Influenza, also known as the flu or grippe, is an infectious viral disease that affects the upper respiratory system, including your lungs, throat and nose. Influenza is a respiratory infection commonly found in humans, pigs, horses and other poultry and wild birds. It is characterized by rapid onset, rapid infection, wide spread and threat to the safety of human life. According to international classification of icd-10, the disease code for influenza is j11-101.
2. Classification of Influenza Viruses
Influenza virus is an RNA virus of Orthomyxoviridae. The influenza virus is spherical, and the newly isolated strain is mostly filamentous, with a diameter between 80 and 120 nanometers, and a filamentous influenza virus with a length of up to 400 nanometers. Influenza viruses include human influenza viruses and animal influenza viruses. Human influenza viruses are divided into type A, B and C, which are the pathogens of influenza.
2.1 Influenza A Virus
Influenza A virus is an RNA virus with an eight-segment, single-stranded, negative-sense genome, belonging to the Orthomyxoviridae family. All subtypes of influenza A are known to exist in birds, particularly in waterfowl, and the virus can infect other animals such as pigs, horses, seals, whales and minks.
The influenza A virus genome consists of eight gene segments encoding 10 proteins: hemagglutinin (HA), neuraminidase (NA), matrix proteins M2 and M1, non-structural proteins NS1 and NS2, nucleocapsid and three A polymerase PB1 (polymerase basic 1), PB2 and PA (polymerase acid) protein . According to the protein structure of hemagglutinin (HA) and neuraminidase (NA) on its surface, it can be divided into many subtypes. So far, influenza A virus has found 16 subtypes of hemagglutinin and 9 subtypes of neuraminidase.
◆ Examples of Type A Virus
Avian Influenza: Avian influenza (AI) is an influenza A virus. Avian influenza viruses generally only infect birds. This interspecific transmission disorder is thought to be determined by a variety of viral genetic determinants, including viral HA and NA genes, as well as other internal genes such as nuclear proteins and PB2 genes. When the virus undergoes genetic reassortment during replication, resulting in structural changes, it is possible to acquire the ability to infect humans. The avian influenza virus subtypes that can directly infect humans are: H5N1, H7N1, H7N2, H7N3, H7N7, H9N2 and H7N9 subtypes.
Swine Influenza: Swine influenza (SI) is an acute, highly contagious, swine respiratory disease caused by the swine influenza virus of the Orthomyxoviridae, which is widely prevalent in pigs. The swine influenza virus is an influenza A virus. The main virus types are the classic swine H1N1, avian H1N1 and human H3N2 strains.
Both human influenza viruses and avian influenza viruses have established stable viral lineages in pigs, which may reflect the presence of both avian and human influenza virus receptors in pig epithelial cells . For these reasons, pigs are considered to be possible intermediate hosts (mixed containers) for the production of pandemic influenza viruses by recombination . The study  showed that the H1N1 virus genome is a mixture of avian influenza, swine flu and human influenza virus genes.
◆ Variation of Influenza A Virus
Type A influenza virus often has antigenic variation, which is highly contagious and spreads rapidly, and can easily occur large-scale epidemics. Many influenza pandemics in history have been caused by influenza A viruses.
The variation of influenza A virus is reflected in the generation of new virus types through genetic recombination. The 1957 pandemic and the 1968 pandemic flu virus was produced by genetic recombination   . In contrast, the 1918 pandemic was thought to be due to a purely avian influenza virus that directly adapted to humans for effective transmission .
Therefore, the pandemic influenza virus is a zoonotic disease, and the avian influenza virus plays a key role in its occurrence.
In addition, influenza virus RNA does not have a corrective function during replication, and its frequency of mutation is higher than other viruses. This is also the main reason for the flu virus variation.
2.2 Influenza B Virus
No other natural hosts have been found other than infected people. Influenza B viruses often cause local outbreaks and do not cause a worldwide influenza pandemic. The nomenclature of influenza B and C viruses is the same as that of influenza A viruses, but there are no subtypes.
