Covid-19 reminded us of the threat of respiratory viruses. It is not the first pandemic to be caused by a respiratory virus and it is not likely to be the last. Influenza viruses are thought to have the potential to cause pandemics in the future.
According to the World Health Organization, 2.9 lakh to 6.5 lakh deaths occur during every flu season. As such, strategies preventing and treating influenza virus infections have become a major area of research.
There are four influenza viruses—A, B, C and D. Only A, B and C infect humans. C infections in humans are infrequent and mild. A and B infections can be severe, making them a public health problem.
Influenza viruses have a single-stranded segmented RNA genome. Influenza A and B have eight genes, each encoding for a different protein essential for the virus’s ability to infect and reproduce in humans. Out of these eight proteins, hemagglutinin (HA) and neuraminidase (NA) are on the surface of the virus.
HA is responsible for entry of the virus into our respiratory system. After replication, the new viral particles are released from the infected cell with the help of NA. Our immune system clears the infection by making antibodies. However, these antibodies are specific to the HA or NA and will either provide only partial protection or no protection against an influenza virus with different HA or NA.
Influenza A viruses cause both pandemics and seasonal influenza virus epidemics. Influenza A viruses are categorised into subtypes based on what HA and NA they have. There are 18 distinct HAs and 11 NAs found in nature, and, theoretically, viruses with any combinations of these are possible. For example, Influenza A H1N1 has type 1 HA and type 1 NA.
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