CDC reports 13 additional cases of blood clots linked to J&J COVID-19 vaccine. All happened before 11-day pause in its use.

Thirteen more cases of an unusual blood clotting disorder have been identified among people who received the Johnson & Johnson vaccine, but no one else has died and no new cases have been seen among people vaccinated after the government's 11-day pause in J&J shots.

The Centers for Disease Control and Prevention said Wednesday that 28 people have now been identified with a disorder being called Thrombosis with thrombocytopenia syndrome.

To qualify for the diagnosis, someone must have a blood clot, known as a thrombosis, in an uncommon location, such as the brain, as well as low levels of platelets in their blood, a condition known as thrombocytopenia.

The combination is rare. The fact that it occurred in so many people within about two weeks of vaccination "suggests a plausible causal association," Dr. Tom Shimabukuro, deputy director of the CDC's Immunization Safety Office, told an agency advisory committee Wednesday.

So far, three people have died of TTS, and one remains in intensive care.

The J&J vaccine was delivered to nearly 8 million Americans before its use was temporarily paused last month. All of the patients with TTS received the shot before the pause, though the 13 additional cases were identified only recently.

Only about 1 million people have received the single-shot vaccine since use of the J&J vaccine resumed April 23. Only about 150,000 J&J shots have since been given to women under 50, the highest risk group for TTS, the CDC data suggests.

In earlier reports, only one TTS patient was male; now, there are six men among the 28 patients. The patients ranged in age from 18 to 59, with more older patients identified recently. Women appeared to be at highest risk in their 30s and 40s, while the men did not cluster in a particular age range, Shimabukuro said.

Most developed symptoms six to 15 days after vaccination, after normal side effects generally recede.

The CDC had initiated the pause in part because it wanted to warn vaccine recipients of the possible risk of TTS, and also to alert doctors that they should not treat TTS patients with heparin, a drug commonly used to treat blood clots but which could make these patients worse.

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There have been no confirmed cases of TTS among the 135 million doses of the Pfizer-BioNTech vaccine, and 110 million doses of the Moderna vaccine that have been delivered since both were authorized for use in December.

A vaccine made by AstraZeneca and developed by Oxford University, which is available in Europe but not the U.S., does cause a similar problem. Both the J&J and AstraZeneca-Oxford vaccines are delivered via a harmless vaccine, a different technology than used among other shots.

It's not clear whether the risks of TTS are higher among those who received the J&J shot or the AstraZeneca-Oxford ones, Shimabukuro said.

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J&J's vaccine is the only current vaccine that requires a single dose, which makes it ideal for certain groups, including those who don't like shots and those unlikely or unable to receive two doses.

The shot's benefits continue to outweigh the rare risk of TTS, concluded a subcommittee of the advisory group that heard Shimabukuro's presentation.

The Johnson & Johnson COVID-19 vaccine was temporarily paused in the U.S. for 11 days in April following rare cases of unusual blood clots.

But two members of the Advisory Committee on Immunization Practices said they are not sure it does, particularly at a time when two other vaccines are available to Americans.

"I remain very concerned," said Dr. Pablo Sánchez, a professor of pediatrics at The Ohio State University – Nationwide Children’s Hospital in Columbus. "One case is too many."

He suggested that the vaccine be used only on people over 60, who do not appear to be at risk for TTS.

Dr. Sarah Long, a professor of pediatrics at the Drexel University College of Medicine and St. Christopher’s Hospital for Children, both in Philadelphia, said she would like women under 60 to be asked to provide written consent before they receive the J&J vaccine, to ensure that they are aware of the possible risks.

Other committee members did not express their opinions but may do so as data, and likely more patients, accumulate over time.

The CDC collects reports on adverse effects from a vaccine at

Contact Karen Weintraub at

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This article originally appeared on USA TODAY: J&J COVID-19 vaccine: CDC reports additional cases of blood clots