A second shot of the Johnson & Johnson vaccine boosts protection against symptomatic and severe covid-19, the disease caused by the coronavirus, the drug company announced early Tuesday.

Those booster shots also generated additional antibodies, molecules churned out by the immune system to help fight off infections.

“It’s very exciting news,” said Vanderbilt University Medical Center infectious-diseases and preventive-medicine professor William Schaffner, though he cautioned that not all the data had been seen and that information must be reviewed by the Food and Drug Administration before recommendations can be made.

“The people who received J&J have been a little concerned that they’ve been left in the shade when it comes to repeat vaccination,” Schaffner said. These results, as presented, seem to signal that those vaccine recipients might “expect a booster in their future,” he said.

Under the FDA’s emergency use authorization, the Johnson & Johnson vaccine is given as a single dose, unlike the two shots required for full immunization with the Pfizer-BioNTech and Moderna mRNA vaccines.

Results published this summer indicate that the Johnson & Johnson vaccine generates lasting amounts of antibodies able to target the delta variant and other variants of concern. In June and July, when delta was ascendant, the effectiveness of the one-shot vaccine was 78 percent against observed covid-19, according to a report published Thursday that has not yet gone through peer review.

A second dose, given 56 days after the first, further improves protection, the company said in a news release Tuesday. According to the company’s Ensemble 2 study, a Phase 3 randomized clinical trial, efficacy was 100 percent against severe or critical covid-19 when assessed at least two weeks after the booster. Efficacy against symptomatic disease was 94 percent in the United States and 75 percent globally.

“We now have generated evidence that a booster shot further increases protection against covid-19 and is expected to extend the duration of protection significantly,” Johnson & Johnson chief scientific officer Paul Stoffels said in a statement.

As new coronavirus variants emerge and the longevity of vaccine protection remains unknown, scientists are researching how booster shots could work. (John Farrell/The Washington Post)

Full data from that trial was not released, the company said. It said it will be submitted for publication in months.

“This is good news and moves us closer to having an answer about the utility of boosters for the millions of Americans who received the J&J covid-19 vaccine,” said Lisa L. Maragakis, an infectious-disease epidemiologist at the Johns Hopkins University School of Medicine. “Of course, we will need to wait to see the actual data and hear from both the FDA and CDC as they review it.”

Antibody levels, too, spiked after booster shots. People who received a Johnson & Johnson booster shot two months after vaccination had antibody levels four to six times as high, while the levels in people who had boosters at six months rose 12-fold in the following four weeks, the company said.

“An increase in antibodies is associated with two good features,” Schaffner said. First, this increase can lead to longer-lasting protection. Second, he said, “not only do you get more antibodies but a wider display of antibodies,” which means an immunized person is likely to be better defended against a variety of variants.

Regulators would have to authorize Johnson & Johnson booster shots before the public could receive them. Health communication expert Scott C. Ratzan, co-founder of the vaccine literacy initiative Convince USA, said that the results looked “promising” but that he was disappointed they were delivered by a news release. “Generally speaking, this is not the way we should be having vaccines approved and discussed,” said Ratzan, who was a vice president at Johnson & Johnson until 2013.

On Sunday, National Institute of Allergy and Infectious Diseases Director Anthony S. Fauci told NBC News that the FDA review of second Johnson & Johnson shots, as well as Moderna boosters, is “a couple to a few weeks away. … We’re working on that right now to get the data to the FDA so they can examine it and make a determination about the boosters for those people.”

Because it does not require a second appointment, the Johnson & Johnson vaccine had been sent to harder-to-reach communities soon after the shot became available. Nearly 15 million Americans received Johnson & Johnson doses, while about 166 million people have been fully vaccinated with the vaccines made by Pfizer-BioNTech and Moderna.