"It is now the dominant form infecting people," Erica Ollmann Saphire of the La Jolla Institute for Immunology and the Coronavirus Immunotherapy Consortium, who worked on the study, told CNN.

"This is now the virus."

The study, published in the journal Cell, builds on some earlier work the team did that was released on a preprint server earlier in the year. Shared information on genetic sequences had indicated that a certain mutant version of the virus was taking over.

Now the team has not only checked more genetic sequences, but they have also run experiments involving people, animals and cells in lab dishes that show the mutated version is more common and that it's more infectious than other versions.

"We do know that the new virus is fitter. It doesn't look at first glance as if it is worse," Saphire said.

The mutation affects the spike protein -- the structure the virus uses to get into the cells it infects. Now the researchers are checking to see whether this affects whether the virus can be controlled by a vaccine. Current vaccines being tested mostly target the spike protein, but they were made using older strains of the virus.

The study, published in the journal Cell, confirms earlier work suggesting the mutation had made the new variant of virus more common. The researchers call the new mutation G614, and they show that it has almost completely replaced the first version to spread in Europe and the US, one called D614.

No effect on patient survival

"Our global tracking data show that the G614 variant in Spike has spread faster than D614," theoretical biologist Bette Korber of Los Alamos National Laboratory and colleagues wrote in their report. "We interpret this to mean that the virus is likely to be more infectious," they added. "Interestingly, we did not find evidence of G614 impact on disease severity."

This could be good news, said Lawrence Young, a professor of medical oncology at the UK's University of Warwick, who was not involved in the study.

"The current work suggests that while the G614 variant may be more infectious, it is not more pathogenic. There is a hope that as SARS-CoV-2 infection spreads, the virus might become less pathogenic," he said in a statement.

The team tested samples taken from patients across Europe and the US and sequenced the genomes. They compared these genome sequences to what's been shared publicly. Comparing these sequences helped them draw a map of the spread of the two forms.

"Through March 1, 2020, the G614 variant was rare outside of Europe, but the end of March it had increased in frequency worldwide," they wrote.

Even when the D614 form had caused widespread epidemics, in places such as Wales and Nottingham in England, as well as in Washington state, G614 took over once it appeared, they found.

"The increase in G614 frequency often continues well after stay-at-home orders are in place and past the subsequent two-week incubation period," they added. There are a few exceptions,

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