Is it true that there has "never EVER" been a virus mutation that resulted in a virus that was more lethal? No, that is a misleading claim. Two experts told Lead Stories that while the delta variant of the COVID-19 virus may be less lethal, that statement ignores the effects a more transmissible strain will have on the health care system and on people, ultimately causing more deaths.
In addition, published studies showed that the Ebola virus did mutate into a more deadly version during the 2013-16 outbreak. One expert told Lead Stories the Ebola virus has not mutated into a less lethal virus and continues to have a 95% fatality rate.
The claim appeared as a Facebook post (archived here) published on June 30, 2021. It opens:
Been preaching this all along. Singing it from the mountain top.
This is what the post looked like on Facebook at the time of writing:
(Source: Facebook screenshot taken on Fri Jul 2 17:52:24 2021 UTC)
The Facebook post was a screenshot of a tweet by Kelly Victory, a Colorado-based doctor.
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. As viruses mutate, they become more contagious/transmissible and LESS lethal. #FactsNotFear #DeltaVariant-- Kelly Victory MD (@DrKellyVictory) June 25, 2021
"When it comes to the evolution of viruses I would be a little more hesitant to make blanket statements," Dr. Rita Burke told Lead Stories via telephone on July 1, 2021. She is an epidemiologist and assistant professor of clinical preventive medicine at the University of Southern California Keck School of Medicine:
It is a much more complicated and nuanced response than just saying the mutations are making it less lethal. With the delta variant we are concerned that it is more transmissible -- more people may get it. If more people show up to the hospital and overwhelm the hospital we will be back in the same situation and some people will die. The two things we have to talk about are transmissibility and virulence.
When you think about a view of a virus, the No. 1 mission is it wants to get as close to as many people as possible. So, if it meets the goals of the virus then that is what it is going to evolve to. Mutations evolve to make it more transmissible so more people get infected. If it becomes more virulent then more people will die and that won't help the virus.
Dr. Otto Yang of the David Geffen School of Medicine at UCLA emailed his answer to Lead Stories on July 1, 2021:
Evolution is caused by the accumulation of mutations that yield a survival benefit. In the case of a virus, in general, being more contagious and less lethal is to its benefit because that maximizes the people it can infect. If a person dies too soon, that makes it hard to go from person to person, which is why Ebola has been relatively easy to contain (so far). So in the long run, that is the trend. Viruses that are adapted well to humans (such as herpes simplex virus) tend to cause relatively mild infections that are not life-threatening in most persons.
Mutations happen randomly, so there could be mutations that are not to the virus' advantage, such as one that makes it less contagious, and those mutations should die out because they are at a competitive disadvantage. A mutation could happen that makes the virus more deadly, which should be to the virus' disadvantage too. However, if a mutation makes the virus both more contagious and more deadly, these could be balanced out, especially if the contagious gain is more than the death loss. So it IS possible for a virus to become more deadly and for that mutant to take over. However, in the long run it is not likely, since becoming less deadly is to the advantage of the virus, and if there is a way for it to maintain being more contagious while becoming less deadly, that is the direction it will evolve.
As for the Delta variant, it is clear it is more contagious. It is not clear if it is more deadly; that's hard to show if the death rate is so variable to begin.
Burke cited Ebola as an example of a virus that has not evolved and become a less virulent virus:
The fatality rate is 95% and it hasn't become any less. It is still able to accomplish its goal even if it is highly virulent.
A 2016 article on Ebola research said the virus had "acquired several new mutations during the course of the 2013-2016 West African Epidemic, making it even better at infecting human cells."