Call them the Johnson & Johnson 16 million.

That’s the number of Americans who got the one-shot vaccine as their first COVID-19 dose and now find themselves in a gray zone. While health officials encourage those who’ve gotten the double-dose Pfizer-BioNTech or Moderna series to get a third shot, J&J recipients are limited to their original one and a single booster.

Of the 16.3 million people who got the J&J vaccine as their first dose, about 3.5 million have gotten boosted, according to the Centers for Disease Control and Prevention. But they can’t go beyond that.

“I suspect there are thousands of J&J recipients in my situation who are questioning our protection," said Donna Alston, 61, of Philadelphia. “I went to my pharmacy last week to see if I could sign up and they said no. Barring additional guidance from the CDC, I’m prohibited from getting additional vaccine.”

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The J&J vaccine was authorized by the Food and Drug Administration on Feb. 27. It wasn’t until Oct. 20 the agency allowed a booster dose of the vaccine for adults age 18 and older at least two months after their initial dose. No third dose has been authorized.

That’s in part because so few J&J doses have been given compared with the 190 million who've been fully vaccinated with a two-dose series of either Pfizer or Moderna.

That's a big data gap, said Dr. Kelly Moore, chief executive officer of Immunize.org, which educates health care professionals about U.S. vaccine recommendations.

“The mRNA vaccines went into widespread use at least three months earlier and established a far larger market share,” Moore said, adding that U.S. regulators must make decisions based on evidence that takes time to gather and not assumptions.

The FDA is collecting that data now, said Dr. Peter Marks, director of FDA's Center for Biologics Evaluation and Research.

In a call with reporters Monday, he said there was strong evidence an adenovirus vector vaccine, such as J&J's, followed by an mRNA vaccine produced a very good immune response. Now, the agency is looking at how well that response lasts. He expects information to be coming within weeks to months.

“As we get those data, we'll analyze them and then potentially make recommendations,” Marks said.

The wait isn’t easy, especially when public health messaging is focused on the Pfizer and Moderna vaccines, said Jill Oliver, 57, of Boulder, Colorado.

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“They keep saying, ‘Three doses, three doses.’ Well, please finish that sentence and say, ‘If you’ve had J&J and are boosted, you’re set,'” she said.

Oliver got her J&J shot in July. She didn’t choose; it was what was on offer.

“All the doctors were saying, ‘The best vaccine is the one in your arm.’ So when it was my turn, I took it,” she said.

Oliver followed the public health advice again, getting her booster – Moderna – on Halloween, less than two weeks after they were authorized.

“I thought I was good,” she said.

Until, that is, data began to show the vaccine's protection could fade over time. Then came the ultra-infectious omicron variant, and suddenly two shots just didn’t seem like enough. Now, she’d like another boost.

Coverage from the J&J vaccine does wane over time, acknowledged Dr. William Schaffner, an infectious disease expert at Vanderbilt University Medical Center. But the protection it offers against severe illness and death remains high, he said.

“I can understand some of those people feeling a little forlorn and at a deficit because of all the discussion about three shots,” he said. “I would just comfort them and let them know that J&J followed by one of the mRNA s gets a huge antibody response.”

In Denver, Emily Moore, 50, got a dose of Pfizer vaccine as a booster to her original J&J. A diabetic, she'd really like a third now, but has been turned away from her pharmacy, which is waiting on further booster guidance.

“I’ve been honest," she said. "I haven’t tried doing it sneakily, or lying and saying I lost my card."

Even for those willing to skirt the truth, it appears harder to sneak extra doses than it once was. In Boulder, Oliver said she had several friends who got J&J as their first shot and then went in to get a second shot without mentioning their first.

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No more.

“Our county has put the kibosh on that," she said. "They started checking people’s cards and their system, so even though a lot of people did it early on, you can’t now.”

For now, it’s a waiting game for J&J recipients hoping for another immune system boost.

In Richmond, Virginia, Kevin Rosengren is on board with following the data. He got his first dose in late March and a booster in late October. He sympathizes with the regulators doing science in real-time.

“We’re getting information fast and furious; it’s a tough call,” he said.

Even so, Rosengren would like to get a third dose of vaccine as omicron sends cases to record levels in the U.S.

“If you just wanted to strap a bag of vaccine to me so it was constantly dripping into my body, I’d be fine with that,” he said.

Contact Elizabeth Weise at eweise@usatoday.com

This article originally appeared on USA TODAY: COVID J&J recipients want a third shot. The CDC has yet to approve one