Nearly all Americans hospitalized for monkeypox infection had weakened immune systems, most often because of H.I.V. infection, the Centers for Disease Control and Prevention reported on Wednesday.
Of 57 hospitalized patients described in the report, 82 percent had H.I.V. More than two-thirds of the patients were Black and nearly one-quarter were homeless, reflecting racial and economic inequities seen in the outbreak overall.
The finding suggests that although most cases of monkeypox are mild, doctors should test patients with suspected cases for H.I.V. as well, and be prepared to offer prompt treatment for both infections.
“Monkeypox and H.I.V. have collided with tragic effects,” Dr. Jonathan Mermin, the C.D.C.’s lead scientist on monkeypox, said in a statement.
Most of the patients in the study were given tecovirimat, or Tpoxx, but treatment for some patients was delayed by as long as four weeks after they first sought care.
As of Tuesday, more than 28,000 cases of monkeypox had been reported in the United States, and nearly 76,000 cases worldwide. A vast majority are still among men who have sex with men, according to the C.D.C.
The number of new monkeypox infections has declined steadily since September. But the number of high-risk people opting for vaccination has also dropped. Only 7 percent of the vaccine doses administered so far have gone to recipients who are Black.
People living with H.I.V. or with other conditions that weaken the immune system fared poorly in previous outbreaks of monkeypox in African countries.
In the new report, C.D.C. scientists analyzed case reports from 57 patients older than 18 who were hospitalized for monkeypox between Aug. 10 and Oct. 10. Roughly 95 percent of the patients were male.
All of them had skin rashes, and most also had severe lesions in the mouth, urethra, rectum or vagina. About one in five experienced symptoms in their lungs and eyes, and in four patients the brain and spinal cord were affected.
Four of the 47 patients with H.I.V. were taking drugs to suppress the virus before they were diagnosed with monkeypox. About one in three had a CD4 count — a proxy for the immune system’s strength — of less than 50, indicating severe immunosuppression.
Two of the patients, one of whom had H.I.V., were being treated for cancer; three were solid organ transplant recipients; and three were pregnant. All of these conditions are linked to a weakened immune system.
One third of the patients were admitted to intensive care units. Of the 12 recorded deaths, five were a result of complications from monkeypox infection, six are under investigation and one was determined to be unrelated.