How many types of influenza viruses are there?

Influenza viruses include three types: A, B, and C. Type A has the greatest antigenic variability. It infects humans and other animals, causing moderate to severe illness, affecting people of all age groups, and often causing global pandemics. Type B has a weaker variability and only infects humans, generally causing mild illness, mainly affecting children, and can cause local outbreaks. Type C antigenicity is relatively stable and only causes infection in infants and young children and sporadic cases in adults.

resistance

Influenza virus has weak resistance to the outside world and is quite sensitive to heat. The infectivity of the virus and the activity of its enzymes can be inactivated at 56°C for 30 minutes, 65°C for 5 minutes, or 100°C for 1 minute. The virus is relatively stable in a low-temperature environment and can survive for 1 week to 1 month if stored in a 4°C refrigerator. It can be stored for at least several years below -70℃.

The virus is relatively stable in the pH range of 7.0 to 7.8, and its infectivity is destroyed at pH 3.0. Ultraviolet rays, x-rays, etc. can inactivate influenza viruses. The virus is sensitive to chemicals such as ethanol, mercuric chloride, chlorine, acid, phenol, formalin, ether, and chloroform. Soaps and detergents can also inactivate influenza viruses.

Classification of influenza viruses

Based on the differences in viral antigenic characteristics and genetic characteristics, influenza viruses are divided into three types: A, B, and C. Influenza A virus is divided into many subtypes based on the differences in H and N antigens. H can be divided into 15 subtypes (H1~H15), and N has 9 subtypes (N1~N9). Among them, only H1N1, H2N2, and H3N2 mainly infect humans, while the natural hosts of many other subtypes are various birds and animals. Among them, the H5, H7 and H9 subtypes are the most harmful to poultry. Generally, avian influenza viruses do not infect animals other than birds and pigs. However, in 1997, Hong Kong reported its first 18 cases of H5N1 human avian influenza infection, including 6 deaths, which attracted widespread attention from the world. Since 1997, there have been several incidents of avian influenza virus infecting humans in the world. Once the highly pathogenic H5N1, H7N7, H9N2 and other avian influenza viruses mutate and become capable of transmission from person to person, they will cause an epidemic of avian influenza in humans, indicating that the avian influenza virus has become a great potential threat to humans.

Influenza virus naming

Based on the antigenic characteristics of hemagglutinin (HA) and neuraminidase (NA), influenza A virus is further divided into many subtypes. So far, 15 different hemagglutinin subtypes and 9 neuraminidase subtypes have been discovered. In 1980, WHO announced a new unified nomenclature for influenza viruses. The naming content and order of influenza A virus are: type/host/isolation location/strain number/isolation year (hemagglutinin antigen subtype and neuraminidase antigen subtype). If the host is a human, it can be omitted; other hosts must be specified. For example: A/Hong Kong/1/68 (h3n2). The nomenclature for influenza B and C viruses is the same as that for influenza A viruses, but since there is no H and N subtype division, they are not indicated, such as B/沪防/1/77; C/猪/京科/10/81. Sometimes, for the convenience of description, the H1N1, H2N2, and H3N2 subtypes are also referred to as A1, A2, A3 and other customary names in China.

Antigenic variation

The most important influenza virus mutation is type A, which is often closely related to global pandemics. Generally speaking, antigenic variation of influenza viruses refers to changes in the structure of h and n antigens. Small variations (quantitative changes) often occur within subtypes, which are called antigenic drift. Although it is only a minor mutation, it allows the virus to easily evade the host's immune system. If a mutation occurs in the hemagglutinin molecule-specific antigenic determinant (antigenic epitope) compared to previously discovered strains, the new strain is considered to be a heterologous variant of the previous strain, which has epidemiological significance and can cause an influenza epidemic.

Antigenic variation occurs only in type A viruses. It may be caused by two viruses from humans and animals infecting the same cell, resulting in genetic recombination between the viruses. The resulting viral hemagglutinin and neuraminidase undergo a completely new combination, rendering the population non-immune. Antigenic shift is responsible for global influenza pandemics. The influenza A virus undergoes a major mutation approximately every ten years. Since 1933, the influenza A virus has undergone four antigenic shifts: H0N1 (original influenza A, A0) from 1933 to 1946, H1N1 (subinfluenza A, A1) from 1946 to 1957, H2N2 (Asian influenza A, A2) from 1957 to 1968, and H3N2 (Hong Kong influenza A, A3) since 1968. Generally, there is an obvious alternation between the new and old subtypes. After a new subtype appears and becomes popular in a region, the old subtype can no longer be separated. There are also large and small variations among influenza B viruses, but these are not divided into subtype changes. No antigenic variation has been found in influenza C virus.