The U.S. Food and Drug administration announced limits on the authorized use of the Johnson & Johnson COVID-19 vaccine on Thursday, citing concerns over rare and potentially life-threatening blood clots that can be a side effect of the vaccine.

The authorized use of the vaccine is now limited to individuals 18 and older for whom other authorized or approved COVID-19 vaccines are not accessible or clinically appropriate and to adults who choose to receive the Johnson & Johnson vaccine because they do not want to take another one.

According to the FDA, an analysis of reported vaccine side effects has determined there is a risk of thrombosis with thrombocytopenia syndrome (TTS), which causes blood clots in combination with low levels of blood platelets. Symptoms of the side effect can appear approximately one to two weeks after an individual receives the Johnson & Johnson vaccine, the FDA said.

But there are some people the FDA says should still be eligible to receive the Johnson & Johnson vaccine, such as those who have had a severe allergic reaction to the mRNA vaccines manufactured by Pfizer/BioNTech or Moderna, those who object to taking an mRNA vaccine, or those with limited access to mRNA COVID-19 vaccines.

“We recognize that the Janssen COVID-19 Vaccine still has a role in the current pandemic response in the United States and across the global community. Our action reflects our updated analysis of the risk of TTS following administration of this vaccine and limits the use of the vaccine to certain individuals," said Dr. Peter Marks, director of the FDA's Center for Biologics Evaluation and Research.

“Today’s action demonstrates the robustness of our safety surveillance systems and our commitment to ensuring that science and data guide our decisions. 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 EUA," he continued.

"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.

The Johnson & Johnson COVID-19 vaccine received FDA emergency use authorization on Feb. 27, 2021. Months later on April 13, 2021, the FDA and the CDC issued a call to "pause" the administration of the vaccine over concerns of blood clotting. At the time, the agency wanted to investigate six reported cases of TTS after nearly 7 million doses of Johnson & Johnson's single-dose vaccine had been administered. The pause was lifted on April 23, 2021, after the FDA and CDC conducted a safety evaluation.

In December, the CDC's Advisory Committee on Immunization Practices issued a "preferential recommendation" stating that unvaccinated adults should get one of the mRNA COVID-19 vaccines before they consider taking the Johnson & Johnson vaccine.

A fact sheet provided by the FDA says that TTS cases have been reported in males and females, in a wide range of individuals age 18 years and older. The highest reported rate of TTS was approximately 8 cases per 1,000,000 vaccine doses administered among females ages 30-29 years. The FDA says that approximately 15% of reported TTS cases have been fatal.

As of Friday, more than 18.7 million people have received the Johnson & Johnson COVID-19 vaccine, according to the U.S. Centers for Disease Control and Prevention. The FDA says it has confirmed 60 cases of TTS, including nine deaths. The FDA estimates the reporting rate of TTS is 3.23 cases per million vaccine doses administered and the reporting rate of TTS deaths is 0.48 per million doses of vaccine administered.