A small but significant percentage of Americans take medications that can hamper their immune system and its response to COVID-19 vaccines, researchers say.

Their analysis of data from more than 3 million adults under 65 with private insurance found that nearly 3% take immunosuppressive drugs. Those include chemotherapy medications and steroids such as prednisone.

Two-thirds took an oral steroid at least once, and more than 40% took steroids for more than 30 days in a year, according to findings published May 20 in the journal JAMA Network Open.

Growing evidence suggests that immunosuppressive drugs may reduce effectiveness of COVID vaccines, increasing patients' risk of severe illness and hospitalization if they get infected.

"This study gives us previously unavailable information about how many Americans are taking immunosuppressive medications," said lead author Dr. Beth Wallace, a rheumatologist at Michigan Medicine-University of Michigan in Ann Arbor.

It also shows that many Americans continue to take oral steroids, which have serious side effects, she said. Other medicines can often be substituted, she added.

The new study comes at a time when doctors are beginning to realize that people on immunosuppressants may have a slower, weaker response to COVID vaccination, and, in some cases, no response at all.

"We don't have a full picture on how these drugs affect the vaccine's effectiveness, so it's difficult to formulate guidelines around vaccinating these patients," Wallace said.

Researchers are investigating several strategies, including temporarily halting use of immunosuppressive medications around the time of COVID-19 vaccination and giving an extra "booster" shot.

It's also unclear what people taking immunosuppressive medications should do to protect themselves now that the U.S. Centers for Disease Control and Prevention has relaxed masking and distancing guidelines for vaccinated people.

"The CDC acknowledges this cohort might not be as protected as other fully vaccinated people, but there are no set recommendations for what precautions they should take," Wallace said. "For now, this is going to be an individual decision people make with their doctor."