Influenza Hemagglutinin (HA) Subtypes and Flu Virus Strains: What You Need to Know

The flu is a highly contagious virus that affects the respiratory system. It can cause fever, muscle aches, and coughing. In some cases, it can lead to more serious health complications such as pneumonia. Each year, the flu virus mutates, meaning people must recieve a new vaccine against new strains of the virus. One important factor in determining how severe the coming flu season will be is the type of hemagglutinin (HA) subtype involved. In this blog post, we will discuss influenza HA subtypes and flu virus strains.

Flu virus Strains:

There are three types of flu viruses: A, B, and C. Influenza A is the most prevalent variety of flu virus. It causes the vast majority of flu-related hospitalizations and fatalities each year. Influenza B is less harmful than influenza A, but it can still be deadly in certain circumstances and does not branch into multiple subtypes. Influenza C is the least widespread variety of influenza virus, which usually produces minor respiratory complaint. Influenza A viruses are divided into subtypes such as H1N1 and H3N2, whereas influenza B viruses are split between lineages: B/Yamagata and B/Victoria, which are seasonal flu viruses.

The flu season generally begins in October and reaches its peak between December and February. However, the flu season is not always predictable. It can start as early as September and continue until May.

Influenza Hemagglutinin(HA)

There are many different subtypes of influenza A viruses. The most common subtypes that circulate among people are HxNx, where x is a number (e.g., H11N12). These subtypes are classified based on the proteins found on the surface of the virus hemagglutinin (HA) and neuraminidase (NA). HA protein is responsible for the process of binding Influenza virus to sialic acid on the surface of target cells, such as those in the upper respiratory tract or erythrocytes, and causes internalization of the virus, while the NA protein helps the virus spread from cell to cell. HA is also in charge of the viral envelope's fusion with the late endosomal membrane once it has been exposed to low pH (5.0-5.5).

HA is a homotrimeric integral membrane glycoprotein. It looks like a cylinder and has a length of about 13.5 nanometres. HA trimer is made of three identical monomers. Every monomer consists of a single HA0 polypeptide chain with both the HA1 and the HA2 regions connected by two disulfide bonds. The alpha helical coiled coil structure of the HA2 region sits on top of the HA1 domain, which is a tiny globular domain made up of a combination of α and β forms.

The inactive precursor protein HA0 is produced by the HA trimer to prevent any premature and unwanted fusion. Host proteases are required to break down the HA trimer so it can be infectious. The 23 residues near the N-terminus of HA2, known as the fusion peptide that is ultimately responsible for viral-host membrane fusion, are hidden in a hydrophobic pocket between the HA2 triad interface at neutral pH. The viral membrane is also represented by the C-terminus of HA2, which is known as the transmembrane domain.

Hemagglutinin(HA) Subtypes

• There are 18 different types of HA and 9 different types of NA, therefore, there are potentially 144 different subtypes of influenza A viruses. Influenza A virus types H1N1 and H3N2 are the two most common subtypes to infect people. HA proteins, are further classified into two groups: Group I and Group II. Group I consists of HxNy viruses, where y is a number (e.g., H11N12). Both human and animal cells are susceptible to these viruses, which can bind to them. Group II consists of HyAx viruses, where x is a number (e.g., H22A), which can only bind to human cells. Human influenza viruses contain the first three hemagglutinins, H1, H2, and H3.
• For each subtype viral infection, the hemagglutinin gene changes frequently, thus resulting in a large number of variants for the same subtype viruses and necessitating the need for virus strain replacement on an annual basis. Because of the virus's propensity to reassort, there may be many more influenza A subtype combinations in the future. Reassortment is the conversion of gene segments between influenza viruses. When two influenza viruses infect a host at the same time, they may swap genetic material and cause reassortment.
• A deadly variant of the H5N1 avian influenza virus that does not infect people can evolve and become capable of invading human cells. The high pathogenicity of the H5N1 strain may be attributed to the H5N1 virus hemagglutinin's ability to promote conversion of the proteinase-resistant form into an active version, which has been linked to the virus' ease of conversion to a virulent form.
• The hemagglutinin subtype of a flu virus is crucial in predicting how severe the flu season will be. Group I viruses (HxNy), for example, which can bind to both human and animal cells, are more likely to cause pandemics than Group II viruses (HyAx), which can only bind to human cells. Furthermore, certain HA subtypes are more dangerous than others. The HxNy subtype, for example, is linked to more serious flu symptoms than the HyAx variety.

Flu Vaccines

The flu vaccine is intended to protect people against the most prevalent flu strains that are expected to flourish throughout the following influenza season. The vaccine is made up of dead or weakened flu viruses that are intended to stimulate the immune system. Each year, the vaccine is updated to protect people against the most common flu strains.

Seasonal flu vaccines do not protect against seasonal influenza C or D strains, or against zoonotic (animal-derived) flu viruses that can cause human infection, such as variant or avian flu viruses.

Influenza Hemagglutinin related products

• Influenza A H1N1 Monoclonal Antibody (CPAB0715)
• Influenza-A H3N2 Monoclonal Antibody [PAT1B7AT] (CPAB0379)
• Influenza-A H5N1 Monoclonal Antibody [IA-H5N1] (CPAB0390)