This is a substance that can give a person immunity from a disease. That means their body can recognize and kill disease-causing organisms before they can start an infection. Many diseases are caused by bacteria or viruses. Some vaccines are made with a whole virus that has been killed or made harmless. Others contain materials that imitate part of a disease-causing agent. These materials are all antigens. Antigens trigger the immune system to respond.
A vaccine introduces these antigens to the bloodstream. The body recognizes that the antigen is an intruder. The immune system responds, producing antibodies. Later, if the person is exposed to the actual virus or bacteria, the antibodies recognize it. These tiny proteins glom on to the antigen. This helps the body recognize and destroy the virus or bacterium before anyone gets sick.
There are vaccines for viruses such as measles and mumps. There are also vaccines against some bacteria, such as the one that causes whooping cough.
In a sentence
Scientists are trying lots of new methods to make a vaccine against the new coronavirus.