(e.g., early production campaigns of SARS-CoV-2 in adherent Vero cells) safety

considerations should have a high priority and may require handling of virus-

containing materials at least in a BSL3 environment. For manufacturing of in-

activated vaccines, inactivation (heat, formaldehyde, β-propiolactone) is necessary

and needs to be carefully validated and confirmed by innocuity assays. For re-

combinant sub-unit vaccines, higher concentrations are needed due to the lower

immune response they induce. Taking all these points together explains that process

optimization for viral vaccine manufacturing can be very lengthy and complicated.

5.8

SOMETIMES NOTHING MAKES SENSE—SOME IDEAS ON

TROUBLE SHOOTING…

This part addresses some ideas on trouble shooting. It is a collection of experience

of the authors, that is certainly not exhaustive.

Considering the high complexity of vaccine production processes with many,

partially correlated, parameters, sometimes seemingly small changes can have a

dramatic impact on process performance. It could happen that the medium supplier

has a new provider for one compound of the cell culture medium. Now this compound

has a slightly different quality or purity. It is prepared or stored in a different vessel

and interacts differently with other medium components. It could equally result in

differences regarding the medium filtrations performed. This could then have an

impact on the performance of the cells, such as lower specific growth rate, reduced

maximum cell concentration, or decreased cell viability. It could also display no direct

impact on the cells, but when thawing the cells in this medium, they do not grow.

Finally, and worst case, all cell growth−related properties are more or less the same,

but virus yields are reduced. According to GMP requirements, the supplier will notify

the customers about this change, as it could be a process relevant change. However,

testing the impact of such small changes is not only time consuming and expensive,

but also very challenging and flaws might not be identified before problems in large-

scale productions arise. Due to these possibilities, the quality and consistency of all

consumables that are used, should be verified on a regular interval when it comes to

manufacturing. This includes regular audits of the respective suppliers. As the

manufacturer is responsible for the quality of the product, everything needs to be

documented very carefully following cGMP guidelines. At the end, the goal is to have

a safe and potent vaccine produced with the same quality for many years.

In particular, cell culture media are critical. Quality can already change de-

pending on where it is produced due to the water quality. Special care is needed

when changing from liquid medium to powder medium that is prepared on-site to

lower the costs and to extend in-house shelf life of lots. How the medium will be

prepared will be different compared to what was done by the supplier (equipment,

size, filters, water). Therefore, the medium might not show the same performance as

the medium bought as liquid. Similar effects can be caused by the change of sterile

filters for medium filtration, change of water quality (water cartridges of the

Millipore systems, via reverse osmosis), change of flasks for cell growth (interac-

tion of surface and medium components, leakage of unwanted compounds, etc.),

change of supplier of bags for disposable bioreactors or change of hollow fiber

Upstream processing for viral vaccines

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