JNJ-78436735 is Janssen Pharmaceuticals Companies of Johnson & Johnson's vaccine. It is a one-dose vaccine for individuals 18 and over, with full protection 28 days after vaccination. The vaccine has been shown to be effective at preventing severe COVID-19 illness, hospitalization, and death. (Photo courtesy of Janssen Pharmaceuticals)

Federal regulators have limited the authorized use of Johnson & Johnson’s COVID-19 vaccine due to its risk of causing potentially life-threatening blood clots, the U.S. Food and Drug Administration said Thursday.

Administration of the single-dose vaccine, developed by J&J pharmaceutical subsidiary Janssen, will be limited to adults who cannot access the other federally authorized vaccines and adults who would not otherwise get vaccinated unless they receive a J&J shot.

While the vaccine remains effective at preventing serious illness and death, FDA officials said the risk of developing thrombosis with thrombocytopenia syndrome, or TTS, within two weeks of receiving the J&J vaccine is enough to limit its use for adults.

“We’ve been closely monitoring the Janssen COVID-19 Vaccine and occurrence of TTS following its administration and have used updated information from our safety surveillance systems to revise the (emergency use authorization),” FDA Center for Biologics Evaluation and Research Director Dr. Peter Marks said in a statement.

“The agency will continue to monitor the safety of the Janssen COVID-19 Vaccine and all other vaccines, and as has been the case throughout the pandemic, will thoroughly evaluate new safety information,” Marks said.

Federal regulators first issued an emergency use authorization for the J&J vaccine in February 2021. In April of that year, both the FDA and U.S. Centers for Disease Control and Prevention recommended a pause in the J&J vaccine’s use after six cases of TTS were reported.

California public health officials also temporarily paused the vaccine’s use, as did local health officials in the Bay Area.

Ultimately, the CDC and FDA confirmed 15 total TTS cases among the 8 million J&J vaccine doses that had been administered at that time.

“The FDA has determined that the reporting rate of TTS is 3.23 per million doses of vaccine administered and the reporting rate of TTS deaths is 0.48 per million doses of vaccine administered,” the agency said in a statement.

As of Friday, 18.7 million doses of the J&J vaccine have been administered in the U.S., roughly 3 percent of the nearly 578 million total vaccine doses received across the country.

Some 220 million U.S. residents have completed their initial vaccination series, roughly 66.3 percent of the population.