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A booster dose of Johnson & Johnson’s COVID-19 vaccine provided 100% protection against severe disease when given two months after the first inoculation, according to widely anticipated data that suggests it increases the potency of the one-time shot.

The booster was 94% effective at preventing symptomatic COVID infections in the U.S. portion of the Phase III trial, and 75% effective overall when it was given 56 days after the initial dose, the company said in a statement. A second study found the additional shot spurred a 12-fold increase in production of antibodies against COVID when it was given six months after the first.

The results come as many developed nations turn to booster shots to address waning protection from some vaccines and the rise of the more infectious Delta variant, which is sparking new outbreaks globally. The move has raised questions about the level of protection afforded by the existing shots and the equitable distribution of the vaccines now available.

“Our single-shot vaccine generates strong immune responses and long-lasting immune memory,” said Mathai Mammen, global head of research and development for Johnson & Johnson’s Janssen drug unit, in the statement. “When a booster of the Johnson & Johnson COVID-19 vaccine is given, the strength of protection against COVID-19 further increases.”

The time period between the initial vaccine and the booster shot was significant, the research showed. When the second dose was given two months after the first shot, it triggered a four-to-six fold increase in antibody levels—less than the 12-fold gain seen when it was given six months after the initial injection.

Most vaccinated Americans have received the potent two-dose messenger RNA vaccines made by Pfizer Inc. and BioNTech SE, or Moderna Inc., with a smaller group getting J&J’s single shot inoculation.

Booster Decision

The U.S. Food and Drug Administration is nearing a decision on whether to authorize a third shot of the Pfizer-BioNTech vaccine for vulnerable people who have already received the normal two-dose schedule. On Sept. 17, a panel of FDA advisers rejected Pfizer’s proposal to broadly recommend a third dose, and instead voted in favor of only giving it to people at high risk, including those 65 and older. The agency, which doesn’t have to abide by the panel’s advice, is expected to make a decision within days.

Moderna, which also makes a two-dose mRNA vaccine for COVID, has applied to the FDA for emergency authorization of its COVID-19 booster shot. The agency is expected to consider the application in the coming weeks.

That has left the smaller number of U.S residents who received the J&J vaccine, which proved less effective than the mRNA shots in clinical trials, waiting for clarity on whether they too will need boosters, and if so—when.

Separately in its statement, J&J said that a single dose of its vaccine provided strong protection that remained stable through July. The real world study compared 390,000 people in the U.S. given J&J’s shot to 1.5 million who were unvaccinated.

The immunization reduced hospitalizations by 81% and infections by 79%, the company said. There was no evidence the protection ebbed over time, including after the delta variant came to dominate the U.S. outbreak, it said.

J&J has provided the available data to U.S. regulators and plans to submit it to regulators elsewhere as well as the World Health Organization.