Vaccines are biologics to provide immunity, typically against infectious diseases such as that caused by the novel coronavirus (SARS-CoV-2). A vaccine usually contains parts of a disease-causing microorganism such as surface proteins or a weakened (attenuated) or killed microorganism. These biological agents stimulate the body’s adaptative immune system to recognize them as threats such that the body can destroy the actual microorganism it encounters in the future.
One of the fathers of vaccines is Edward Jenner, an English physician, who observed that dairy workers were largely spared from fatal and disfiguring smallpox because of their continual exposure to a similar virus in cows. This discovery formed the scientific basis of using vaccination to prevent smallpox, and subsequently other infectious diseases like influenza.
The majority of our vaccines are produced from chicken eggs. The candidate vaccine viruses are injected into fertilized chicken eggs and allowed to replicate. Following which, the viruses are harvested from the fluids in the eggs, purified, and ready to use. However, this method has huge limitations in terms of scalability, environmental footprint, and vaccination potency. Each fertilized chicken egg can produce only approximately 100-300 vaccine doses and these eggs must come from special pathogen-free chickens. During times like the COVID-19 pandemic, such an approach is not likely to meet surges in global demand for vaccines. A 2017 study also suggested that egg-based vaccine virus strains can evolve away from human strains toward ones that work better in chickens. The authors concluded that egg-dependent antigenic changes might explain why vaccine effectiveness during the 2016-17 influenza season was low.
Cell-based vaccine production
Faced with limitations of egg-based vaccine production, researchers have turned to other approaches, and in 2012, the US Food and Drug Administration (FDA) first approved the use of cells for vaccine production. Instead of chicken eggs, the candidate vaccine viruses are injected into cultured mammalian cells. The virus then hijacks the cell’s machinery to replicate many copies of itself. Viruses are finally purified from extracts of cell cytoplasm. In 2013, the FDA went on to approve the culture of synthetically engineered virus in cells for producing recombinant vaccines.
There are several benefits of using mammalian cells for vaccine production. For instance, unlike egg-based methods, viruses cultured in mammalian cells are likely to retain mutations that confer them advantage in infecting human cells. This makes them more antigenically matched and useful as human vaccines. Additionally, the quality of mammalian cells can be more consistently monitored using modern biomedical technologies, unlike eggs. Having quality cells is important to ensure that viruses can replicate efficiently for high vaccine yield.
Bioreactors to support vaccine manufacturing
Moving from the traditional egg-based method that we have depended on since the early 1940s to mammalian-cell-based vaccine manufacturing has its challenges. As a result, bioreactor design has evolved to meet the technical requirements for vaccine manufacturing, and several companies have developed innovative products and workflows.
In any biological system, there is always a risk of contamination, which can adversely affect the quality, and more importantly, safety of the products. Many bioreactors now offer closed-system operation as opposed to conventional open-system bioreactors, to ensure the sterility of their contents. Oxygen and nutrients can be introduced to culture while waste such as cell extracts can be removed from culture through filtered pipes. Compared to open-system counterparts, closed-system bioreactors reduce the risks of fluid splashing and accidental transfer of contaminants from the outside environment into bioreactors and better protect the operators. They also decrease machine down time, leading to cost savings.
As vaccine manufacturers move toward more flexible and agile production, single-use disposable plastic bag bioreactors are also becoming more popular compared to multi-use stainless steel bioreactors, especially at the initial testing phase. This is because single-use bioreactors are usually a cheaper investment and are available in lower volumes, enabling manufacturers to test different cell lines and growth conditions quickly and at a lower cost to optimize vaccine production. Importantly, as manufacturers may also be testing and producing human and animal vaccines at the same time, single-use bioreactors can minimize potential cross-contamination.
Another important aspect of using bioreactors for biomanufacturing is sensing. To achieve high yield, host mammalian cells must be healthy to support virus replication. Cells, being biological agents, are highly sensitive to their environment. Having good chemical sensors measuring temperature, oxygen, pH, nutrients such as glucose and amino acids, and cell waste is thus crucial. Sensors are generally more reliable in multi-use bioreactors than single-use bioreactors. This is because expensive and more accurate pH electrodes are generally not incorporated into disposable single-use bioreactor bags. To minimize per-use cost while taking into account the bag flattening assembly and delivery process, single-use bioreactors are typically using non-invasive methods like electromagnetic waves sensing with radio-frequency identification tags, and optic fibers embedded into patches to detect for chemical changes in their cellular contents.
Mitigate mechanical stress
Besides being sensitive to their chemical environment, cells are also affected by mechanical forces in their environment. A previous Lab Manager article discusses how cells growing at the bottom of static bioreactors experience more hydrostatic pressure, which can affect their growth. Additionally, excessive fluid shear stress in a stirred-tank bioreactor can also negatively affect cellular functions. To optimize mechanical stresses while providing sufficient agitation for uniform distribution of nutrients, companies have introduced new designs such as rocking bioreactors. For adherent mammalian cells, micro-carriers—usually polymeric beads—are also being used to culture these cells in bioreactors to optimize space use, maximize surface area to volume ratio to promote cell growth, and reduce unnecessary mechanical stresses.
The growing popularity in the use of mammalian cells for vaccine production is likely to increase the use of bioreactors. This, alongside developments in stem cell and immune cell therapy, will promote greater innovations in bioreactor designs to meet various needs in cell manufacturing. A challenge for vaccine and bioreactor manufacturers is balancing scalability and flexibility such that companies can rapidly produce and test the clinical effectiveness of various vaccines, and once the candidate vaccine is chosen, production can be ramped up fast and to a large enough scale. To achieve this balance, good quantitative models are needed to know how each chemical and physical condition affects cell activities in bioreactors when they are scaled up and scaled down. This requires researchers from different backgrounds (e.g. biologists, fluid engineers, and analytical chemists) to work together.