Janice Higgins isn’t the type to ignore advice from health professionals, especially when it comes to protecting herself from the coronavirus. When officials recommended masks, she wore them. When the vaccines arrived, she got the first one available to her.

But the Biden administration’s plan for coronavirus booster shots is testing her faith in the process.

Higgins is one of 14 million Americans who received the Johnson & Johnson vaccine, which isn’t covered under the booster plan announced Aug. 18. Officials’ decision to focus first on recipients of the Pfizer-BioNTech and Moderna shots panicked her, she said. She already worried that she had received the weakest of the three vaccines and questioned how it would stand up against the ferocious delta variant.

“I feel like I’m being neglected,” said Higgins, 69, of Hadley, Mass. “I’m trying to do the right thing and I’m being stymied by the health establishment.”

Scores of other Johnson & Johnson vaccine recipients voiced similar concerns, saying they felt left behind by the White House’s move toward boosters for everyone but them. Officials noted the single-dose regimen was authorized later and they expect Johnson & Johnson booster shots would be needed, too — a fact that they said would become evident when data on long-term effectiveness is made available.

They also emphasized that for most people, one dose of Johnson & Johnson is a strong shield against the worst effects of covid-19. A small, non-peer-reviewed study by the company showed it was effective against the delta variant, too, and other research indicates immunity remains strong eight months after injection.

“For the majority of people, outside of the higher-risk groups, the Johnson & Johnson does currently protect against severe disease, hospitalizations and death quite well, even with the delta variant,” said Purvi Parikh, an allergist and immunologist at NYU Langone Health. “So at this moment in time, you should not panic or stress.”

But that’s little comfort to those eager to bolster their protection as soon as possible. Many fret about declining immunity and struggle with the thought of waiting on the sidelines while infections surge and friends and neighbors line up for their third doses. Their unease highlights the confusion surrounding the White House’s booster plan, which must still be approved by health regulators.

“We’ve kind of left people who got J&J high and dry, we have no idea what to tell them,” said a federal health official who spoke on the condition of anonymity because they were not authorized to discuss the issue.

The concerns mounted last Thursday, when the heads of the Food and Drug Administration and the Centers for Disease Control and Prevention privately warned the White House that the plan may have to be slowed, according to people familiar with the matter. Acting FDA commissioner Janet Woodcock and CDC Director Rochelle Walensky said their agencies would need more time to collect and analyze the data, and that third shots may initially have to be limited to Pfizer-BioNTech recipients.

Both officials had publicly endorsed the broader plan last month.

Monica Gandhi, an infectious-disease expert at the University of California at San Francisco, said the messaging from the administration “has managed to terrify the vaccinated and make many of the unvaccinated dig in.”

“We scared people to pieces,” she said. “Now we’re in this confusing place.”

“I sympathize with them,” Gandhi added of Johnson & Johnson recipients. “This is a true minority population.”

In their announcement last month, White House officials said every adult American should get a booster shot eight months after they completed their initial dosage, starting the week of Sept. 20, if the FDA and the CDC approve the plan. They pointed to a growing body of data showing that immunity declines over time, and said boosters would likely be necessary, especially as delta rips across the country.

Top infectious-disease expert Anthony S. Fauci on Sept. 5 said Pfizer booster shots will likely be ready by Sept. 20, but Moderna's could be delayed. (Reuters)

The White House said the move at first would only apply to recipients of the Pfizer-BioNTech and Moderna vaccines, which were the first shots rolled out in the United States and represent those given to the majority of vaccinated Americans. Those shots use molecules called messenger RNA to trigger an immune response, while the Johnson & Johnson vaccine uses a harmless virus called an adenovirus to achieve the same result.

“For people who received the Johnson & Johnson vaccine, we anticipate vaccine boosters will likely be needed,” Surgeon General Vivek H. Murthy said in the Aug. 18 briefing by the White House covid-19 response team. “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 for J&J booster shots.”

Johnson & Johnson said in late August that data from a pair of studies it conducted over the summer showed a booster dose of its vaccine generated a ninefold increase in the types of antibodies that neutralize the coronavirus. Earlier research showed that antibody responses from a single shot remained strong eight months after immunization, according to the company.

"With these new data, we also see that a booster dose of the Johnson & Johnson covid-19 vaccine further increases antibody responses among study participants who had previously received our vaccine,” Mathai Mammen, global head of research and development at Johnson & Johnson, said in a statement. “We look forward to discussing with public health officials a potential strategy for our Johnson & Johnson COVID-19 vaccine, boosting eight months or longer after the primary single-dose vaccination.”

Some high-profile health experts have criticized the White House’s plan as premature. They say the officials seemed to be preempting health regulators and argue that the data on the risks or benefits of boosters is still too thin to justify such a massive expansion of vaccinations when millions of Americans haven’t received their first shots.

The FDA is set to hold a key meeting on Pfizer-BioNTech’s booster data in mid-September, just a few days before the planned start of the booster campaign. It’s not clear how it will affect the rollout or what it will mean for Johnson & Johnson recipients.

