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Types of Influenza Viruses

Most of us have experienced flu at least once. It is a highly contagious infection that aims at the respiratory organs and involves a broad spectrum of people every year, regardless of age, ethnicity, or general health conditions. Several pandemics have spread worldwide and resulted in insane mortality rates. However, things have taken a turn for the better, thanks to the numerous medical advancements. The traits of influenza known to many of us include a one-week-long sickness along with a bunch of symptoms such as fever, headaches, fatigue, cough, and muscle aches. That being said, the flu is not caused by merely one type of virus. 


Influenza Virus Types

The flu is not a new phenomenon and dates back over a millennia ago. Hippocrates mentioned a kind of illness that shares similar grounds with influenza circa 410 B.C. According to him, this influenza-like disease was widespread around northern Greece. Besides the long time that it has been around, a broad range of species can get afflicted by influenza, meaning that it is not merely a human infectious disease. In short, four types of influenza viruses can cause illness in people and animals: influenza A, B, C, and D. Moreover, each can be divided into subtypes. In the same way, we can continue with this process of breaking the viruses down for a couple of more stages to access complete information about these viruses. This helps us initially in naming various types and developing efficient vaccines that provide robust protection.


Influenza A

Type A influenza is considered the most distinguished and the deadliest cause of flu infection and outbreaks. It plays a significant role in epidemics (flu season) and is the only type infectious enough to bring about flu pandemics. Pertaining to the glycoproteins on the surface of the virus, we can split this type into two subtypes: neuraminidase (N) and hemagglutinin (H). On top of that, there are 11 neuraminidases and 18 hemagglutinins which tells of 130 or more type A influenza combinations. That, however, does not mean umpteen varieties of influenza A threaten us. The common subtypes that involve people entail only N1 and N2, including A(H1N1) and A(H3N2). These protein strains tend to mutate constantly, which keeps the viruses a continuous threat. In addition, the classification can be broadened by two more groups introduced through further surveillance, including clades and sub-clades. These subdivisions do not belong merely to type A influenza viruses but also to type B viruses.


Influenza from Birds to Humans

The other combinations are recognized in animals, such as wild birds (avian flu) and pigs (swine flu). This characteristic is specific to type A influenza viruses and might result in reassortment in the genetic forms of the viruses. That means a mutation in gene segments that occurs when various viruses co-infect a host cell and leads to newly generated viruses. Avian influenza, or bird flu, is prevalent primarily among wild birds. It has three main subtypes that contain H5, H7, and H9, with the most common subtype of A(H5N1). Nevertheless, there have been reports of individuals who got severely infected with the subtype mentioned above. It is worth noting that these cases are rare. Besides, the transmission of the virus has been proven to be through direct contact with an infected animal, meaning that a person cannot pass on bird flu to another.


Influenza from Pigs to Humans

Like birds, pigs can get the flu via type A influenza viruses. The difference lies in the fact that pigs are prone to more threats, as they can get infected by human and avian influenza viruses too. Most swine flu epidemics occur during the late fall and winter. That should not be a serious concern for us since the cases of human beings infected by swine flu are very few. Nevertheless, one of the most outspread pandemics that took place back in 2009 to 2010 was initially caused by A(H1N1). This subtype primarily caused respiratory infection in pigs, and few people were immune to it. As a consequence, a large population of people, mainly children and young adults, went through swine flu infection. Over a decade, this virus has joined up with other viruses as one of the chief causes of seasonal flu.


Influenza B

Type B influenza viruses go hand in hand with type A viruses in terms of function and rate of transmission. They are one of the main initiators of epidemics that occur each year, and we should expect similar impacts from type B influenza viruses. Influenza B viruses can be transferred only from human to human and are categorized differently. The classification consists of two lineages of B/Victoria and B/Yamagata instead of subtypes. There are various type B influenza viruses from either of these lineages that function more or less in the same manner. That being said, the percentage of influenza B viruses from these two lineages can vary according to the variants of time and place. Moreover, influenza B viruses are less intended to change genetically than influenza A viruses.


