HIV and cancer patients, organ transplant recipients and those taking immunosuppressant drugs comprise about 2.7% of the U.S. adult population.
Growing evidence had suggested select immunocompromised patients mount a diminished protective immune response, even after two doses of vaccine.
Research published in the New England Journal of Medicine on Wednesday among 120 organ transplant recipients who received a third dose of Moderna's vaccine indicated a substantial boost in neutralizing antibodies and T-cell counts, compared to a group receiving saline placebo.
The FDA statement, which was released Thursday night, said the specific individuals impacted are "solid organ transplant recipients or those who are diagnosed with conditions that are considered to have an equivalent level of immunocompromised."
Other fully vaccinated individuals do not need an additional vaccine dose right now, the statement said.
"FDA’s approval of the third COVID-19 vaccine dose is excellent news for organ transplant recipients," Anastasia Henry, executive director of the American Transplant Foundation, told Fox News. "This is definitely a needed step to ensure transplant recipients are protected. Transplant patients are excited about this option. We encourage transplant recipients to follow-up with their medical providers promptly to make sure they can get their booster shot and discuss future precautions such as masks and social distancing."
It's harder for vaccines to rev up an immune system suppressed by certain medications or diseases, so those patients don't always get the same protection as otherwise healthy people -- and small studies suggest for at least some, an extra dose may be the solution.
"This action is about ensuring our most vulnerable ... are better protected against COVID-19," Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention, said ahead of the FDA's announcement.
Fox News' Edmund DeMarche and the Associated Press contributed to this report