But sometimes the virus infects people with weak immune systems. In their bodies, the virus can thrive for months. Case studies on these immunocompromised people have shown that the virus can accumulate a large number of mutations as it replicates in their bodies for a long period of time.

Over time, researchers have found, natural selection can favor mutant viruses that can evade the immune system. Researchers have also suggested that the evolution of the variant might have been additionally driven by medicine given to such patients. Some mutants might be able to withstand drugs such as monoclonal antibodies.

Other scientists have suggested that the virus could have gained new mutations by spreading through an animal population, like minks, before re-entering the human population. Such “animal reservoirs” have become a focus of intense interest as more animal infections have been detected.

Is the variant already circulating in the U.S.?

Not yet, as far as anyone knows. But that does not mean it hasn’t already reached the United States. British scientists have established a much stronger system to monitor coronaviruses for new mutations. It’s conceivable that someone traveling from the United Kingdom has brought it with them. Now that the world knows to look for the variant, it may turn up in more countries.

Will the variant render the new vaccines ineffective?

No. Most experts doubt that it will have any great impact on vaccines, although it’s not yet possible to rule out any effect.

The U.S. Food and Drug Administration has authorized two vaccines, one from Moderna and the other from Pfizer and BioNTech. Both vaccines create immunity to the coronavirus by teaching our immune systems to make antibodies to a protein that sits on the surface of the virus, called spike. The spike protein latches onto cells and opens a passageway inside. Antibodies produced in response to the vaccines stick to the tip of the spike. The result: The viruses can’t get inside.

It is conceivable that a mutation to a coronavirus could change the shape of its spike proteins, making it harder for the antibodies to gain a tight grip on them. And B.1.1.7’s mutations include eight in the spike gene. But our immune systems can produce a range of antibodies against a single viral protein, making it less likely that viruses can easily escape their attack. Right now, experts don’t think that the variant will be able to evade vaccines. To confirm that, researchers at the Walter Reed Army Institute of Research are analyzing the changes to the structure of its spike protein.