Influenza C and D

Compared to the previously discussed types of influenza viruses, influenza C viruses are less prevalent. They were discovered in 1947 and can cause upper or lower respiratory infections and milder symptoms, making them less clinically threatening viruses. In addition to that, type C influenza viruses appear as a typical cold and very often spread among children younger than two years of age. With regard to such subdued effects, it is easy to guess that influenza C viruses are not robust enough to bring about epidemics. Lastly, influenza D viruses can run through cattle and pigs. Not so long ago, around 2016, type D influenza viruses were counted as subtypes for influenza C viruses. Yet, they are now a distinguished type with zero association with human beings. Although the main reservoirs of influenza D viruses are cows and pigs, they can go further as to infect other animals too.


Differences Between the Influenza Types

All influenza viruses target the respiratory system, usually the airway epithelial cells. However, they are different in several aspects, like the rate of spread, the severity, the age of the target group, and the various species of reservoir they infect. Influenza A outstrips the rest of the influenza types in many of these elements. It can infect humans, birds, pigs, and more and cause widespread epidemics and global pandemics. Besides, in comparison to influenza B, it can bring about more acute symptoms. Though highly contagious, influenza B is responsible for a considerably smaller percentage of overall infections yearly compared to influenza A. This proportion is much meager regarding influenza C, which threatens children more than other age groups. Neither type B nor type C can be transmitted between animals and humans, though there are a few cases of sea lions and seals diseased with influenza B and pigs and dogs with influenza C. Additionally, influenza D afflicts mainly cattle and sometimes pigs.


Transmission and Symptoms

The presupposed image we have about seasonal flu is more or less accurate about any influenza type that infects human beings. Any infected person could transmit the virus by producing droplets when sneezing, coughing, or even talking. The chances are you get infected indirectly as well, like coming to touch with a contaminated phone or door knob. Moreover, the symptoms usually begin furtively and lightly in the form of a runny nose or a tickle in the throat. Depending on your general health condition and a certain level of immunity that you might have acquired through a flu shot or natural antibodies, the symptoms may stay intact or step up to more severe ones, such as fever, muscle and headaches, chills, and overwhelming fatigue. The difference lies primarily in the severity of the symptoms, with influenza A causing the toughest recovery time.



The treatments and medications for different influenza types are also the same. Prescribed antiviral drugs could mitigate the symptoms along with home remedies, such as sufficient rest, painkillers, and fluids. That said, nothing can be as effective as a flu shot. Since influenza A and B are continually changing, lifetime protection is impossible, and thus, there is little room for neglecting flu vaccination. The currently available flu vaccines are all-embracing and provide immunity against a handful of different influenza viruses that cause epidemics. They include one influenza A(H1N1) virus, one influenza A(H3N2) virus, one influenza B/Yamagata lineage virus, and one B/Victoria lineage virus. Protection against these viruses underlies further protection against other antigenically similar influenza A and B variants. On top of that, the recently introduced biosimilars function equivalently and offer the same degree of protection. They require less development and research, resulting in more reasonable price tags. Many companies are now switching to biosimilar production as the trend is on the rise in developing countries. To give an example, Opal Biopharma is one of the leading companies in this field that has dedicated its R&D center and front-rank technologies to manufacturing biosimilar products.



Influenza A, B, C, and D comprise the four-piece spectrum of influenza viruses. The first two are more critical than the others as they are the only types that give rise to epidemics. Influenza A viruses are more diverse in subtypes and the range of species that can get infected. Additionally, Type A influenza virus strains can get through human beings even if they are essentially nonhuman, which is not the case with the other three types. The second most common type is influenza B which causes disease only in humans and, more specifically, in children. According to the data at hand, influenza A viruses are responsible for 75% of all flu virus infections, leaving the remaining 25% for the influenza B type. The most efficient way to fend off flu infection is yearly vaccination. A flu shot allows the body to produce antibodies against several influenza A and B viruses. Although the influenza vaccine does not guarantee complete immunity, it can alleviate the symptoms and the duration of infection to a large extent.

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