization help in determining whether a vaccine that contains certain vaccine viruses will be effective for the newly circulating virus.

What Is Genetic Characterization?

The process of comparing the genetic sequence of the currently circulating flu virus with genes of the older flu viruses and also with other viruses used in vaccines is called genetic characterization.

Genetic characterization is used for the following purposes:

• To determine the evolution and changes in the flu virus.

• To determine the genetic similarity between the influenza virus.

• To assess the efficacy of the flu vaccine (by determining how good the vaccine is in protecting against the particular flu virus based on genetic similarity).

• To evaluate genetic changes in the flu virus that infest animal populations (their genetic changes may enable these viruses to infect humans).

• To determine the relationship between genetic changes and viral properties, genetic changes that cause more severe diseases, or resistance to antiviral drugs, etc.

1. Phylogenetic Tree-

Viruses can be organized graphically to study the genetic differences between groups of viruses. This graphical representation is called a phylogenetic tree. These are similar to family trees we use for human studies. The nucleotide changes within the gene are used for comparing viruses. Viruses that have common ancestral origins belong to the same clade.

In the phylogenetic tree, the length of the horizontal line represents the degree of genetic differences between the virus. The more the distance between the viruses on the phylogenetic tree, the more the viruses are different from each other.

The data from genetic characterization, antigenic characterization, and other data (like human serology data) are used for vaccine selection (for determining the vaccine viruses for each year’s flu vaccine). The Centers for Disease Control and Prevention (CDC) collects genetic data by genetic characterization of the flu virus yearly.

2. Flu Genome Sequencing-

Previously, a sequencing technique called the Sanger method was used by scientists to study the genetic evolution of the flu virus. A viral sample taken from a patient may contain many flu viruses having small genetic differences from one another. The Sanger method helps in determining the predominant genetic sequence from the sample. Currently, newer techniques are used for genetic characterization. Next-generation sequencing is one such technique that helps to detect small genetic variations in the flu virus.

Conclusion

As the virus is undergoing continuous genetic changes, an older vaccine may not be effective for the newly circulating virus. The data collected from genetic characterization, antigenic characterization, and other data (such as serology data) are essential for determining the viral composition of each year’s flu vaccine.