2.3 Influenza C Virus
It can infect pigs as well as people. Type C influenza virus mainly appears in the form of dispersal, mainly affecting infants, and generally does not cause pandemic.
Figure 1 Typical structural features of influenza virus
8. How to Prevent The Influenza?
Flu is highly capable of being transmitted from person to person, and it is more important to actively prevent and control than limited effective treatments.
Vaccination is considered the most effective way to prevent influenza. The centers for disease control and prevention (CDC) recommends everyone six months or older get a flu shot. The annual seasonal flu vaccine contains protection from three or four flu viruses that are expected to be the most common during the flu season that year. The best time to get a flu shot is before the start of the annual flu season.
Type of influenza vaccine. Whole virus inactivated flu vaccine, pyrolysis inactivated flu vaccine, cold adaptation live attenuated vaccine and subunit vaccines based on influenza surface glycoprotein molecules HA and NA (mainly producing specific antibodies against HA and NA).
Universal influenza vaccine. The rapid variability of influenza viruses severely limits the long-term protection of vaccines. Existing influenza vaccines do not induce cross-protection between different subtypes and are faced with the problem of timeliness and effectiveness.
Corti et al  published the results of the flu study on Science - a superantibody FI6 antibody that neutralizes all influenza A viruses. The antibody is isolated from human plasma cells and is demonstrated by animal experiments to protect against the entire influenza A virus. This result may be a turning point in the development of a universal influenza vaccine.
In February 2018, the National Institute of Allergy and Infectious Diseases launched the Universal Influenza Vaccine Strategic Plan. Regarding the study of universal influenza virus, the first concern is the highly conserved M2 and NP proteins in all antigens of influenza A virus, which is the main candidate antigen for the current universal influenza vaccine. The M2 protein is a transmembrane protein, and the NP protein is a major component of the ribonucleoprotein RNP. Currently, vaccines developed based on M2, M2e and NP include fusion protein subunit vaccine , DNA vaccine  , recombinant virus-like particle (V1F) vaccine  and peptide vaccine  . These vaccines, based on M2, M2e and NP, can be induced to produce broad-spectrum cross-immune protection for different types or subtypes of viruses in animal models.
Influenza vaccine dose. Children between 12 and 35 months were given two doses, 0.25ml per dose, with an interval of one month. For children over 36 months and adults, 1 dose is given, 0.5ml per dose. Both children and adults are intramuscularly injected into the upper arm deltoid muscle. Never inject intravenously.
Vaccination may cause some symptoms, such as flu urticaria, caused by flu shots.
Vaccination prohibited population
Allergic to eggs or other ingredients in vaccines (such as neomycin).
Patients with Guillain-Barre syndrome.
Pregnant women. However, recent studies on the safety of vaccines in pregnant women have shown that TIV is safe for pregnant women,  . The standard trivalent inactivated influenza vaccine (TIV) is recommended for all pregnant women in the Northern Hemisphere and some regions of the European Union .
Patients with acute febrile illness.
Those with severe allergic constitution.
The person whom the doctor considers not suitable for inoculation.
Currently, the vaccination rate of influenza vaccine is generally low worldwide. In developed countries, the vaccination rate of influenza vaccine is about 30-40%. In addition to vaccines, the flu surveillance network is also important to prevent the outbreak of influenza.
8.2 Influenza Surveillance Network
In 2018, The Global Virome Project was launched and the flu epidemic was an important direction of the project. In fact, as early as 1952, the World Health Organization (WHO) established a global influenza surveillance network. Previously, Google Trend was able to determine the prevalence of influenza in a certain area based on the index of the user's search keyword.
8.3 Control The Spread of Infection
Influenza vaccines are not 100% effective, so it is important to take these measures to reduce the spread of infection:
Pay attention to personal hygiene and wash your hands.
Keep the environment clean and ventilated, minimize to places where people are dense and air is dirty.
Avoid contact with patients with respiratory tract infection.
Increase or decrease clothes according to the temperature, eating a balanced diet, strengthening exercise, ensuring sleep, and improving physical fitness and immunity.
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