A decision on booster shots for the approximately 13.8 million Americans who received the Johnson & Johnson single-shot COVID-19 vaccine is likely to take weeks, according to people familiar with the issue.

Federal health officials are waiting on results from a government-backed clinical trial and from studies by Johnson & Johnson to determine whether those who received the vaccine should get a second shot of that vaccine or a booster using another vaccine, CBS Newsreported. The government trial has tested pairing a wide array of vaccine combinations, including a Pfizer or Moderna booster shot for people first vaccinated with the Johnson & Johnson vaccine. The trial's scientists hope to finalize early results by the end of August.

U.S. health officials emphasized that data on the J&J vaccine are coming later because it was approved later than the Pfizer or Moderna vaccines. "The J&J vaccine was not administered in the U.S. until March of 2021, and we expect more data on J&J in the coming weeks. With those data in hand, we will keep the public informed with a timely plan," Surgeon General Vivek Murthy said Wednesday.

In response to concerns about waning immunity with all COVID-19 vaccines, the Biden administration this week said people who received the two-dose Pfizer and Moderna shots should get booster shots of those vaccines eight months after their second dose, CBS News reported.

Johnson & Johnson has said that early data from its own research suggest the single-shot vaccine triggers "strong and stable" immunity for at least eight months and that the vaccine remains effective against the highly contagious delta variant that now accounts for nearly all cases in the United States. "We are engaging with the U.S. FDA, CDC, and other health authorities and will share new data shortly regarding boosting with the Johnson & Johnson COVID-19 vaccine," the company said in a statementlate Wednesday.