Fact check: Yes, viruses can mutate to become more deadly

·6 min read

The claim: Viruses never mutate to become more lethal

As the U.S. faces down the COVID-19 Delta variant – now fast-becoming the nation's dominant strain – reports of a new Lambda variant that first emerged in Peru in August 2020 are gaining the attention of public health officials worldwide.

News of this coronavirus variant is concerning, especially what it could mean for new COVID-19 cases, hospitalizations and deaths, all of which had been declining in the last few months. But one tweet being shared across social media claims new viral strains are nothing to be worried about.

"One more time, for the folks in the back: In the history of virology, there has never, EVER, been a viral mutation that resulted in a virus that was MORE lethal," reads a tweet by Dr. Kelly Victory shared in a June 30 Facebook post.

Victory, a Colorado-based physician known for making false assertions about the pandemic, claims instead that as viruses mutate, they "become more contagious/transmissible and LESS lethal."

She repeated this assertion in a comment to USA TODAY and further claimed this common viral path toward increased transmissibility and decreased virulence, or the ability to cause disease or other harm in the host, is "exactly what we have seen with all the COVID-19 mutations thus far."

Victory's June 25 tweet has been shared widely on Twitter and across Facebook, where it's accumulated more than 3,000 interactions, according to data provided by CrowdTangle, a social media analytics tool owned by Facebook.

Viruses do walk a fine line between transmissibility and virulence. A virus needs to be able to replicate and transmit its progeny but at the same time not cause too much harm to its host, which would mean it doesn't have an opportunity to spread.

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This evolutionary trade-off likely can keep a virus in check, but it's not as black-and-white as Victory makes it out to be. Experts say viral mutation is a complex calculus but there are several instances where viruses have evolved into more lethal strains.

How exactly do viruses mutate?

Unlike their human or animal hosts, viruses aren't capable of replicating on their own. Bringing their own genetic instructions with them, viruses typically hijack their host cell's manufacturing machinery – proteins called polymerases, which act as genetic photocopiers, and ribosomes, which assemble the viral proteins – to do so.

Mutations are introduced during this replication process, with the frequency of the mutations depending on the type of virus.

"DNA viruses (viruses that use DNA to encode their genes) tend to be the most stable... they actually rely on the host's same polymerases to copy their genome," Kari Debbink, virologist and assistant professor of biology at Bowie State University, told USA TODAY. "Because (humans) have such a big genome, we have to make sure we don't make a lot of mistakes... so our (polymerases) are actually able to proofread and fix mistakes."

This proofreading ability is not shared by viruses that use a different form of DNA called RNA and, as such, are more prone to mutation.

Coronaviruses, which are RNA viruses, are a bit unique though. The polymerases they use, called RNA polymerases, have an extra protein that helps them out in proofreading, said Debbink and virologist Juliet Morrison, an assistant professor in microbiology and plant pathology at the University of California Riverside.

"It's not 100%, so sometimes you'll have changes in nucleotides (the chemical building blocks of DNA and RNA) that will not get corrected," Morrison told USA TODAY.

Whether these random adjustments are passed on to progeny depends largely on how much they improve survival or confer a competitive edge, said infectious disease specialist Dr. Joel Chua of the University of Maryland's Institute of Human Virology.

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Most of the time, they don't confer any advantage and instead kill the virus. Other times, they introduce minor genetic variations, a phenomenon known as antigenic drift, but this doesn't mean these mutations won't be virulent to their host. In influenza, an RNA virus, small changes in the genes encoding its surface proteins are the reason why it appears regularly every season, and why flu shots are updated each year.

Many viruses have evolved to become deadlier

Victory claims no virus has ever become more deadly over time, but experts say that's not the case.

"The statement that 'in the history of virology, there has never, ever, been a viral mutation that resulted in a virus that was more lethal' appears to be quite untrue," Timothy Sheahan, virologist and assistant professor for the Gillings School of Global Public Health of the University of North Carolina, told USA TODAY.

Sheahan pointed to several examples such as the Ebola virus, which was discovered in 2016 to have undergone a mutation that not only made it more transmissible but likely more infective. This variant eventually died when the epidemic ended in 2016. The West Nile virus was found in 1999 to have mutated into a highly virulent strain, killing crows on multiple continents.

The 1918 Spanish flu pandemic, estimated to have killed at least 50 million people worldwide, is another example, said Chua.

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A May study out of the Robert Koch Institute in Berlin, Germany, found evidence that the virus responsible did mutate into more lethal variants. These deadlier strains, responsible for three later influenza outbreaks, likely made the virus better at spreading between humans rather than birds, its natural hosts, and better at evading the immune system.

Our rating: False

Based on our research, we rate FALSE the claim that viruses never mutate to become more lethal. Ebola, the West Nile virus and the Spanish flu from the 1918 pandemic are all examples of viruses that became more lethal after mutating, experts say.

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This article originally appeared on USA TODAY: Fact check: Viruses can mutate to become more deadly

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