A single dose of the Moderna coronavirus vaccine given as a booster rapidly increased the level of antibodies in people who had already been vaccinated, the company announced on Wednesday.

The antibodies produced by the booster were effective against the original form of the virus, as well as against the variants of concern first identified in South Africa and Brazil. A second booster specifically designed to counter B.1.351, the variant identified in South Africa, produced an even stronger immune response against that variant.

The results are from an early stage of Moderna’s trial of the boosters and have not yet been published or vetted by other scientists. Moderna plans to post the findings to the preprint server bioRxiv, the company said on Wednesday.

“We are encouraged by these new data, which reinforce our confidence that our booster strategy should be protective against these newly detected variants,” Stéphane Bancel, Moderna’s chief executive officer, said in a statement.

The boosters, tested in 40 participants, were administered as a third shot six to eight months after the two-shot immunization with the current vaccine. Antibodies from the initial vaccination were detectable in 37 of those people, but in about half the participants, the antibodies performed poorly against the variants that have pummeled South Africa and Brazil. The boosters raised the levels of the antibodies against both variants, although the boosters were still slightly less effective than against the original form of the virus.

The mRNA platform used in the Pfizer and Moderna vaccines can be readily tweaked, enabling the companies to produce newer versions within weeks. Moderna began modifying its vaccine to combat the variant identified in South Africa, after reports emerged that the existing vaccines are slightly less effective against that variant. The variant carries a mutation that helps the virus sidestep the immune system.

Moderna is testing three strategies for enhancing the immunity produced by the current vaccine: using the current vaccine as a booster; using the booster designed specifically to combat B.1.351; and a combination of the two in a single vaccine.

Wednesday’s announcement refers to results achieved two weeks after immunization with either a booster of the original vaccine or of the booster modified for B.1.351. The company plans to release data from later time points, and from tests of the combination booster, as they become available.