The US Centers for Disease Control and Prevention have confirmed 28 cases, including six in men, of a rare blood clotting disorder in adults who have received the Johnson & Johnson Covid-19 vaccine, the media reported on Thursday.

The number of cases has increased from 15 last month, all of which were in women, the New York Times reported. Although the latest report has identified a handful of cases in men, women — especially those between the ages of 30 and 49 — remain at high risk.

“The trend is that the reporting rates are higher in females compared to males in all age categories,” Tom Shimabukuro, the deputy director of the immunisation safety office at CDC, was quoted as saying.

Shimabukuro presented the new cases on Wednesday at a meeting of a panel of advisers to the CDC, the report said.

Patients with the rare but serious disorder develop blood clots, often in the brain, as well as low levels of platelets — blood components that promote clotting.

The disorder is a “rare, clinically serious and potentially life-threatening condition,” Shimabukuro said.

Among the 28 confirmed cases, 12 people who developed the disorder had obesity, 7 had high blood pressure, 3 had diabetes, and 3 were taking estrogen, though it is not yet clear whether any of those factors might substantially increase the risk of the disorder, the report said.

The CDC and the US Food and Drug Administration had, last month, recommended halting the use of Johnson & Johnson’s Covid-19 vaccine, after rare blood clot cases emerged in six recipients.

However, the pause was lifted 10 days after a thorough safety review. The agencies had also issued a warning about the potential risks to the vaccine’s label, which notes that a connection between the vaccine and the condition is “plausible.”

Twenty-two of the confirmed cases so far have been in women, and six have been in men. All were adults between the ages of 18 and 59 who received the vaccine before the national pause. (There was also one additional case recorded in a 25-year-old man who participated in the clinical trial), the NYT said.

Three people have died and four remain hospitalised, including one who is in intensive care. No new deaths have been documented since last month’s meeting, Shimabukuro said.

There have been no confirmed cases of the clotting disorder following the Pfizer-BioNTech or Moderna vaccines, which employ a different technology, he added.

The roll-out of the single-dose Johnson & Johnson vaccine has been paused in Germany, Denmark, Italy and Canada over the blood clot concerns.