COVID-19 Vaccines May Not Be Effective In 3% Of US Population: Here's Why KEY POINTS Immunosuppressive drugs, steroids "affect the body's response to the vaccine" Health experts say immunocompromised Americans may need boosters regularly COVID-19 vaccines may not work fully or at all in 3% of the population in the United States, new research showed. Early data suggests that the vaccines offer a weaker response in people who take immunosuppressive drugs and, potentially, steroids. Immunocompromised Americans -- which make about 3% to 4% of the U.S. population -- may not be protected against the novel coronavirus despite being fully vaccinated. According to the research, 15% to 80% of people with underlying medical conditions, including specific blood cancers or previously organ transplants, are generating fewer antibodies, The Washington Post reported.