Why flu changes every season?

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The answer to the question “Why influenza changes every year?” is not that difficult. It is quite similar to that of why two human beings are not alike? The answer to both the questions can be derived from genetics. Two human beings are not the same because their genetic makeup is not identical, and the same applies for two influenza viruses isolated during any two consecutive outbreaks. The viruses undergo a number of changes in their genetic makeup during the process of infecting a human being. The genes undergo mutation, rearrangement and recombination; and as genes are responsible for the antigens expressed on the capsule of the virus, there occur changes in the surface hemagglutinin (H) and neuraminidase (N) antigens resulting in emergence of a new antigenic variant. There are different mechanisms by which the genetic variations occur in influenza virus, two patterns have been observed- ANTIGENIC DRIFT and ANTIGENIC SHIFT. The virus endowed with these two potential lethal weapons creates havoc in the environment.

ANTIGENIC DRIFT is a gradual, sequential change in antigenic structure that occurs regularly at frequent intervals. The new antigens are different from the previous antigens but they are related to them. As a result, they react with antibodies against predecessor virus strains to varying degrees depending on the degree of similarity between the new and old strains. The immune system is faltered by these “new” strains as it is unable to identify them. This inability on the part of the immune system adds to the advantage of the virus in establishing the infection. Antigenic drift involves MOSTLY the H antigens and sometimes the N antigen, but never both.


The mechanism behind antigenic drift involves point mutations in the RNA segments of the virus and selection of the strains not reacting with the antibodies against the predecessor viruses. By this process, only those strains are selected that are capable of producing infection in humans. This pattern of antigenic variation accounts for the periodic epidemics of influenza and the small scale outbreaks in the society.

On the other hand, ANTIGENIC SHIFT is a major change in the genetic constitution of the virus resulting in an abrupt, drastic and discontinuous variation in antigenic structure of the virus. This results in a virus strain that is completely unrelated to its predecessors. Antigenic changes usually affect BOTH the H and N antigens.

MECHANISMAND OUTCOME OF ANTIGENIC SHIFT-Antigenic shift involves a different mechanism as genetic mutations cannot explain such major variation in antigens. Possibly, there occurs genetic recombination in the genes. It may occur when two strains affect a single cell and there happens exchange of genetic materials between them during this process. These antigenic shifts are the founding stones in starting a pandemic. It is also proposed that pandemic strains might originate from some animal or avian influenza strains that gain the ability to infect humans through recombination between human strains and these non-human strains.

 IN A NUTSHELL, antigenic drift produces variants that somehow resemble their old counterparts while antigenic shift produces completely NOVEL strains of influenza virus. This explains the role of the latter in causing large-scale pandemics compared to the former which usually result in small-scale outbreaks.


Another unique feature of flu virus is that once a new antigenic variant has emerged, it completely displaces the pre-existing strain. As a result of which influenza spreads from one person to another easily as all lack immunity against the virus. it has been observed over the years that the new strains are more virulent and refractory to conventional drug therapy. The same applies to vaccination also. Since vaccination is based on certain particular strains, it becomes completely ineffective against the disease caused by a new strain. This is why influenza vaccines become ineffective in a year or two and new vaccine production is required every year.


It has been observed that avian influenza virus can acquire properties of human strains causing clinical disease amongst humans. This is evident from recent bird flu outbreaks in human population.

Due to the above properties the influenza virus becomes versatile and often succeeds in duping and overwhelming our immune system.

Published by me at Helium under pen name Dev Senapati.


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