Overview

Influenza, commonly known as the flu, is a highly contagious infection that targets your respiratory system. It is prevalent worldwide, but due to the easy transmission of the virus, it has caused multiple worldwide pandemics in 1918, 1957, 1968, and 2009. The majority of people recover from the infection without treatment; however, it can still be detrimental on a global scale. Hence, learning about the basics of influenza and its’ common strains can help you be aware of how it could affect your health. 

What is Influenza? 

Influenza (the flu) is a highly contagious and infectious virus that targets your nose, throat, and lungs. As a virus, its primary mechanism of infecting someone involves entering your cells and hijacking them. On the outer surface of the virus, various proteins facilitate its entry. The main viral protein involved is Haemagglutinin (HA), a protein that is made up of two different subunits.¹ HA can bind to Sialic Acid (SA), a protein found on the surface of your cells. SA is typically observed to bind to different sugar compounds with two types of ‘linkages’. These linkages are like links in a chain that hold two things together. In this case, these linkages connect SA and sugars. The two main types of linkages are known as either alpha(2,3) or alpha(2,6) linkages.² Once HA binds to SA, it activates a mechanism that allows for the entry of the virus into your cells.

Once inside, the virus uses your cellular machinery to build viral replicas within the nucleus of infected cells³. The nucleus of a cell contains all its’ genetic information, a blueprint essential for life and cellular function. The virus manipulates your genetic information to reprogram the code to prioritise viral protein production instead of the regular cellular function. Large amounts of the virus are then created within the cell and they disperse from the original infected cell and spread from cell to cell, infecting other regions of your body. 

Common strains of Influenza

Influenza is categorised into 4 overarching types which are all genetically different, influenza A, B, C, or D. 

Influenza A

These viruses are the most common subtype of influenza and can infect humans as well as various animals, including birds and pigs. This strain is considered to be the most severe and causes global outbreaks.

Influenza A viruses are further classified into subtypes based on the combinations of different types of HA and another surface protein, Neuraminidase (NA). For example, there is a strain of influenza A commonly referred to as Swine flu or A(H1N1). H1N1 acts as a codename that tells us the specific components of the virus. In this case, H1N1 means the virus contains type 1 HA and type 1 NA. Knowing the specific types of HA and NA helps curate an appropriate treatment plan for the virus. Influenza A has 18 types of HA and 11 types of NA currently known,⁴ meaning there are a plethora of possible strains of influenza A. 

The World Health Organisation (WHO) states that the current most common influenza A strains are A(H1N1) and A(H3N2). A(H1N1) is also referred to as A(H1N1)pdm09 as it caused the global pandemic in 2009. Only strains of influenza A have been observed to cause pandemics. For example, the influenza pandemic in 1918 was due to an H1N1 virus, the 1957-1958 pandemic was due to the H2N2 virus, and the 1968 pandemic was due to H3N2.

Influenza B

In contrast to influenza A, influenza B is categorised based on its lineage instead of its’ subtype. These two lineages are genetically different and are known as the B/Yamagata lineage or the B/Victoria lineage.⁵ Both of these lineages circulate globally in today’s day and age, making up 23.4% of the average influenza cases worldwide.⁶ 

Influenza C 

This type of influenza infects humans but is less common and causes milder respiratory illnesses in comparison to influenza A and B.⁷ Hence, these strains are unlikely to cause epidemics/pandemics. These viruses are not categorised based on HA/NA subtypes, but six different lineages of influenza C have been classified so far.⁸

Influenza D

Five subtypes of influenza D have been identified.⁹ These viruses mainly infect cattle, pigs, and other non-human species.¹⁰ However, little is known about its effect on human health and its’ ability to cause illness. 

Strain variation

Antigenic drift and antigenic shift are two key processes that explain how influenza strains are created over time. These mechanisms are primarily seen in influenza A. The ability to change their genes and certain characteristics of themselves helps them avoid being identified by the immune system.

