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A third shot of Moderna’s COVID-19 vaccine at half the dosage and a third shot of a reworked version of the vaccine both appear to raise antibody levels against worrisome variants of the virus that first emerged in South Africa and Brazil, according to early study results released Wednesday by the Cambridge company.

But the reworked messenger RNA vaccine tailored to fight off newer strains of the coronavirus appeared to work better against the B.1.351 variant first identified in South Africa. The company looked at the level of antibodies in the blood of fully vaccinated volunteers who got one of the two boosters.

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“As we seek to defeat the ongoing pandemic, we remain committed to being proactive as the virus evolves,” said Moderna CEO Stéphane Bancel. “We are encouraged by these new data, which reinforce our confidence that our booster strategy should be protective against these newly detected variants.”

The volunteers received the booster shots about six to eight months after they had gotten the second dose of the original vaccine, as part of a mid-stage clinical trial that started last year.

Some volunteers received a 50-microgram dose of Moderna’s authorized vaccine, half the usual dosage. Others got the vaccine reprogrammed to combat variants.

Both booster shots increased immune-system components known as neutralizing antibodies against the two variants of concern. But the tailor-made booster appeared to produce more robust protection. Both boosters were generally well tolerated.

Moderna is continuing the study and expects more data soon.


Jonathan Saltzman can be reached at jonathan.saltzman@globe.com.