Controlling the pandemic will require shots that prevent viral spread, but that feature is difficult to measure.
www.nature.com
Transmission testing usually occurs subsequent to safety and efficacy testing as you do not have enough vaccinated subjects to draw a conclusion on transmissability. Also you usually need a larger cohort to extrapolate from. But it's happening now and it looks quite promising.
I'd be careful about recommending a fat soluble vitamin without baseline testing ;D But masking, washing hands, eating well are all excellent advice.