I'm not a doctor so please don't take this as medical advice, but... there shouldn't be anything in kava that interferes with vaccines. If a shell or three of kava takes the anxiety off getting a shot, I say go for it.
On the other hand, the first generation of covid vaccines don't protect you against every strain. It's a virus that mutates often, so I suspect we'll see seasonally or annually updated vaccines sort of like annual flu shots. Even if you're vaccinated, please continue to wash your hands, wash the things you bring into your home,
and wear a mask around other people outside your "bubble".