A C.D.C. panel is meeting on a blood clot risk linked to J.&J.’s vaccine.

As more cases of the clotting disorder were adjudicated by federal health officials in recent months, F.D.A. and C.D.C. officials grew increasingly alarmed by the numbers presented to them by the C.D.C.’s immunization safety office, which monitors reports in the Vaccine Adverse Event Reporting System, or VAERS, a decades-old system that relies on self-reported cases from patients and health care providers.

The reports of the condition grew worrisome enough in recent weeks that federal officials determined they needed to call an emergency meeting of the C.D.C. advisers.

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In April, soon after Johnson & Johnson’s vaccine started being administered in the United States, federal officials briefly recommended halting use of the shot because of concerns about the risk of blood clots. At the time, the condition had emerged in six women, all of whom developed the illness within one to three weeks of vaccination. One of the women had died. By May, 28 cases had been confirmed.

The updated F.D.A. fact sheet for providers says that “currently available evidence supports a causal relationship” between the condition and Johnson & Johnson’s vaccine.

The new data come during a surge in virus cases driven by the Delta coronavirus variant and Omicron, the version of the virus that has already become dominant in some countries and is spreading fast in the United States.

Several laboratory experiments suggest that a single dose of Johnson & Johnson’s shot may offer little defense against infection with Omicron. The company said late last month that it is testing blood samples from clinical trial participants who have received its shot as a booster to see how their vaccine-induced antibodies fare against Omicron.

The shot has largely fallen out of favor in the United States, despite early hopes that its one-and-done format would be easy to deploy in more isolated communities, and among people skittish about receiving two doses.

Federal health officials in October authorized booster shots for people who had received a single shot of Johnson & Johnson vaccine at least two months earlier. They allowed for a “mix and match” approach, allowing people to get a second shot of a Pfizer or Moderna vaccine. Among people in the United States who originally received a single Johnson & Johnson shot and then got a second shot, fewer than 28 percent have gotten Johnson & Johnson as their booster.

Christina Jewett contributed reporting.