Yes, viruses can evolve to become more deadly

AAP FactCheck July 8, 2021

The Statement

Social media posts claim that viral mutations have never resulted in viruses becoming more deadly.

A July 1 Instagram post from an Australia-themed account includes a meme that states: “In the history of virology there has never, ever been a viral mutation that resulted in a virus that was more lethal. As viruses mutate they become more contagious and less lethal.”

The post mirrors the wording of a June 26 tweet, which had been retweeted more than 2400 times at the time of writing. It was posted by controversial activist Dr Kelly Victory, who has been identified as sharing various coronavirus-related conspiracy theories in the past.

Screenshots of the tweet have been shared on various social media platforms (see here, here, here and here), including by minor political party the Great Australian Party.

The Instagram post
 A meme claims there has never been a virus that mutated to become more deadly. 

The Analysis

Contrary to the posts’ claim, there are multiple examples of viruses that have mutated to become more virulent with the potential to prove more deadly to hosts.

Viral variants have been gaining increased attention following the rapid spread of the Delta variant of the virus that causes COVID-19, which the WHO says is 55 per cent more transmissible than earlier variants of the coronavirus (Edition 46, Pg 8).

Viruses themselves are constantly changing parasites that are unable to replicate without a host cell. There are multiple ways in which they can mutate, including antigenic drift, when the genes of a virus go through a series of small changes until a host organism may no longer recognise the virus and be immune to it.

A more dramatic change called an antigenic shift may also occur. This can occur in cases where a virus that previously only infected animals gains the ability to infect humans, for example. Few potential hosts may be immune to the newly mutated virus, giving the pathogens the potential to cause epidemics.

Virologists say these mutations can lead to viruses becoming more or less virulent (paragraph 5), which is typically defined as the ability of a virus to multiply and cause harm to its host.

In the case of the Delta variant of SARS-CoV-2, first documented in India in October, it is currently unclear whether it is more deadly in addition to being more transmissible, although a June Public Health Scotland analysis found it was approximately twice as likely to lead to hospitalisation than the Alpha variant, which was discovered in the UK in September.

World Health Organisation executive director Michael Ryan, who heads the agency’s emergencies program, said in a 21 June press conference (55min 54sec) the Delta variant of the coronavirus was “faster” and “fitter”, and would “pick off the more vulnerable more efficiently than previous variants”.

Edward Feil, a professor of microbial evolution at The Milner Centre for Evolution at the University of Bath who recently wrote about evolution of the coronavirus, said the posts’ claims about viruses never becoming more deadly were incorrect.

“I’m afraid that this claim is not in the least bit true – although it would be great if it was … (but) then it would be kind of hard to explain why so many virulent viral infections still exist,” he told AAP FactCheck via email.

Prof Feil cited studies identifying mutations that led to increased virulence in zika virus, hepatitis B and canine and phocine (seal) distemper viruses.

Influenza viruses can also mutate and become more virulent, periodically leading to deadly epidemics and pandemics, such as the 1918 Spanish flu that is estimated to have killed at least 20 million people worldwide – including many young, otherwise healthy adults.

Prof Feil noted that increased virulence did not necessarily mean the same thing as a virus’s mortality rate, however the two concepts were linked. The term “lethality”, used in the meme, was “not a term widely used in the scientific literature,” he said.

“If it means mortality (ie the chance that you will die if you catch it), then there will be a broad link with virulence, but exactly how that works will depend on the specific virus and mutations,” he said.

Troy Day, a professor of mathematics and biology at Queen’s University in Canada told the Associated Press that for viruses to thrive it was advantageous for them to become more transmissible and less lethal, “but the problem is that it’s not always possible”.

Jemma Geoghegan, an evolutionary biologist and virologist with the University of Otago who has also written on the evolution of virus virulence, told AAP FactCheck the claim in the meme was “completely false”.

“A virus will evolve to become more or less virulent if it provides a selective advantage for the virus. There are examples of virulence increasing and decreasing. Bird flu, for example, has become highly pathogenic in humans,” she said.

“Some of the best examples are in animals: there is good evidence to suggest myxoma virus has increased virulence.”

A woman sneezing
Influenza viruses have periodically evolved to become more deadly. 

The Verdict

The claim that viruses never mutate to become more lethal is incorrect. There are numerous examples of viruses affecting animals or humans that have evolved to become more virulent, which can lead to increased mortality.

False – Content that has no basis in fact.

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