We just got our first evidence that Moderna's coronavirus booster shot works to fight variants

• Moderna said Wednesday that a third dose greatly boosted antibody levels in volunteers in a study.
• An updated vaccine tailored against the B.1.351 coronavirus variant worked best, the company said.
• These are the first clinical results from any vaccine developer testing an updated COVID-19 shot.

Moderna is voicing confidence in a new version of its coronavirus vaccine tailored to fight variants of the virus. The biotech on Wednesday released data from a clinical trial testing its booster doses.

These are the first clinical results from any vaccine developer testing an updated COVID-19 shot. Moderna started research earlier this year to see whether it could develop a booster shot that would better protect against the B.1.351 variant, first found in South Africa.

Earlier research, done in petri dishes, suggested the B.1.351 variant was able to partially reduce the effectiveness of leading vaccines, including those from Pfizer and Moderna. The current shots likely still work against variants, but experts have said it's less clear how long that protection will last.

The latest findings could support the rollout of variant-specific booster doses, potentially as soon as the end of 2021 or 2022 in some countries, Moderna executives recently said.

While no one knows exactly when people would need booster shots, experts generally think the presence of variants that can partially degrade the efficacy of vaccines will speed up the need to give additional shots, particularly to the most vulnerable populations.

Researchers tried 3 kinds of boosters in the clinical trial

In the new clinical trial, Moderna tested a third dose in people who had gotten the two-dose vaccine six to eight months ago.

The study volunteers received one of three options: another dose of Moderna's original vaccine, a new version tailored to the B.1.351 variant, or a multivalent version with a 50-50 mix of the original and B.1.351 vaccines.

In a press release on Wednesday, Moderna described initial results from the original vaccine and the B.1.351 shot. The company plans to later report on the multivalent version and to test an even lower dose strength. The research has not been published in a peer-reviewed medical journal, and Moderna said it had submitted the paper to a preprint server.

Before the volunteers got the third dose, Moderna tested blood samples to see how durable the vaccine response had been through six to eight months. While 37 of the 40 people had detectable levels of antibodies, the virus-fighting proteins that are the backbone of our immune response, only about half of the participants had detectable levels of antibodies against the B.1.351 variant and P.1, the variant first identified in Brazil, Moderna said.

One booster shot was most effective and led to fewer reactions

The study found that the people who were given the B.1.351-specific booster had the most effective immune response. Antibody levels were about 62% higher in that group than in the group given another dose of Moderna's original shot, Moderna said.

"We are encouraged by these new data, which reinforce our confidence that our booster strategy should be protective against these newly detected variants," Moderna CEO Stéphane Bancel said in a statement. "The strong and rapid boost in titers to levels above primary vaccination also clearly demonstrates the ability of mRNA-1273 to induce immune memory."

Moderna said the booster shots were well tolerated overall, with people having reactions similar to those after the second dose.

The B.1.351 booster shot appeared to produce slightly less severe reactions, Moderna said. About 11% of the people who got the variant-specific booster reported a grade-three adverse event, which is severe enough to affect daily living, compared with 15% who got the third dose of the original shot.

The most common side effects were fatigue, headache, muscle pain, and joint pain.