Vaccines never tested in people with immune deficiencies While there's no clear number of how many Americans are taking medications that might prove to be a challenge for the Covid-19 vaccine, a new study can help give an estimate. In a paper published last month, Dr. Beth Wallace, a rheumatologist and her team at the University of Michigan, looked at a database of more than 3 million Americans to see how many took immunosuppressants. Based on that data, Wallace estimates that about 60 million Americans are taking immunosuppressants that could interfere with the vaccine.