The following sections describe in detail the individual steps of USP, resulting in a

virus harvest ready for purification.

5.3

VACCINE CANDIDATES

Viral vaccine formulations that contain or are derived from whole virus particles are

mainly produced in animal cell cultures. The choice of a suitable host cell system

depends on the vaccine type (see Table 5.1): Live-attenuated, inactivated, split, or

sub-unit vaccines are mainly produced in mammalian, avian, or fish cell lines. For

manufacturing of virus-like particles (VLPs) and recombinant protein vaccines, apart

from mammalian cells, insect cells, but also yeast or bacteria, are used. Alternatively,

viral antigens can be introduced to the immune system with mRNA or DNA tech-

nology or via viral vectors. Therefore, the choice of production system depends on the

vaccine type, but is also strongly influenced by the virus, the biosafety level (BSL) of

the virus, the required quantity, as well as the target group of vaccinees (age, place,

human or veterinary, costs). Moreover, the disease itself is a further decision cri-

terium: depending on severity and prevalence of the disease, the need for a vaccine is

more evident. Thus, higher costs are accepted and more companies will take an effort

to develop a manufacturing process. Regulatory hurdles, subsidization from gov-

ernments or other funding agencies (e.g.,, priority review vouchers, orphan incentives,

and expedited programs), and “time to market” will further decide on production

innovations or if manufacturers will go back to what historically works and is safe.

Some viruses, like Hepatitis C virus (HCV), are only poorly or not replicating at

all in available cell lines or in primary cells. Producing such viruses for inactivated

vaccines, where a high virus concentration per dose is needed, will be difficult.

Here, often the lack of vaccines indicates such production issues. For HCV, virus

replication typically results in less than five infectious virions/cell (cell-specific

virus yield: CSVY) [4].

Regarding efficacy and duration of immunity, live-attenuated vaccines are often

preferred over other vaccine types. However, developing attenuated virus strains

and demonstrating their safety is challenging. This is reflected by the relatively low

number of such vaccines in the market: yellow fever virus (YFV), polio virus, some

influenza virus strains, modified vaccinia virus ankara (MVA) virus, varicella zoster

virus, rotavirus, pox virus, and the vaccine against measles, mumps, and rubella

(MMR). Reverse genetics uses of non-human viruses as vectors (e.g., vesicular

stomatitis virus [VSV]) is a very promising alternative for the development of

vaccines against some highly pathogenic viruses. As before, the replication of these

vectors in cell cultures is often poor. Examples here are the production of measles

viruses for a Chikungunya vaccine [5], VSV for an Ebola vaccine [6], or YFV for a

dengue vaccine [7].

Some vaccines are administered as a multivalent vaccine. For instance, recent

influenza vaccines may contain up to four influenza A and B subtypes (quadrivalent

influenza vaccines). Other vaccines are combining even different viruses, such as

the MMR vaccine. For dengue vaccines, four virus subtypes need to be in the

vaccine in the correct ratio to each other to not cause complications [8]. Obviously,

Upstream processing for viral vaccines

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