Here are the definitions, as per Wikipedia—
A vaccine is a biological preparation that provides active immunity to a particular infectious disease. A traditional vaccine typically contains an agent that resembles a disease-causing microorganism and is often made from weakened or killed forms of the microbe, its toxins, or one of its surface proteins. The agent stimulates the body’s immune system to recognize the agent as a threat, destroy it, and to further recognize and destroy any of the parasites, microbes or viruses associated with that agent that it may encounter in the future.
A drug is a chemical substance, typically of known structure, which, when administered to a living organism, produces a biological effect.
By definition, the Pfizer, Moderna, Astra Zeneca and J&J injectables are more drug-like than vaccines. Unlike traditional vaccines, they do not provide complete immunity but only decrease symptoms. They do not contain the actual agent to which the immune responses will be directed, but contain genetic information that instructs the cells of the body to actually manufacture the agent. In this case, a spike protein.
Relative to a drug classification, the four injectables for COVID-19 prevention are chemical substances of a known and well defined nucleic acid structure, and which are manufactured as biological products based on the gene sequence information for the spike protein of the SARS-CoV-2 virus. When they are administered, they do produce a biological effect in the form of stimulating immune responses including specific antibody production to increase protection against the virus, and like drugs, they can produce adverse events.
When future requirements are considered, drugs are often understood to require future doses. Vaccines typically do not since they are meant to train the acquired immune system to prevent future infection. Pfizer, Moderna, Astra Zeneca and J&J are again better categorized as drugs as we see third booster shots are already being recommended.
It may be that the term ‘drug’ fits the medical purpose, but the term ‘vaccine’ suits the marketing department and government promotion. With this understanding of their potential therapeutic outcomes, it is understandable why there is debate as to the classification of the injectables for COVID-19 prevention as vaccines or drugs.
Recently the US FDA has changed their definition of a vaccine to “a preparation that is used to stimulate the body’s immune response against diseases.” With such a loose definition, even vitamin D3 can be considered as a “vaccine.”