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Moderna's coronavirus vaccine is still doing its job. But when it comes to the COVID-19 variant first found in South Africa, there could be some room for improvement.

More transmissible variants of COVID-19 first appeared overseas in the past month, and have since been found in the U.S. Both the Pfizer and Moderna vaccines have been found effective against the B.1.1.7 variant that was first found in London, and Moderna has confirmed it found no reduction in antibodies when its vaccine was used against that strain. But when researching the B1.351 strain, Moderna saw a sixfold reduction in antibodies produced, a study released Monday indicates.

Despite the shortfall, Moderna affirmed that the antibodies produced with the vaccine "remain above levels that are expected to be protective" against either strain. But "as an insurance policy," the company is working on a booster shot that could make the vaccine more effective against the South Africa-based strain, Moderna's chief medical officer Dr. Tal Zaks told The New York Times. "I don't know if we need it, and I hope we don't," he added.

So far, there has been no evidence to suggest the B1.351 strain is more deadly than the more widespread virus, only that it spreads faster. The B.1.1.7 variant also spreads up to 50 percent faster than the original strain, and the U.K. has found early evidence that it could be more deadly.