Powell was fully vaccinated, but a source close to the matter confirmed to CNN he had multiple myeloma, a type of blood cancer that would have affected his immune response to the vaccine, and made it difficult to fight the virus.

Peggy Cifrino, Powell’s chief of staff, said Powell, 84, also had Parkinson’s disease, a neurodegenerative disorder.

Although the Covid-19 vaccines provide strong protection against severe disease and death in healthy people, multiple myeloma patients are among the immunocompromised groups who may not respond as well, studies have shown. One study published in Nature in July showed that only 45% of multiple myeloma patients developed an adequate response to the vaccine, while 22% had a partial response. One-third had no response.

Dr. Jonathan Reiner, a CNN…