Covid-19 may have become more contagious as it has mutated, the largest genetic study carried out in the US into the virus has suggested, as scientists warn it could be adapting to interventions such as mask-wearing and social distancing.
One variant of the novel coronavirus is now one of the most dominant in America, accounting for 99.9 per cent of cases in one area studied.
The paper concluded that a mutation that changes the structure of the "spike protein" on the surface of the virus may be driving the outsized spread of that particular strain.
Researchers have been sequencing the genomes of the coronavirus at Houston Methodist, one of the largest hospitals in Texas, since early March, when the virus first appeared in the city. To date, they have documented 5085 sequences.
In the first wave of the outbreak in Houston around March, some 71 per cent of the viruses were characterised by the mutation, which originated in China and is known as D614G.
By the second wave, which began in May and is ongoing, the D614G mutation leaped to 99.9 per cent prevalence.
A tiny tweak in the spike protein of the dominant variant switches an amino acid from aspartic acid to glycine. The new mutation appears to be outdistancing all of its competitors.
The researchers, who include some from the University of Chicago and the University of Texas at Austin, found that people infected with this strain had higher "loads" of virus in their upper respiratory tracts, which allows a virus to spread more effectively.
One of the authors offered that D614G has been increasingly dominant in Houston and other areas because it is better adapted to spreading among humans.
Their paper, published on Wednesday by preprint server MedRxiv, however, did not find that it was more deadly.
A similar study published in the UK had similar results, finding that D614G was increasing in frequency at "an alarming rate" and had rapidly become the dominant Covid-19 lineage in Europe and had then taken hold in the US, Canada and Australia.
By failing to control the spread in the US - which has the highest number of cases in the world - the virus has been given more opportunity to mutate in a shorter amount of time.
David Morens, a virologist at the National Institute of Allergy and Infectious Diseases (NIAID), told the Washington Post the findings point to the possibility that the virus has become more transmissible and that this "may have implications for our ability to control it".
Morens cautioned that it was only one study that had not yet been peer-reviewed and "you don't want to over-interpret what this means". But the virus, he said, could potentially be responding - through mutations - to such interventions as hand-washing and social distancing.
"Wearing masks, washing our hands, all those things are barriers to transmissibility, or contagion, but as the virus becomes more contagious it statistically is better at getting around those barriers," said Morens, senior adviser to Dr Anthony Fauci, the director of NIAID.
As a rule, the more genetic diversity a virus has the more prepared it is to evolve away from future treatments and vaccines.
Other virologists downplayed the importance of the study, saying much is still unknown about the various mutations of the virus and how virulent they are.
Studying mutations in detail, however, could be important for controlling the pandemic. It might help to pre-empt the most worrying of mutations - those that could help the virus to evade immune systems, vaccines or antibody therapies.