Some worry the Johnson & Johnson shot isn’t enough. A new study offers answers — and questions.

Choosing the Johnson & Johnson vaccine was about convenience for Amy Riley. She was knee-deep in planning a virtual event for her communications job when she became eligible for vaccination and couldn’t chance losing two days to potential side effects from a two-dose mRNA vaccine. Plus, she wanted to be fully immunized before her niece’s graduation.

Riley, 41, said she knew Johnson & Johnson’s shot had lower efficacy numbers than the other approved vaccines, but she remembers health officials assuring the public that the one-dose vaccine was just as good.

Now, she’s not sure she believes them, frustrated by how little information seems to be available about her vaccine brand’s effectiveness against the delta variant.

“I would still get it over not getting vaccinated, if that was the choice,” Riley, a Riverside County, Calif. resident, said in an interview with The Washington Post. “I just feel like, well, did I make the wrong decision?”

Riley is among the more than 13 million Americans who have received the Johnson & Johnson jab, some of whom now question how well it will protect them against the delta variant. She and others who fret about their one dose have reasonable concerns given previous data and a rocky rollout. New research offers encouraging evidence about how the Johnson & Johnson vaccine stacks up against its competitors — and the delta variant, according to infectious-disease specialists. However, there are still lingering questions about booster shots.

Earlier clinical trials showed the Johnson & Johnson vaccine was 66% effective overall in preventing moderate to severe disease four weeks after the shot with effectiveness varying depending on location. Its competitors, on the other hand, recorded 90%-plus effectiveness against the coronavirus. Anthony Fauci, the nation’s leading infectious-disease expert, has said all three vaccines are effective.

Vaccine experts cautioned recipients against comparing vaccines as the numbers didn’t tell the full scope of what was measured, but the memory of that lower number still lingers in the minds of Johnson & Johnson vaccine recipients such as Riley.

She’s not alone in her fears about the introduction of the vaccine to the market.

Keith Alberts, 61, of Cleveland, said he was nervous about the single shot and its effectiveness compared with other options, but it was the only one available at a vaccination site. He said he would have gone to another location to receive a Pfizer or Moderna jab had he known. Now he has “zero confidence” in his current immunization, he told The Post.

“Everybody is talking about Pfizer. Everybody is talking about Moderna. But you don’t hear anything about [Johnson & Johnson],” he said. “I’m dubious about its ability to protect.”

A trial out of South Africa has caught the attention of people, particularly infectious-disease experts, with regard to the effectiveness of Johnson & Johnson’s vaccine against the delta variant.

The Sisonke trial looked at the effectiveness of the single-dose shot in South Africa’s front-line health-care workers.

Linda Gail-Bekker, one of the researchers for the trial, said the vaccine was found to be to be up to 95% effective in preventing delta variant-caused death and up to 71% effective in fending off hospitalizations.

The results, which have not been peer reviewed or published in a scientific journal, show that the vaccine is not only safe but also effective in fighting the delta variant, she said.

Johnson & Johnson said in a statement that the preliminary results add to the body of evidence that confirms its efficacy in staving off death and severe illness.

“A single-shot vaccine that provides protection and prevents hospitalization and death is an important tool in the global fight against covid-19,” the company said. “Evidence from our Phase 3 ENSEMBLE study demonstrates the efficacy of the J & J single-shot coronavirus vaccine, including against viral variants that are highly prevalent. Regardless of race and ethnicity, age, geographic location and comorbidities, these results remain consistent.”

The company also pointed out a double-blinded study published in the New England Journal of Medicine that showed its vaccine gave antibody and T-cell immune response protection against the coronavirus. The defense lasted up to eight months after immunization.

Preliminary findings out of South Africa are somewhat reassuring though there are still some unanswered questions, said Tom Russo, professor and chief of infectious disease at the University at Buffalo Jacobs School of Medicine and Biomedical Sciences.

Russo said researchers on the South Africa study didn’t provide data about symptomatic disease and didn’t define what it considered to be mild or moderate infections though the overall results are encouraging.

“There’s room for improvement there with the 71% [of preventing hospitalizations],” he said, noting that he’d like to see the full details of the study. “Data suggest Johnson & Johnson is a very solid vaccine that protects against the delta variant. It may not match up as well against the mRNA vaccine, but we need a lot more data on both of these vaccines.”

As more data trickles out about vaccines and how well they protect people against mutations of the coronavirus, some people are wondering if they should get a shot of another vaccine to protect themselves.

Alberts said he would like to get a dose of one of the mRNA vaccines, but that is not permitted in the United States.

The United Kingdom, Canada and Spain are among a growing list of countries mixing vaccines to boost immunity.

Mary Kay Vellucci, 63, said she is displeased that the federal government hasn’t approved topping off the vaccine with alternatives despite other nations forging ahead.

“What bothers me is the fix for that — the science for that — exists, but the FDA approval for it doesn’t,” she said. “So it’s not the vaccination that I have a problem with; it’s just that you can’t get what you need.”

The hesitancy to push ahead is a bit more complex than a simple solution appears to warrant, said Art Reingold, division head of epidemiology at the University of California at Berkeley.

There’s not enough data to suggest that individuals should take it upon themselves to mix vaccines, an action that’s prohibited, Reingold noted. That lack of data can make leaders extra cautious before making a broad endorsement.

“The risk-benefit with a vaccine is a little different [compared with medication given to people who might already be ill],” he said. “If we’re giving shots to tens of millions of people who are really healthy, they need to be really safe.”

Reingold also noted the issue of vaccine surplus and waste as so much of the world is still suffering under the pandemic’s thumb without vaccine relief to help.

Tedros Adhanom Ghebreyesus, director general of the World Health Organization, said last week that countries should hold off on giving booster shots as the vaccine gulf between rich and poor countries expands.

“I understand the concern of all governments to protect their people from the delta variant,” he said. “But we cannot accept countries that have already used most of the global supply of vaccines using even more of it, while the world’s most vulnerable people remain unprotected.”

Russo said people should be confident that vaccines are largely working.

“What’s going on in this country is the pandemic of the unvaccinated,” he said, citing that 99% of coronavirus deaths have been among the unvaccinated. “This means that our vaccines are getting it done now.”

Lisa Mednick Powell, a 64-year-old musician from Joshua Tree, Calif., is a Johnson & Johnson vaccine recipient confident in her jab.

She said she took the first available vaccine because her family was concerned that her asthma coupled with her career in the music business would put her at higher risk .

Five months after her shot, she has no regrets, she said.

If there is an opportunity to get a booster, particularly with an mRNA vaccine, she said she probably will take it.

“But I’m not afraid because I got the Johnson & Johnson vaccine,” she said. “I’m very glad I got it.”