A vaccination is a way of priming your immune system to have the necessary antibodies at hand so that it may effectively defend the body from an invading organism. This is particularly advantageous because it can take the body 3-6 weeks from the time it is introduced to a challenging organism until it has produced the reserves necessary to successfully defend itself from a quickly multiplying organism.
A vaccine primes the system by introducing either a live organism, or crucial parts of a inviable organism. Both are tricky. By giving a live virus you may actually be introducing the COVID-19 infection to the individual, due to an incompetent immune system or dosage misjudgment. To introduce a nonviable organism or introducing the body to fragments, you have to have a thorough understanding about how that organism interacts with the body and exactly which fragments would prime the body sufficiently.
To date, there are at least 78 vaccine studies underway to assess both the safety of the vaccine on individuals and the effectiveness of the vaccine to prepare the body to defend itself from a full COVID-19 illness. This can be particularly challenging for this type of virus, as RNA are known to mutate quickly, due to translation errors during replication. Which is to say, the process of testing the vaccine requires multiple phases to ascertain both it’s safety and effectiveness, as well as a period of post vaccination evaluation to assure there are no long term side effects evident from the treatment. Typically this process can require over two years. I understand provision may be made to allow the investigational vaccines to be accessible to high risk individuals, possibly as early as July 2020, undoubtedly with a solid waiver signed prior to providing the treatment.