What is Flu?
What causes Flu?
How is it different from a cold?
What are the symptoms of the Flu?
Influenza can cause mild to severe illness and at times can lead to death. Flu is different from a cold. Flu usually comes on suddenly. People who have flu often feel some or all of these symptoms.
Fever/Chills
Cough, Sore throat
Running nose, Headache, Body aches
Some may have vomitings or diarrhoea.
There are 4 types of Influenza Virus- A, B, C, D.
Influenza A viruses are classified by subtypes based on their properties of Hemagglutinin(H) and Neuraminidase(N).
These HA and NA are actually what we listen to the name of the viruses as H1N1, H3N2 etc.
HA and NA are the surface proteins on the virus.
There are 18 subtypes in HA and 11 different subtypes in NA.
Thus the name H1N1 means the virus HA of subtype 1 and NA of subtype 1.
Since there are 18 and 11 subtypes in HA and NA respectively by applying Permutations and Combinations we may get 198 subtypes, but till now 131 subtypes have been detected in the nature.
Some important concepts we need to understand-Antigenic Drift and Antigenic Shift
Antigenic Drift and Shift are often specifically applicable to influenza virus.
Influenza viruses are constantly changing by two ways- Drift and Shift.
Antigenic Drift
There will always be small changes occuring in the genes of the virus resulting in the changes in the HA and NA proteins coded by these genes.
These small changes occur continually over time as the virus replicates.
These small changes will result in a new strain of virus that is closely related to the previous influenza virus. So the antibodies produced by our immune system against other influenza virus will likely recognise and respond to antigenically similar virus.
These small changes accumulate over period which changes the proteins in such a way that the proteins become unrecognisable to the preexisting host immunity(developed during the previous attack of flu).
This is the reason why we may get flu every year even though we develop immunity against the previous one.
Therefore it is advised to get flu vaccination once a year.
This is the reason why the composition of the vaccine will be reviewed every year following the changes in the virus.
Antigenic Shift
Antigenic Shift is an abrupt, major change in an influenza virus resulting in new HA/NA proteins.
Antigenic Shift is important for the emergence of new viral pathogens as it is a pathway that viruses may follow to enter a new niche.
Antigenic Shift can happen when an influenza virus from an animal population gains the ability to infect humans.Since they contain HA/NA proteins that is so different from the same subtype in humans, as they have no immunity to this new virus,resulting in a pandemic.
Example- 2009 H1N1 outbreak. The shift occurred from the virus either genes from North American swine, Eurasian swine resulting in a pandemic.
2. When two different strains of influenza infect the same cell simultaneously, their capsids and lipid envelopes are removed exposing their RNA.The host cell forms new viral particles which combine the antigenic features of two strains resulting in new virus.
For example the HA protein of one strain and the NA protein of the other strain may combine in the new virus.
Since it is a new one, humans will have no immunity against it leading to a pandemic.
Thus, we can conclude that, the risk of flu pandemics exists forever. And we need to be prepared to face a flu pandemic at any time.