Relief, eager anticipation for many Americans ahead of FDA decision on booster shots

For Mitchell Kronenberg, answers on the Johnson & Johnson vaccine can't come soon enough.

A 42-year-old dad living outside Charlotte, North Carolina, Kronenberg enrolled in a clinical trial for the single-dose vaccine and got his shot last January. Since then, he's been patiently waiting for U.S. federal regulators to tell him what to do next to stay safe: Should he get another Johnson & Johnson shot? Switch to Moderna or Pfizer? Is his single-shot vaccination enough to protect him from spreading the virus to his unvaccinated 4-year-old son?

"Why is it taking so long? You have people out on a ledge out here," he said of the process.

The U.S. Food and Drug Administration is expected to decide as early as Wednesday whether the 15 million Americans who got the single-dose J&J shot by Janssen Pharmaceutical Companies should get a second dose. The decision comes after studies suggested effectiveness against moderate and severe symptoms climbed from 70% with one dose to 94% with two doses.

Also anticipated is a decision on possible third booster shots for Moderna recipients and whether Americans can mix vaccine brands when getting a booster.

With a decision expected soon, an advisory panel to the Centers for Disease Control and Prevention planned to meet Thursday to offer the public a final recommendation.

Typically a wonky endeavor ignored by most Americans, the FDA and CDC regulatory process is now being closely followed by millions of Americans. People describe joining private Facebook groups to swap the latest research and repeatedly getting their antibody levels checked, even if experts warn it's an incomplete measure of a person's immunity.

Some have even opted to mix booster shots on their own even without the federal government's blessing.

"Every day, I was researching, Googling, reading everything I could get my hands on," said Lynne Conway, a 59-year-old development officer for an animal shelter in Ithica, New York.

Conway said she experienced mild heart inflammation following her first dose of Moderna last March. After ruling out other causes, her doctor recommended she get the J&J so she can be fully protected against COVID-19. She got that shot in August, despite there being no official green light from regulators to mix shots in such a way.

"The relief of finally deciding to get the (J&J) vaccine, and have it over with, was monumental," she said.

J&J recipients are among those who expressed being most on edge emotionally, with the 70% effectiveness after one shot -- lower than the two doses of Pfizer or Moderna.

Jadzia Pierce, who lives outside of Washington, D.C., said she was relieved regulators were inching toward a decision. In her early 30s, Pierce isn't at serious risk of major complications from COVID-19.

But she also would like to visit higher-risk relatives this holiday season and to know she's not at risk of infecting them.

"Even if the answer is (that it puts) other people around at risk, then I guess I just won't go home," she said. "I just feel like safety is the most important thing at this point and that's really all that matters to me."

J&J declined to comment for this article, citing pending action by regulators. But last Friday, after a meeting of FDA advisors, a top company official told ABC News the J&J vaccine likely protects people by triggering the protection of a person's T-cells, which is harder to measure than antibodies.

J&J's "biggest, most impressive components are T-cells, which contribute to both efficacy and durability of our vaccine," said Mathai Mammen, global head of R&D at the Janssen Pharmaceutical Companies of Johnson & Johnson.

For its part, the FDA says its staff is working around the clock to comb through technical data. The CDC too has to hear from its own advisory panel before making a decision.

Kronenberg, who has to travel for work as a director for a medical device company, said he still worries about spreading the virus to his son. He says he understands wanting to get it right but wishes the various agencies could get in a room together and hash it out immediately, rather convene separate meetings and reviews that can drag on for weeks.

Pierce, also in limbo having received the J&J, said she's torn on whether to be frustrated with the regulatory bureaucracy.

"I'm trying to be patient. I do realize that this stuff takes time, and I don't want to be following advice that is too rushed and not complete," she said.

ABC News' Sony Salzman contributed to this report.