What is the term for a substance that can stimulate a primary immune response but cannot produce the symptoms of a disease?

A substance that can stimulate a primary immune response without producing the symptoms of a disease is best referred to as a vaccine. Vaccines are typically made from weakened or inactive parts of a pathogen (such as proteins or sugars) or from a closely related pathogen that does not cause disease, which allows the immune system to recognize it and respond without the risk of illness. This preparatory response helps the body to develop immunity, so if exposed to the actual pathogen in the future, the immune system can respond effectively.

The concept of a pathogen refers to any organism that can cause disease, including bacteria and viruses, but does not fit the criteria of this question, as it would indeed provoke actual disease symptoms. An antigen is a broader term for any substance that can provoke an immune response, including components of pathogens as well as other molecules, but it does not necessarily imply a protective role like a vaccine. An allergen is a type of antigen that results in allergic reactions and is generally not associated with immunity to infections, making it an inappropriate choice in this context. Therefore, the term that most accurately describes the substance in question is a vaccine, as it encapsulates the role of both stimulating an immune response and preventing disease symptoms.