A(H1N1)

Due to the outbreak of 2009 flu pandemic, our health is endangered and I still see quite a number of people wearing mask in shopping mall. Everyone mentioned about A(H1N1) virus, but do you know what A(H1N1) means?

It actually refer to Influenzavirus A subtype H1N1. Influenzavirus A is one of the five genera under the family Orthomyxoviridae. Other genera are Influenzavirus B, Influenzavirus C, Isavirus, and Thagotovirus. Only Influenzavirus A classified into different subtypes, based on antigen associated with outer viral proteins or spikes, namely hemagglutinin (HA) and neuraminidase (NA). There are 14H and 9H subtypes, so the subtypes are named according to the combination of HA and NA, e.g. H1N1, H5N1, etc.

Why suddenly there is an appearence of A(H1N1) virus and it started to infect people around the whole world? Influenzavirus A has 8 different helical nucleocapsod genome, i.e., it has 8 RNA genome segments. HA is encoded by RNA segment 4 while NA is encoded by RNA segment 6. HA and NA are the factors that cause antigenic variation.

Appearence of new subtype is due to the antigenic shift (major anitgenic change), whoch occurs infrequently, maybe once every 10-20 years. When two genetically different influenzaviruses (e.g human influenzavirus and animal influenzavirus) infect in the same cell, there might be a reassortment of genomes between these two different strain, because genome segment are enveloped in a random manner. The progeny virus contains some RNA segments from one parent and other RNA segments from another parent. As a result a new virus with different genetic constitute produced.

This new virus has a different anitgenic structure, so most humans do not have antibogies against this virus, i.e. most people are immnunologic naive or susceptile towards this new virus. Hence there will be a pandemic outbreak of disease.