Viral Vector based vaccines for Covid-19

Published at: 25.01.2022 15:57

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We are at the beginning of 2022 but the pandemic situation is not over yet. We are still learning to adapt to all the changes it brought. We’ve been under lockdown several times. Some people keep working from home until others decide to take the vaccine. However, are we all really familiar with the existing vaccines and those that are in the developing process?

Let’s take a look at the Viral Vector-based ones. What are they?

Viral vectors are playing a major role, especially using adenovirus-based vectors for vaccine development.
Scientists make those kinds of vaccines by using genetically modified viruses (the vector). They are using the host translational machinery to express the foreign antigens. For example, vesicular stomatitis virus (VSV), lentivirus, influenza viruses, measles, and adenovirus vectors are using vector designs when it comes to vaccine development. Viral vector vaccines can be widely classified into non-replicating viral vectors and replicating vector vaccines.

Self-amplifying RNA virus vectors are applied for lipid nanoparticle-based delivery of RNA as COVID-19 vaccines. And that’s not all – specialists use Viral Vectors for vaccine development against both cancer and viral infections. Let’s take for example the EBOV vaccine. It is an Ebola virus glycoprotein (EBOV-GP) – based on the VSV-ZEBOV vector which was approved by FDA in December 2019.

A huge spectrum of viral vectors is subject to vaccine studies and vaccine development against Covid-19, especially in preclinical studies in animal models and in clinical trials. For example – there are 162 preclinical studies at the moment, 35 of them are based on VLPs or viral vectors.

According to WHO (World Health Organization), 14 vaccine candidates out of 52 currently in progress, are based on viral vectors. Furthermore, protection is achieved after rodents' immunization with LV, MVA, NDV, and VSV vectors. The classic delivery route comprises intramuscular administration, but also intranasal sprays have shown good results for adenovirus, LV, and influenza virus vectors.

How do they work?

Vaccines based on viral vectors are different from most standard vaccines – they don’t contain antigens. They use the cells in our bodies to produce them. That process happens by using a modified virus (that’s the vector) to convey genetic code for antigen, in the case of Coronavirus spike proteins discovered on the surface of the virus, into human cells. The cells are producing large amounts of antigen when they are infected. This triggers an immune response. The vaccine mimics the process during natural infection with defined pathogens, especially viruses. This gives the advantage of waking a powerful cellular immune response by T cells and the production of antibodies by B cells.

What about manufacturing?

Usually, the viral vectors are growing in cells, which are attaching to a substrate, instead than in cells that are floating free. This is a bit difficult to do on a large scale. Suspension cell lines are currently under development that would enable viral vectors to grow in large bioreactors. This is why making a vector vaccine is a complicated process – it includes multiple components and steps, each of which increases the risk of inoculation. Extensive testing is a must after every step, even if it is increasing the costs.

So far most other viral vectors have reached only phase I, but several adenovirus-based vaccine candidates have successfully passed through phase I and II clinical studies. And the most advanced vaccine candidates are already classified in tens of thousands of volunteers.

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