Antigenic drift 

Subtle changes in the DNA of certain proteins are caused by mutations over time, resulting in a new strain of influenza.¹¹ In this case, a mutation is like a typo. For example, if we take the word ‘near’ and then change the ‘n’ to an ‘h’, the result is ‘hear’. Of the four letters, only the first letter changes and the other three stay the same. However, this change in letters brings a whole new meaning to the word. Hence, if you think about mutations within the DNA of HA or NA, these changes bring about different components and overall structures of these proteins. 

Our immune system recognises and targets specific structures of viral proteins such as NA or HA, also known as antigens. Hence, if these antigens mutate or become structurally different, it becomes more difficult for our immune systems to recognise and respond effectively against them. As a result, individuals previously exposed to influenza or vaccinated against it may become infected with a drifted strain. This process results in seasonal flu outbreaks. 

Antigenic shift

In contrast to antigenic drift, the antigenic shift is sudden and occurs abruptly. This happens if an influenza virus that was previously known to only infect a certain species of animal, gains the ability to infect humans (zoonoses). Bird flu or avian flu is a prime example of zoonoses. According to the NHS, the four main avian flu strains have been observed to infect humans: H5N1, H7N9, H5N6, and H5N8. 

Another example of the antigenic shift is if two or more strains of influenza-infected the same cell, they would create a new subtype of the virus. It would contain a mixture of the antigens from the strains, creating a more genetically diverse strain of influenza. 

Transmission

Understanding the different modes of transmission of the virus is crucial for implementing preventive measures to reduce the spread of the virus. The seasonal flu spreads rapidly, especially in crowded areas. Infected individuals who cough, sneeze, or talk produce droplets that contain the influenza virus. These droplets can then be dispersed into the air and infect people through two main possible methods of transmission:¹²

Direct or Indirect Contact: Being in direct proximity with someone infected with the virus is a common mode of transmission. This can involve shaking hands with the infected person or touching surfaces contaminated with the viral droplets and then touching your mouth, nose, or eyes. 

Airborne transmission: Inhalation of the air droplets containing the influenza virus is another significant mode of transmission. When an infected individual coughs or sneezes, they disperse many droplets containing the virus into the air that you breathe into, leading to the further spread of the infection. 

Prevention

As seen before, influenza is capable of causing global pandemics. This means that following preventative measures help protect you from the flu. The following methods are a combination of preventative strategies from the NHS and Mayo Clinic to help prevent the spread of influenza:

Vaccination: The vaccine for the flu is updated every year and is available as an injection or nasal spray. It can protect those who have been vaccinated against the influenza viruses that are predicted to be the most common during the upcoming flu season. However, it is also important to take the following additional measures to help lower the spread of infection. 

Practice good hand hygiene: Wash your hands frequently, with soap and warm water, for at least 20 seconds especially after coughing, sneezing, or touching surfaces in public spaces. If water and soap are unavailable, alcohol-based hand sanitiser can be used until you can wash your hands. 

Cover your coughs and sneezes: Cough or sneeze into a tissue or your elbow and wash your hands directly afterwards. Additionally, if you go into public spaces, make sure to wear a face mask to help prevent the spread. 

Clean surfaces: Regularly clean surfaces that are often touched to prevent the spread of the virus from the surface to you. 

Social distancing: In crowded areas, the virus spreads even faster as people are nearby. By avoiding these busy spaces, you can lower your chances of infection. 

Summary

Influenza is a highly contagious respiratory illness that manifests into various strains, categorised as influenza A, B, C, and D. These strains undergo frequent genetic mutations through antigenic drift and shift, leading to seasonal outbreaks and occasional pandemics. Transmission occurs via water droplets that contain the virus, emphasising the importance of personal hygiene. Vaccination remains the most effective preventative treatment. However, additional precautions such as wearing masks, social distancing, and maintaining proper hand hygiene are also crucial in managing the spread of the influenza virus.