Net reproductive rate, the average number of female offspring produced by all the females in a population over the course of a generation.

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Influenza virus

Influenza are surrounded by an outer envelope, a lipid bilayer derived from the plasma membrane of the infected cell.

Viral specific Protein 

Hemagglutinin (HA) 

Neuraminidase (NA)

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HA trimers are responsible for the attachment of the virus to host cells, binding to the sialic acid. 

NA is an enzyme that cleaves N-acetyl neuraminic (sialic) acid

Within the envelope, consists of eight different strands of single- stranded RNA (ssRNA) associated with protein and RNA polymerase. 

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There are 18 different antigenic subtypes for HA and 11 for NA.

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Variation in Influenza Strains

Antigenic variation in HA and NA is generated by

Antigenic drift involves a series of spontaneous point mutations that occur gradually.

Antigenic shift results in the sudden emergence of a new subtype of influenza.

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Original antigenic sin

Tendency to focus an immune attack on those structures that were present during the original.

This means that our immune systems effectively ignore the subtle changes occurring each year in pathogens