What are viral vector-based vaccines and how could they be used against COVID-19?

How do such vaccines 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. A similar principle underpins viral vector vaccines-only in this case, the host cells only receive code to make antigens. 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 pathogen you’re trying to vaccinate against). The virus itself is harmless, and by getting the cells only to produce antigens the body can mount an immune response safely, without developing disease.