Moderna Inc. said its vaccine will protect against two known variants of the Covid-19 virus, but it plans to start human studies of a booster shot for a strain from South Africa that may cause immunity to wane more quickly. In laboratory tests, Moderna’s vaccine produced antibody protection against the B.1.1.7 strain first identified in the U.K. at levels comparable with older forms of the virus. But against the South Africa variant, known as B.1.351, the neutralizing antibodies produced were six-fold lower, the company said in a statement. Despite that gap, Moderna’s shot should protect against either strain, according to the company. While the South Africa variant hasn’t been seen in the U.S., the U.K. mutation -- which British officials said last week may be deadlier -- is spreading rapidly among Americans. Both strains are thought to be more transmissible than the original virus. “We expect that whatever immunity you get over time will wane. The question is will it wane faster if you have lower levels to begin with,” said Tal Zaks, Moderna’s chief medical officer, in an interview. Moderna shares gained as much as 12.5% in New York trading, the biggest intraday gain since Dec. 1. Over the past 12 months, the company’s stock price has soared more than 500%. The results came from tests Moderna conducted with the National Institute of Allergy and Infectious Diseases. While the shot produced neutralizing antibodies above levels that protect monkeys, the new studies will determine if that holds true with humans. The company plans to quickly begin testing a new booster shot formulation of its vaccine, designed to target the South African strain, giving it as a third shot in a few thousand people who participated in its earlier studies, said Chief Executive Officer Stephane Bancel in an interview with Bloomberg TV. The results could be available by the late summer or early fall, he said. The booster shot could get regulatory authorization based on lab measurements of antibody response it stimulates, rather than requiring another massive study comparing illness rates in people. “Our goal would be to have it ready as a cautionary step,” Bancel said. “It might not be needed but we cannot be wrong.” If the South Africa variant becomes problematic in the fall, he said, the company wants to have a booster ready in single dose form “to be given at least to people at high risk like the elderly.” It isn’t clear how often people might need booster shots, according to Moderna’s Zaks. It’s possible that annual booster shots could be needed, just as they are for influenza. But Zaks said it’s also possible that a single booster shot could protect people for a long time, he said. “We don’t know that yet,” he said. “Our job is to prepare for every eventuality that may occur.” On Monday, Anthony Fauci, the top U.S. infectious-disease expert, said he’s concerned about the ability of vaccines to protect against new variants of the coronavirus and that drugmakers may need to tweak their shots to address new mutations. “There seems to be considerable more threat to vaccine efficacy, even though the cushion of efficacy is sound enough that the vaccines we’re using now will be good against both the mutant in South Africa as well as those in the U.K.,” Fauci said. Subscribe to our YouTube channel: https://bit.ly/2TwO8Gm Bloomberg Quicktake brings you live global news and original shows spanning business, technology, politics and culture. Make sense of the stories changing your business and your world. To watch complete coverage on Bloomberg Quicktake 24/7, visit http://www.bloomberg.com/qt/live, or watch on Apple TV, Roku, Samsung Smart TV, Fire TV and Android TV on the Bloomberg app. Have a story to tell? Fill out this survey for a chance to have it featured on Bloomberg Quicktake: https://cor.us/surveys/27AF30 Connect with us on… YouTube: https://www.youtube.com/user/Bloomberg Breaking News on YouTube: https://www.youtube.com/c/BloombergQuickTakeNews Twitter: https://twitter.com/quicktake Facebook: https://www.facebook.com/quicktake Instagram: https://www.instagram.com/quicktake