For John Halloran and his family, the wait is the hardest part.

“The messaging we’re getting is, ‘We don’t know yet,’” said Halloran, a college professor from Chicago. “That’s nerve-racking.”

The 42-year-old and his wife didn’t hesitate to get Johnson & Johnson shots when offered them in March. Being vaccinated brought back a sense of normalcy. The couple went to church, took their school-age daughters to gymnastics, and visited safely with their parents. They were looking forward to hosting an in-person birthday party for their youngest child for the first time since the pandemic began.

But the explosion in delta cases has renewed their worries, as has the administration’s call for Pfizer-BioNTech and Moderna recipients to plan on receiving third shots. After the White House’s announcement, they considered pairing their Johnson & Johnson doses with one of the other vaccines — a practice not authorized by health officials — but decided against it, in part because they dreaded what the insurance negotiations would be like if something went wrong.

For now, they say they accept the officials’ rationale for holding off on their booster shots. But Halloran is wary of returning to in-person work. His daughters are going back to the classroom, too, and he’s afraid they will bring the virus home — or worse, become part of the growing wave of pediatric covid-19 cases.

“If we knew we’d have additional protection,” he said, “we’d have peace of mind.”

Other Johnson & Johnson recipients say they, too, felt the urge to get a shot of the Pfizer-BioNTech or Moderna vaccine after the White House’s announcement.

Leslie Jansen, a 37-year-old mother from Chesapeake, Va., said she became convinced Johnson & Johnson was inferior to the other vaccines after reading reports of its lower effectiveness and links to rare blood clots in a small number of patients. News of the booster plan only compounded her regrets about not getting one of the other shots.

“I feel like we’ve been left in the cold. I feel like the mRNA shots offer better protection,” said Jansen, who got the Johnson & Johnson vaccine earlier this year. “It seems weird that you can’t just switch.”

Some countries have allowed people to get the mRNA shots after receiving the AstraZeneca vaccine, which uses the same technology as the Johnson & Johnson one. But no U.S. health agencies are recommending a mix-and-match approach, and the potential risks and benefits are unclear.

Research on mixed doses is underway at the National Institutes of Health. For now, Gandhi, of UC San Francisco, said there’s no indication that combining the adenovirus shot with an mRNA shot is unsafe, but noted there’s little data on the practice.

“I think we should have some humility in that nobody really knows” if a combination approach helps, she said.

She added that Johnson & Johnson vaccine recipients might take comfort in the fact that health officials aren’t recommending extra shots right now, saying it showed they remain confident in the vaccine’s effectiveness. “If one had shown more risk than the other, they would have said so,” she said.

Parikh, the NYU immunologist, said she agreed with the Biden administration’s decision to wait on recommending Johnson & Johnson boosters, but said she didn’t have reservations about extending supplemental mRNA shots to Johnson & Johnson patients with compromised immune systems. Data out of Britain suggested mixing vaccines is safe, she said, and “exceptions should be made depending on their individual risk.”

Ileana Benitez, a kidney transplant patient, was among those holding out hope that officials would allow her to combine the single dose of the Johnson & Johnson vaccine she received in the spring with an mRNA shot.

The 41-year-old takes immunosuppressants to prevent her body from rejecting her donated tissue, which in turn makes it hard for her immune system to produce a full response to the vaccine. Cooped up at home in Kensington, Md., she feared that even a trip to the grocery store or the open house at her daughter’s elementary school could put her in jeopardy.

In mid-August, Benitez fell between the cracks when the administration opened booster shots of the mRNA vaccines to immunocompromised people. She watched as friends in her Facebook group for transplant patients signed up for extra doses right away, and wished she could join them.

Later, when officials said they were opening Pfizer-BioNTech and Moderna boosters to the general public, she felt spurned again. “Just wait, because we don’t know” was the message she said she heard from officials.

“At the same time I’m seeing everybody getting a booster,” she said. “I know that delta is very dangerous for me, and I don’t feel like there’s guidance for me on what to do.”

Recently, as her daughter’s school year was starting, Benitez decided to get a Pfizer-BioNTech shot on her own, although her doctors recommended against it.

“I don’t know what will happen with my immune system. But I know that if I get covid, it will not be fun or easy,” she said. “I need to do something, and I will do it.”

Higgins, the 69-year-old Hadley resident, also tried to sign up for a Pfizer-BioNTech booster, but workers at her local pharmacy turned her away. “They looked at me like I was trying to do something illicit,” she said.

Until she hears more guidance from officials, she said, she’ll be reluctant to spend much time in public. She recently cut back on her Pilates classes and trips to the nail salon. And she stopped making regular visits to a thrift shop that supports a local hospice.

“The little things that are part of your life and become important connections to the community — they all got broken by the pandemic,” she said. “The sooner we feel protected, the more we can get back into those things.”

Lena H. Sun, Tyler Pager, Laurie McGinley and Ben Guarino contributed to this report.