The U.S. Food and Drug Administration is limiting the use of Johnson and Johnson's Janssen COVID-19 vaccine as a last resort for adults who cannot accept a shot from another manufacturer.

Though the potential benefits of the vaccine outweigh the risks, the FDA said Thursday that it is restricting the use of the Janssen vaccine. It will be limited to individuals 18 and older and medically ineligible for another approved vaccine and to those who only have access to the J&J vaccine and want one, according to an FDA news release.

U.S. regulators on Thursday strictly limited who can receive Johnson & Johnson’s COVID-19 vaccine due to the ongoing risk of rare but serious blood clots.

The Food and Drug Administration said the shot should only be given to adults who cannot receive a different vaccine or specifically request J&J’s vaccine. U.S. authorities for months have recommended that Americans get Pfizer or Moderna shots instead of J&J’s vaccine.

The Food and Drug Administration (FDA) announced on May 5 that it will restrict the use of the Johnson & Johnson COVID-19 vaccine to adults who can’t receive mRNA vaccines, because of the risk of potentially life-threatening side effects with the J&J shot.

The agency said the COVID-19 vaccine can now only be administered to people aged 18 years of age and older who can’t receive either the Moderna or Pfizer vaccines, both of which are two-dose vaccines and use mRNA technology. J&J’s single-shot vaccine uses adenovirus technology.

Vaccine advisers to the US Centers for Disease Control and Prevention voted 15-0 Thursday to change recommendations for Covid-19 vaccines to make clear that shots made by Moderna and Pfizer/BioNTech are preferred over Johnson & Johnson's vaccine.

The new recommendation: "mRNA vaccines are preferred over the Janssen Covid-19 vaccine for the prevention of Covid-19 for those 18 years of age and over."

Apanel that advises the Centers for Disease Control and Prevention gave a rare preferential recommendation to the Covid vaccines based on messenger RNA technology on Thursday — a decision aimed at steering people away from the Johnson & Johnson Covid-19 vaccine because of concerns about a rare but serious side effect.

The recommendation passed by a unanimous vote, with all 15 members of the Advisory Committee on Immunization Practices supporting it.

U.S. drug regulators this week formally strengthened a warning to Americans regarding a severe condition linked to Johnson & Johnson’s COVID-19 vaccine.

Blood clots and low blood platelet levels, known as thrombosis with thrombocytopenia syndrome (TTS), are now listed as a contraindication, or a medical reason for somebody not to get the shot.

The single-dose Johnson & Johnson vaccine that last week won approval from FDA advisers for a booster shot probably should have been a two-shot vaccine from the start, the nation's top infectious disease physician said Sunday. 

"What the advisers to the FDA felt is that, given the data that they saw, very likely this should have been a two-dose vaccine to begin with," Dr. Anthony Fauci told ABC's "This Week."

As the greenlight looms for another dose of Johnson & Johnson's Covid-19 vaccine, experts on Friday urged those who received it to get a booster shot as soon as it's available because it will provide them with the best protection against the coronavirus.

"J&J is a very good vaccine. I also believe it's probably a two-shot vaccine," said Dr. Ashish Jha, dean of the Brown University School of Public Health. "It's really urgent that people get that second shot pretty quickly.

The Food and Drug Administration (FDA) has agreed to extend the shelf life on Johnson & Johnson’s single-shot COVID-19 vaccine from four-and-a-half months to six months.

In a July 28 concurrence letter (pdf), the FDA said Johnson & Johnson had provided sufficient data to support the extension of the company’s vaccine to six months, including for already expired batches, provided they have been stored within the required temperature range of between 36–46 degrees Fahrenheit (2–8 degrees Celsius).

A decision to lift the suspension of administering the Johnson & Johnson vaccine might occur by Friday, White House chief medical adviser Dr. Anthony Fauci said Sunday on ABC's "This Week."

"But no indication they will stop using it?" "This Week" Co-Anchor Martha Raddatz asked Fauci.