evidence: The active ingredients are the parts of the vaccine made from viruses or bacteria, sometimes also called ‘antigens’. They challenge the immune system so that it makes antibodies to fight the disease - see our animation on 'How do vaccines work?'. [...] Some vaccines contain whole bacteria or viruses. In these cases, the bacteria or viruses will either be severely weakened (attenuated) so that they cannot cause disease in healthy people, or killed altogether (inactivated). Many vaccines contain only parts of viruses or bacteria, usually proteins or sugars from the surface. These stimulate the immune system but cannot cause disease. See our page on 'Types of vaccine'. [...] The Department for Social Justice of the Catholic Bishops’ Conference of England and Wales released a statement addressing the use of HEK-293 cells in the COVID-19 vaccine. They say that "one may in good conscience and for a grave reason receive a vaccine sourced in this way", and "that one does not sin by receiving the vaccine".