The novel coronavirus is accumulating genetic mutations, one of which may have made it more contagious, according to a study involving more than 5,000 COVID-19 patients in the US.
The research, published in the journal mBIO, however, did not find that these mutations have made the virus deadlier or changed clinical outcomes.
The researchers noted that the mutation, called D614G, is located in the spike protein that pries open our cells for viral entry.
"The virus is mutating due to a combination of neutral drift -- which just means random genetic changes that don't help or hurt the virus -- and pressure from our immune systems," said Ilya Finkelstein, associate professor at The University of Texas at Austin, US.
The researchers noted that during the initial wave of the pandemic, 71 per cent of the novel coronaviruses identified in patients in Houston had this mutation.
When the second wave of the outbreak hit Houston during the summer, this variant had leaped to 99.9 per cent prevalence, they said.
This mirrors a trend observed around the world, according to the researchers.
The reason why strains containing this mutation outcompete those that didn't have it may be that natural selection would favour strains of the virus that transmit more easily, the researchers said.
However, some scientists have suggested another explanation, called "founder's effects."
In that scenario, the D614G mutation might have been more common in the first viruses to arrive in Europe and North America, essentially giving them a head start on other strains, according to the researchers.
The spike protein is also continuing to accumulate additional mutations of unknown significance, they said.
The team also showed in lab experiments that at least one such mutation allows spike to evade a neutralising antibody that humans naturally produce to fight SARS-CoV-2 infections.
The researchers said this may allow that variant of the virus to more easily slip past our immune systems.
Although it is not clear yet whether that translates into it also being more easily transmitted between individuals, they said.
The scientists noted a total of 285 mutations across thousands of infections, although most don't appear to have a significant effect on how severe the disease is.