Moderna’s vaccine is effective against new variants of the coronavirus that have emerged in Britain and South Africa, the company announced on Monday. But it appears to be less protective against the variant discovered in South Africa, and so the company is developing a new form of the vaccine that could be used as a booster shot against that virus.

“We’re doing it today to be ahead of the curve should we need to,” Dr. Tal Zaks, Moderna’s chief medical officer, said in an interview. “I think of it as an insurance policy.”

He added, “I don’t know if we need it, and I hope we don’t.”

Moderna reported findings from a study that used blood samples from eight people who had received two doses of the vaccine, and two monkeys that had also been immunized.

The British variant had no impact on the levels of neutralizing antibodies — the type that can disable the virus — produced after vaccination. But with the South African form, there was a sixfold reduction in those levels.

Even so, the company said, those antibodies “remain above levels that are expected to be protective.”

Moderna collaborated on the study with the Vaccine Research Center at the National Institute of Allergy and Infectious Diseases, part of the National Institutes of Health.

The results have not been published or peer-reviewed yet, but have been submitted to bioRxiv, which posts preliminary studies online.

The company’s action is part of a race to control a shape-shifting virus that has already created global havoc and now threatens to mutate in ways that will make it even harder to fight.

Several new variants of the virus have emerged, with mutations that worry scientists. A form first detected in Britain is about twice as contagious as the virus identified in China a year ago, and researchers have begun to suspect that it may also be more deadly.

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Answers to Your Vaccine Questions

While the exact order of vaccine recipients may vary by state, most will likely put medical workers and residents of long-term care facilities first. If you want to understand how this decision is getting made, this article will help.

Life will return to normal only when society as a whole gains enough protection against the coronavirus. Once countries authorize a vaccine, they’ll only be able to vaccinate a few percent of their citizens at most in the first couple months. The unvaccinated majority will still remain vulnerable to getting infected. A growing number of coronavirus vaccines are showing robust protection against becoming sick. But it’s also possible for people to spread the virus without even knowing they’re infected because they experience only mild symptoms or none at all. Scientists don’t yet know if the vaccines also block the transmission of the coronavirus. So for the time being, even vaccinated people will need to wear masks, avoid indoor crowds, and so on. Once enough people get vaccinated, it will become very difficult for the coronavirus to find vulnerable people to infect. Depending on how quickly we as a society achieve that goal, life might start approaching something like normal by the fall 2021.

Yes, but not forever. The two vaccines that will potentially get authorized this month clearly protect people from getting sick with Covid-19. But the clinical trials that delivered these results were not designed to determine whether vaccinated people could still spread the coronavirus without developing symptoms. That remains a possibility. We know that people who are naturally infected by the coronavirus can spread it while they’re not experiencing any cough or other symptoms. Researchers will be intensely studying this question as the vaccines roll out. In the meantime, even vaccinated people will need to think of themselves as possible spreaders.

The Pfizer and BioNTech vaccine is delivered as a shot in the arm, like other typical vaccines. The injection won’t be any different from ones you’ve gotten before. Tens of thousands of people have already received the vaccines, and none of them have reported any serious health problems. But some of them have felt short-lived discomfort, including aches and flu-like symptoms that typically last a day. It’s possible that people may need to plan to take a day off work or school after the second shot. While these experiences aren’t pleasant, they are a good sign: they are the result of your own immune system encountering the vaccine and mounting a potent response that will provide long-lasting immunity.

No. The vaccines from Moderna and Pfizer use a genetic molecule to prime the immune system. That molecule, known as mRNA, is eventually destroyed by the body. The mRNA is packaged in an oily bubble that can fuse to a cell, allowing the molecule to slip in. The cell uses the mRNA to make proteins from the coronavirus, which can stimulate the immune system. At any moment, each of our cells may contain hundreds of thousands of mRNA molecules, which they produce in order to make proteins of their own. Once those proteins are made, our cells then shred the mRNA with special enzymes. The mRNA molecules our cells make can only survive a matter of minutes. The mRNA in vaccines is engineered to withstand the cell’s enzymes a bit longer, so that the cells can make extra virus proteins and prompt a stronger immune response. But the mRNA can only last for a few days at most before they are destroyed.

Other variants with different mutations have arisen in South Africa and Brazil, and preliminary studies in the laboratory suggested that those forms may have some degree of resistance to the immunity that people develop after recovering from the infection or being inoculated with the Moderna or Pfizer-BioNTech vaccines.

The British variant has been found in at least 20 states, but the Brazilian and South African versions have not been detected in the United States.

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Dr. Zaks said that the new version of the Moderna vaccine, aimed at the South African variant, could be used if needed as a booster one year after people received the original vaccine.

The need for such a booster may be determined by blood tests to measure antibody levels or by watching the population of vaccinated people to see if they begin falling ill from the new variant.

“We don’t yet have data on the Brazilian variant,” Dr. Zaks said. “Our expectation is that, if anything, it should be close to the South African one. That’s the one with the most overlap. New strains will continue to emerge and we’ll continue to evaluate them.”

Noting that Moderna took 42 days to produce the original vaccine, he said the company could make a new one “hopefully a little faster this time, but not much.”

Discussions with regulators about what would be required to bring a new version of the vaccine to the public were just starting.

“It’s early days,” Dr. Zaks said.

This developing story will be updated.

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