Flu is caused by the influenza virus. The virus mutates wildly because of its primitive structure. It thrives in the wild aquatic birds and does not affect them. The first well documented crossover to humans was in 1918 when the Spanish Flu sickened 500 million people. It killed 100 million people spread across every corner of the globe (5% of the human population).
Influenza has two crucial proteins on its surface – hemagglutinin (HA or H) and neuraminidase (NA or N). HA helps the virus enter a human cell. There each virus creates hundreds of new viruses. NA helps the new viruses exit the cell. For immunological reasons, all vaccines target HA. Tamiflu blocks NA and seals the new viruses inside the cell. Then the immune system kills the cell and the viruses inside. That is why, we need to take Tamiflu early in the infection. Once the virus spreads inside the body, it overwhelms Tamiflu.
There are four groups of the influenza virus (Type A-D), 18 variants of HA (H1-18) and 11 variants of NA (N1-N11). So, H3N2 has H3 and N2 on its surface. The aquatic birds have Type A and all variants of H and N. Humans are mostly infected by Type A-B, H1-H3 and N1-N2.
Other variants such as H5N1 and H7N9 sicken the domestic poultry. It is commonly referred as bird or avian flu. Avian flu is highly lethal to humans because we do not have immunity against it. It has been possible to manage the Type B through vaccinations, in part, because it does not cross into non-human vertebrates.
The annual flu vaccine has four components. There are two components each of Type A (H1N1 and H3N2) and Type B (Yamagata and Victoria) strains. Of these, H1N1 and H3N2 are the most pervasive (11% and 78%). H3N2 is the most devastating of all, including wreaking havoc this year. The 2017-18 flu has been characterized as the worst epidemic since the 2009 H1N1 swine flu.
H3N2 entered the human population during the 1968 H3N2 Hong Kong pandemic. The vaccinologists are still struggling with it 50 years later. They have been changing the vaccine frequently, with little success.
Developing the Type-A component of the flu vaccine has been challenging for the scientists due to several reasons. Some of those are 1) the virus has a primitive RNA structure which allows it to mutate rapidly and 2) it crosses into wild and domestic birds and comes back into the humans with a renewed vigor.
Sometimes a single cell (mostly in swine) gets infected with viruses from both birds and humans. As an example, it can happen on a farm where pigs, poultry and humans live closely. Then the fragments of the viruses from birds and humans freely intermingle inside the cell and create totally new strains of the virus. We are utterly defenseless against these radically new strains. That leads to a pandemic such as the 2009 H1N1 swine flu. The epidemiologists are still investigating the root cause of this year’s H3N2 outbreak.