• Viruses accumulate genetic errors, or mutations, over time. 
  • One mutated coronavirus strain that emerged in January — not the original strain that appeared in China  — has become dominant worldwide.
  • A new, preliminary study suggests that mutated strain is more contagious than the original. 
  • But most scientists say the conclusion is premature.
  • Visit Business Insider's homepage for more stories.

No pathogen on Earth is being more closely monitored than the coronavirus.

Scientists have been regularly collecting and genetically sequencing samples of the virus to track how it's changing. Over time, that monitoring has revealed, one version became more prevalent than the rest: a strain with the mutation dubbed D614G.

According to a new, preliminary study from researchers at Houston Methodist Hospital, that mutated strain was responsible for nearly every COVID-19 infection there this summer, during in Texas' second peak of infections. 

James Musser, the senior author of the new research, thinks that means the mutated strain — which contains the amino acid glycine, or G — is more transmissible than the original.

"There's a preponderance of evidence that there's biologically something different in G variant organisms, Musser told Business Insider. "It more readily infects."

His research is the latest in a string of studies suggesting coronaviruses with the D614G mutation are more contagious than their genetic predecessors.

But many other scientists question that conclusion.

"The study provides more evidence for what we already know about this mutation: That it's the most common variant," Emma Hodcroft, a geneticist at the Nextstrain project, told Business Insider. "That doesn't mean the virus is effectively mutating."

Hodcroft's team has tracked the coronavirus' genetic changes since the start of the pandemic, and she said they have yet to identify a mutation that would meaningfully change how infectious or lethal the virus is.

A dominant form of the virus

Geneticists classify the original version of the coronavirus as the "D lineage," whereas strains with the D614G mutation are categorized as the "G lineage." The G lineage didn't crop up until January, Hodcroft said. Since then, according to her Nextstrain colleague Richard Neher, it has come to dominate "almost all places in the US, Europe, and Latin America."

The particular mutation that differentiates these strains is a swap at the amino acid labeled 614 — the part of the virus' genome that codes for the shape of its spike protein.

That spike is what the coronavirus uses to invade our cells, so it's possible a tweak there could make it easier for the virus to infect our bodies. Indeed, a June study found that the D614G strain is three to six times better at infecting human cells in the lab than its predecessor. Other preliminary research has also suggested that the mutation enhances the virus's ability to invade cells.

That's what Musser believes, too.

His research — the largest genetic study of the virus to date in the US — involved more than 5,000 virus samples from Houston collected between March and July. The team classified the samples collected between March 5 and May 11 as part of the city's "first wave" of infections. In that group, the data showed, 82% contained the D614G mutation. But in the set of samples collected between the end of that first wave and July 7, the figure jumped to 99.9%.

Musser said this shows the G variant "out-competes the D variant."

But there could be other reasons the G variant became most common

Hodcroft said there's another possible explanation for why the G strain has become so prevalent: luck.

Data shows that the D614G mutation arose just before the coronavirus spread to Europe. So the G lineage might have coincidentally just ended up being the version that spread to countries in Europe and North America. Many of those countries did not enact lockdowns for weeks after their first cases appeared, which allowed that version of the virus to proliferate.

"One important thing to keep in mind is the G mutation arose in January 2020, so early on in the pandemic. It became the strain that most people got," Hodcroft said, adding, "we've been dealing with this strain the whole time."

In that sense, according to immunologist Kristian Andersen, Musser's study offers "no novel insights." She added, however, that the team did sequence "an impressive number of genomes."

'An advantageous mutant'

Even if G variants of the coronavirus are better at infecting new cells, that still may not necessarily impact how well the virus spreads from person to person.

"This G strain can be stopped in all the same ways the D strain can: masks, hand washing, and socials distancing," Hodcroft said.

Musser's research also describes other amino acid swaps observed at different sequences of the coronavirus' genome. The more a virus replicates, he said, the greater the chance that "an advantageous mutant" may arise. But the slight changes Musser's team observed are still not reason to panic, according to Hodcroft.

All viruses accumulate mutations over time, she said: "That's normal. That's how viruses work."

Most mutations her team is seeing are harmless, Hodcroft added, and the coronavirus is mutating slowly. Hodcroft's project sees a maximum of 20 to 25 differences between sequences that contain about 30,000 genetic building blocks.

"People are getting the same virus now that we saw in the spring," she said.

Houston researchers kept their data to themselves

Nextstrain tracks mutations by collecting as many sequences as possible. Typically, researchers from around the world share those sequences on GISAID, a publicly accessible repository.

"Our sequencing endeavors permit us to have more knowledge about the enemy we're fighting," Musser said.

But his team did not upload their sequences to the server right away. Instead, they waited to do that until after their preliminary study went out.

Musser said the group tried to share the data sooner, "but unfortunately the website could not accommodate the unusually large magnitude of genomes."

Eventually, he added, the information was broken into parcels and uploaded successfully.

"All of this took time to figure out, of course," he said.

But Hodcroft said the existence of Musser's data in the first place came as a surprise to her. All summer, she said, she'd been hunting for more genetic data from Texas in order to glean a better understanding of infections there.

As far as she knew, only 600 or so samples were available from the state. Then suddenly, a paper appeared with an analysis of more than 5,000 sequences. 

"It's a bit of a gut punch to every scientist putting their sequences out there," Hodcroft said. "These scientists kept the data for themselves."

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