seasonal epidemics, and occasional pandemics [2]. Every year, between 290,000–

645,000 people die of complications from seasonal influenza, particularly young

infants, and adults older than 65 [3]. While it is possible to estimate when seasonal

influenza epidemics will occur around the world, predicting their magnitude and

severity, as well as the exact composition of the infectious agent, is a challenging task

for public health authorities. Additionally, since vaccination is the most effective way

of protection, defining vaccine composition based on circulating strains and orga-

nizing worldwide manufacturing and supply of vaccines is a formidable effort.

Influenza pandemics, on the other hand, occur every 10–40 years as a result of the

introduction of a new and antigenically distinct influenza strain, that usually has

zoonotic origins [4]. Whereas it is currently impossible to predict exactly when or

how it will occur, understanding the genetic and epidemiological characteristics of

past pandemics is critical to develop and optimize influenza surveillance tools [5].

Since the beginning of the 20th century, humanity has seen four global influenza

pandemics: in 1918–1920, the Spanish flu is estimated to have killed up to 50 million

people around the globe; between 1957–1958, the Asian flu resulted in the death of

1.5 million people; the Hong Kong flu, was responsible for 1 million deaths from

1968–1969; finally, the swine flu, a milder and more recent pandemic, is estimated to

have caused around 200,000 deaths globally in its first year of circulation [4,6].

9.2

THE INFLUENZA VIRUS

Influenza viruses are members of the Orthomyxoviridae family, being single-

stranded negative-sense enveloped RNA viruses that present a segmented genome

composed of seven to eight segments. Although wild virions are pleomorphic,

virions that were subjected to multiple passages acquire a roughly spheroidal shape,

approximately 100 nm in diameter [7,8]. While influenza A and B viruses (IAVs

and IBVs, respectively) are responsible for worldwide recurrent influenza outbreaks

and have a similar structure (both presenting eight genome segments), influenza C

viruses are more divergent (with only seven genome segments) and cause a less

severe disease in humans [9]. More recently, influenza D virus (IDV), a novel

influenza C-like virus, was isolated from pigs. Although rare, serological surveys

revealed the presence of antibodies against IDV’s hemagglutinin in humans

working with cattle, species considered as the natural reservoir for IDVs [10]. As

ICVs and IDVs account for less severe disease in humans, those will not be further

discussed in this chapter.

The influenza viral envelope is composed of a host-derived lipid bilayer that

presents three different transmembrane proteins, HA (hemagglutinin), NA (neur-

aminidase), and M2 (matrix 2). HA, a glycosylated integral membrane protein, is

the most abundant on the virus surface (around 80%), being responsible for the

initial attachment of the virus to host cell receptors (bearing a terminal sialic acid)

and later merging of viral envelope and host cell membrane. NA, which represents

approximately 17% of viral surface proteins, cleaves the sialic acid residues in host

cell receptors to release new virions, allowing for the spread of the virus [4,7].

Protective immunity against influenza viruses is mediated mainly by neutralizing

antibodies against these two surface proteins, which prevents the infection and

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Bioprocessing of Viral Vaccines