J&J boosters are on the horizon for everyone who got that shot - regardless of age, job, or health

• An FDA expert panel on Friday unanimously recommended the authorization of a J&J booster shot.
• If approved, the booster will be available to anyone who received the J&J shot.

The roughly 15 million Americans who received the one-shot Johnson & Johnson vaccine are poised to be eligible to roll up their sleeves for a second dose. On Friday, the Food and Drug Administration's expert panel voted unanimously to recommend the authorization of J&J's booster shot.

Unlike Pfizer and Moderna's booster shots, which the FDA greenlit only for certain groups, J&J's extra dose would be recommended to anyone who got the first shot, regardless of their age, occupation, or health status, once it's authorized.

The FDA panel's recommendation is a bit of good news after J&J's single-dose vaccine suffered a kind of fall from grace during its rollout. At one point, a factory error led vaccine ingredients to be improperly mixed, and then the CDC issued a pause on the vaccine while it examined the risk of an an extremely rare blood clot condition. Regulators later added a warning label for women under 50.

After that, recipients waited through months of silence from the Centers for Disease Control and Prevention and the FDA about the possibility of J&J boosters. All the while, health officials regularly updated Americans about the possibility of third Pfizer and Moderna doses.

Finally, in late September, J&J released trial results showing that two doses of its vaccine led to 94% protection against mild to severe COVID-19, up from the 74% efficacy found in the trial of its single shot. The data, which was not peer reviewed, put a two-shot J&J vaccine on par with Pfizer and Moderna's mRNA vaccines.

The FDA found a few issues with those results, however. On Wednesday, the agency released documents that suggested J&J's booster shot application lacked sufficiently robust clinical trial data. In addition to using a low sensitivity lab test in its trial, the FDA found, J&J's application included a study of just 17 participants who'd gotten boosters after six months - a very small sample size.

But J&J submitted stronger evidence, including a dataset of 16,000 participants, in support of giving booster shots after two months. (That dataset has not yet been independently reviewed by the FDA, however.)

Another factor for J&J recipients to consider is that despite the FDA committee's unanimous recommendation in favor of a booster shot, it appears that mixing and matching vaccines - topping off one dose of J&J with a Pfizer or Moderna shot - may trigger a better immune response than two J&J doses.

A preprint study, funded by the National Institutes of Health and released on Wednesday, looked at 458 people who had been vaccinated with one of the three US vaccine brands, then gotten a mix-and-match booster. Researchers found that J&Jers who got a J&J booster saw a 4.2-fold boost in neutralizing antibody levels, on average. By comparison, J&Jers who were topped off with a Moderna or Pfizer dose saw 76-fold and 35-fold increases in their antibodies, respectively.

"What the study shows is that regardless of what an individual received originally, getting boosted with one of the three vaccines that we evaluated, the one from Moderna, the one from Janssen, the one from Pfizer, led to good antibody responses in each of the groups," Dr. Robert Atmar, an infectious-disease professor at Baylor College of Medicine who led the study, previously told Insider.

Hilary Brueck, and Andrew Dunn contributed reporting.