As vaccine makers turned to boosters, new studies by Johnson & Johnson and Pfizer on Wednesday showed that additional vaccinations can dramatically increase antibodies to the coronavirus.

Both companies stated that they would submit the new data to the Food and Drug Administration for evaluation. If the agency approves, the Biden government plans to make booster vaccinations available to all Americans eight months after vaccination.

Johnson & Johnson’s one-shot vaccine was missing from the government booster plan announced last week. With the new data, however, the company hopes to be part of the first distribution of additional footage, which could come as early as September.

“We look forward to discussing with public health officials a possible strategy for our Johnson & Johnson Covid-19 vaccine, which will be boosted eight months or more after the primary single-dose vaccination,” Dr. Mathai Mammen, global head of Janssen Research & Development at Johnson & Johnson, said in a statement.

In February, the FDA gave Johnson & Johnson emergency approval for its injection, which in its US clinical study showed 72 percent effectiveness against infections. This study was done before the delta variant became widespread. But in another study published earlier this month, South African researchers found the vaccine was up to 95 percent effective against death from Delta and reduced the risk of hospitalization by 71 percent.

In its new study, Johnson & Johnson tracked 17 volunteers from last year’s clinical study. When they received a booster dose after six months, their antibodies to the coronavirus rose nine times as high as they did after the first dose. The data has not yet been published in a scientific journal.

Small studies of Moderna and Pfizer BioNTech booster syringes have found comparable jumps in antibody levels, as the companies reported in the latest earnings calls.

On Wednesday, Pfizer released new data from 306 people showing that a third dose given five to eight months after the second triggered a strong immune response. The antibody level against the coronavirus among the volunteers has more than tripled, the company said.

Since the three vaccines were not tested in a head-to-head comparison, it is not possible to determine which one provides the greatest boost.

It’s also hard to say whether these big leaps in antibodies will result in a highly effective booster.

Updated

Aug. 25, 2021, 6:11 p.m. ET

A number of studies have shown that higher antibody levels offer better protection, especially against the Delta variant. But other parts of the immune system such as T cells are also important. These data cannot therefore give an exact estimate of how effective a booster vaccination against Covid-19 will be.

“It is too early to assess the protection,” said Dr. Dan Barouch, a virologist at Beth Israel Deaconess Medical Center in Boston, who led some studies for Johnson & Johnson but was not involved in the booster study.

The new data from Johnson & Johnson may allay some early concerns about whether more than one dose of the vaccine might be effective. The Johnson & Johnson vaccine uses an adenovirus to transport coronavirus genes into cells. When the company started its studies, some experts wondered if people were making antibodies to the adenoviruses that could render a booster useless.

“Something that we would have previously considered a big barrier may not be that big a barrier,” said Lynda Coughlan, a virologist at the University of Maryland School of Medicine who was not involved in the study.

The Johnson & Johnson vaccine is the only vaccine approved as a single dose in the United States or Europe. Since November, the company has been conducting a clinical study to investigate how much protection people get from two doses two months apart. This study should deliver results in the next few weeks.

After the volunteers in this study received the second dose, their antibody levels increased by a factor of three. The much larger increase in the new booster study is likely due to the longer latency between doses: a six-month hiatus gives the immune system time to develop a more mature response to the coronavirus.

This spring, Johnson & Johnson’s vaccine distribution in the United States was severely hampered by manufacturing mishandling by a contractor at a Maryland factory. Only eight percent of Americans vaccinated – or about 14 million people – have received a Johnson & Johnson vaccine, according to the Centers for Disease Control.

Since then, US supply of the vaccine has grown. A federal official said the government had sufficient reserves to provide boosters to anyone who received a first dose of Johnson & Johnson, should it be approved.

News of potential Johnson & Johnson boosters for Americans could sting other countries still waiting for their first doses of the vaccine. South Africa, for example, ordered 31 million doses of the vaccine, but only two million people there received it.

In an interview with CNBC last month, a Johnson & Johnson executive said the company plans to produce 500 to 600 million cans worldwide by 2021.

It remains to be seen how long the high levels of antibodies produced by the booster will last. “We don’t have long-term human studies, but my prediction would be that these responses should be sustained after the boost,” said Dr. Coughlan.

Noah Weiland contributed the reporting.