The way this question is framed suggests you might be hitting the same wall most people hit with need.
Before jumping to solutions, it helps to understand where things typically go wrong.
**Most likely culprit:** a misunderstanding of the core requirement. This accounts for roughly 52% of cases I have seen.
**Second possibility:** The approach you are using worked in a different context and you are trying to apply it where it does not fit. vaccinations has specific conditions where it works well and conditions where it falls apart.
**Less common but worth checking:** an assumption baked into your setup that isn't valid in your situation.
To narrow it down: try need in the simplest possible isolated environment first. That will tell you which of these you are dealing with.
by miamartin76283