Two new variants of COVID-19 have been found in the United States, one bearing three new mutations not seen before.
The other has the same mutation as the strain currently ripping through the UK, but evolved independently.
The US has had the worst outbreak of the SARS-CoV-2 virus - which causes COVID-19 - in the world, and is approaching 400,000 deaths.
The more hosts a virus infects the more chance it has to mutate. The variant with three mutations was picked up in Columbus, Ohio, and it's believed to have become the dominant strain over the past few weeks.
"This new Columbus strain has the same genetic backbone as earlier cases we've studied, but these three mutations represent a significant evolution," said Dan Jones, vice chair of the division of molecular pathology at the Ohio State Wexner Medical Center.
"We know this shift didn't come from the UK or South African branches of the virus."
They said it's likely the mutations have made this strain, dubbed COH.20G/501Y, more infectious than the original virus. They're all on the spike protein, the part of the virus which allows it to hook onto our cells and invade them.
The other one - with the same mutation as the virulent UK strain - has only been detected in one patient, so its spread remains unknown. The same mutation has been picked up in South Africa and Brazil, all appearing to have evolved independently.
"The big question is whether these mutations will render vaccines and current therapeutic approaches less effective," said Peter Mohler, chief scientific officer at the Ohio State Wexner Medical Center.
"At this point, we have no data to believe that these mutations will have any impact on the effectiveness of vaccines now in use.
"It's important that we don't overreact to this new variant until we obtain additional data."
Eric Feigl-Ding of the Federation Of American Scientists said earlier this week it was unlikely the spike mutation in the UK virus would stop existing vaccines working. But if it did, vaccine manufacturer Moderna said it wouldn't be too much work to adjust its formulation thanks to its use of state-of-the-art mRNA technology.
Dr Jones said it appears the virus is evolving more frequently now - perhaps because it's infected so many people. There are currently 24 million known active cases in the world, with about 700,000 new infections discovered every day.
The death toll on Saturday passed 2 million.
