I have been taking tacro and mycophenolate for my heart transplant antirejection for 18 years. I discussed with my cardiologist at the VA if i should be tested for antibodies and if i should get a booster. He investigated and has not received any guidance from either the CDC or VA on either subject. He basically said that there is no standard for what the antibody level should be after receiving the Covid19 vaccine. He will continue to monitor studies and will keep me informed of any changes. He is aware of the John's Hopkins study.
BTW He is a specialist in post transplant patient support. The good news may be that heart transplant patients have a less severe immune response to Covid19. He experienced no deaths of transplant patients due to Covid19