Explainer: Viral vector vaccines

NEW DELHI: Viral vector vaccines use a harmless virus to deliver a piece of genetic code to our cells, allowing them to make a pathogen’s protein. The DNA contained in the virus encodes antigens that, once expressed in the infected human cells, elicit an immune response to react to future infections.
Overview
Viral vector vaccines use a modified version of a different virus to send instructions to our cells.These vaccines generally consists of a live attenuated virus that is genetically engineered to carry DNA encoding protein antigens from an unrelated organism.
Trials
Human clinical trials were conducted for viral vector vaccines against several infectious diseases, including Zika virus, influenza viruses, respiratory syncytial virus, HIV, and malaria, before the vaccines targeting SARS-CoV-2, which causes Covid-19
Types of viral vector diseases
Mainly, there are two types of viral vector-based vaccines: non replicating and replicating. Non-replicating vector vaccines are unable to make new viral particles and they only produce the vaccine antigen. Replicating vector vaccines also produce new viral particles in the cells they infect, which then go on to infect new cells that will also make the vaccine antigen. The Covid-19 viral vector vaccines under development use non-replicating viral vectors.
How are these different?
Viral vector-based vaccines differ from most conventional vaccines in that they don’t actually contain antigens, but rather use the body’s own cells to produce them. They do this by using a modified virus, the vector, to deliver genetic code for antigen into human cells. By infecting cells and instructing them to make large amounts of antigen, which then trigger an immune response, the vaccine mimics what happens during natural infection with certain pathogens, especially viruses. This has the advantage of triggering a strong cellular immune response by T cells as well the production of antibodies by B cells.
How do these trigger immunity?
Viruses survive and replicate by invading their host’s cells and hijacking their protein-making machinery, so it reads the virus’ genetic code and makes new viruses. These virus particles contain antigens, molecules that can trigger an immune response.
The viral vector acts as a delivery system, providing a means to invade the cell and insert the code for a different virus’ antigens. The virus itself is harmless, and by getting the cells only to produce antigens the body can mount an immune response safely.
Viruses developed as vectors
Various viruses have been developed as vectors, including adenovirus (a cause of the common cold), measles virus and vaccinia virus. These vectors are stripped of any disease-causing genes and sometimes genes that can enable them to replicate, meaning they are now harmless. The genetic instructions for making the antigen from the target pathogen are stitched into the virus vector’s genome.
Manufacturing and assembling
A major bottleneck for viral vector vaccine production is scalability. Traditionally, viral vectors are grown in cells that are attached to a substrate, rather than in free-floating cells, but this is difficult to do on a large scale. Suspension cell lines are now being developed, which would enable viral vectors to be grown in large bioreactors.
Assembling the vector vaccine is also a complex process, involving multiple steps and components which increases the risk of contamination. Extensive testing is required after every step, increasing costs.