The immune system broadly has two functions: recognition & defence.
Throughout life the body is exposed to viruses, bacteria, other pathogens as well as mutated cells (pre-cancerous cells). The first time we are exposed to a foreign “organisms” small parts of that material (antigens) are presented on the surface of special (antigen-presenting) cells. These cells will then react with immune cells that are subsequently primed to recognise and destroy cells presenting that antigen. These cells are then stored as effector (ready to act) or memory cells (reactivated on exposure).
Vaccine technology aims to teach the immune system without the body having to confront an actual deadly infection. Traditional vaccine technology includes: live-attenuated, killed whole-cell and subunit antigen vaccines. What all of these vaccines have in common is that they create a pronounced antibody response to the infectious agent. These approaches have been successful in generating a broad arsenal of effective vaccines. However, some diseases have proven frustratingly unsusceptible to antibody-based approaches. These diseases have in common that they require a strong cell-mediated (T cell mediated) response as well as a humoral response (for instance, malaria & influenza), which are the potential targets for viral vector technology.
The Vaccitech methodology involves delivering either one or two heterologous viral vector vaccines, each encoding the same antigen, several weeks to months apart.
Crucially and uniquely, there is a very strong response by both CD8+ and CD4+ T cells. The vaccines are also able to induce a potent and durable antibody responses from the humoral immune system. The “prime” vaccine uses a chimpanzee adenoviral vector encoding a particular pathogen antigen (the part of the invader the immune system will come to recognize) or cancer antigen (the part of a cancer cell that differs from a normal cell). The same antigen is then produced and recognized from another viral vector vaccine which serves as a booster. MVA (modified vaccinia Ankara) is the chosen booster vaccine in most cases, although other viral and non-viral boosts are under evaluation.
Of note, for both of these platforms the issues of manufacturing, a problematic area for novel vaccines, have been largely overcome. The Vaccitech influenza MVA vaccine has been manufactured at the 200 L scale, and multiple chimpanzee adenoviruses have been manufactured at the 200 L scale.