backbone and at least 6.5–7 log 50% egg-infected dose (EID50) for vaccines derived

from the Leningrad backbone [17]. While all of the commercial LAIVs are currently

produced in embryonate hens’ eggs, its production using cell-based systems have

been demonstrated for different adherent cell lines, including MDCK [44–46] and

Vero cells [47].

9.4.3

RECOMBINANT VACCINES

Recombinant influenza vaccines are composed of one or more viral proteins, in its

free form or assembled in virus-like particles (VLPs), produced using recombinant

expression systems [48] and represent 5.4% of global production capacity [17].

Because the production of this type of vaccine does not require adaptation of the

influenza strain, a good antigenic match between vaccine and circulating strains is

obtained. Additionally, unlike IIVs and LAIVs, recombinant vaccines do not re-

quire high-level biocontainment facilities, and can be produced in shorter periods of

time [18]. Flublok (Sanofi-Pasteur), the first RV for influenza to be approved by the

FDA, is composed of recombinant HA protein (full length, comprising the trans-

membrane domain, HA1 and HA2 regions) produced in sf9-derived insect cells

using the baculovirus expression system [49,50]. Baculoviruses are DNA viruses

that infect insect cells and induce the production of large amounts of the viral

protein polyhedrin. Recombinant baculoviruses, in which a gene of interest replaces

the polyhedrin gene, can then be used for the expression of large quantities of a

foreign protein of interest [19,48]. Whereas this vaccine requires larger doses of HA

protein (45 μg per strain, 180 μg for QIVs), the antibody response that is induced

was shown to be comparable to that of IIVs or LAIVs [28,49]. Cadiflu-S (CPL

Biologicals), a recombinant vaccine recently licenced in India, contains three im-

munogenic proteins from influenza (HA, NA, and M1) incorporated into a virus-

like particle, also produced using the baculovirus expression system [17].

9.4.4

EMERGING TECHNOLOGIES FOR INFLUENZA VACCINE PRODUCTION

While IIVs, LAIVs, and RVs represent all the influenza vaccines currently on the

market, other emerging vaccine technologies, such as DNA, mRNA, and viral

vectored vaccines, are presently in clinical trials (Table 9.4). These novel tech-

nologies that gained ground during the COVID-19 pandemic, are potentially faster

to produce, could increase vaccine efficacy and, therefore, are likely to impact

global influenza vaccine manufacture [51].

9.5

INFLUENZA VIRUS QUANTIFICATION

While efforts in the improvement of upstream processes for influenza production

are crucial for the development of cell-culture−based influenza vaccines, the

quantification of influenza viruses remain one of the greatest bottlenecks for both

traditional and cell-based vaccine manufacturing. A number of different techniques

are available for the quantification of either HA content, total viral particles, or total

infectious particles, with different stages of the vaccine process development

Manufacturing of influenza vaccines

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