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Home » Virus evolution — debunking anti-vaccine myth that they become less virulent

Virus evolution — debunking anti-vaccine myth that they become less virulent

Apparently, a 125-year-old debunked idea about virus evolution has circulated around the anti-vaccine world. They believe that if viruses are left on their own, they always evolve to become less virulent to humans. That’s why they falsely claim that the Omicron variant is almost nothing and very soon SARS-CoV-2, the virus that causes COVID-19, will evolve into something that we can ignore.

The old Skeptical Raptor is going to take a deep breath and hope he doesn’t lose any brain cells repeating that to all of you. Anti-vaxxers and COVID-19 deniers are wrong, completely and utterly wrong. It’s as if they never took a class on virology, evolution, or anything else germane to the discussion.

I know that any of you spending time reading this article are already listing out a dozen things that debunk this myth. Because we all know that first, that’s not how evolution works, and second, there are dozens of viruses that are known from the dawn of human history that have remained virulent over thousands of years.

Let’s take a look at this nonsense. Maybe I’ll give you some information to debunk some anti-vaxxer or COVID-19 denier nonsense.

Let’s talk evolution

I don’t mean to be a cynic, but my guess is that the Venn diagram of science denialism shows a huge overlap between the vaccine, COVID-19, and evolution deniers. The right-wing religious part of the USA believes in the pseudoscience called Creationism.

Creationism refers to the belief that the universe and everything in it was specially created by a god through magic, rather than a natural, scientifically explained, process. Creationism explicitly relies on the claim that there is a “purpose” to all creation known only to a creator.

There is little argument that contradicts the conclusion that creationism is nothing more than a religious belief. Furthermore, no matter what argument is made by so-called “creation scientists,” creationism can never be tested scientifically because it relies upon a supernatural being, which means it can never be falsified, one of the basic principles of the scientific method.

But let’s get back to evolution.

Evolution is rather simple. It is the change in heritable characteristics of a population of organisms over time as a result of natural selection or genetic drift. Essentially, random mutations confer some characteristics to an organism that makes it either more fit (able to produce offspring and thus transfer its genes to the next generation) or less fit (it either dies before reproducing or makes it less able to reproduce). There are actually some mutations that provide no immediate benefit to the organism but could subsequently do so in response to a change in their environmental niche.

Evolution is random. There is no plan. There is no overarching reason for evolution. And evolution certainly couldn’t care less about humans one way or another. Evolution benefits no one except the organisms and how they reproduce in a particular environmental niche.

Just to be clear, the fact of evolution is observed in fossils and living organisms. Natural selection is observed in populations of organisms. Genetic drift is also observed. These are facts only dismissed by evolution deniers. The theory of evolution just describes the mechanisms of evolution. Despite the irrational claims of creationists, there has never been anything presented by them that scientifically dismisses the fact and theory of evolution. Nothing.

The scientific evidence supporting the fact and theory of evolution is literally a mountain. There are well over a million papers written about evolution, even though all scientific theories are provisional, meaning they could be dropped if there were a similar quantity and quality of papers disputing evolution, we might rethink it. But there isn’t.

microscopic shot of a virus
Photo by CDC on

Virus evolution

Virus evolution is subject to the same rules of evolution as any other organism. Whatever new trait through a mutation gives the virus a better chance to survive, reproduce, and spread will be selected for and will become predominant.

Viruses are a fascinating way to study evolution. Their generations last maybe an hour, and they mutate frequently. The immune system is one of the most powerful natural selection mechanisms for virus evolution — if the immune system can destroy a virus before it reproduces, then it either dies out, or some mutation allows it to avoid the immune system. Then, it can infect and reproduce until the immune system recognizes it and attacks it with new antibodies.

The idea that viral infections tend to become less lethal as they evolve was first proposed by notable bacteriologist Dr. Theobald Smith in the late 1800s. His hypothesis about virus evolution was later dubbed the “law of declining virulence.”

Smith’s idea was that to ensure their own survival, pathogens such as viruses evolve to stop killing their human hosts. Instead, they eventually evolve to produce a mild infection, allowing people to walk around, spreading the pathogen. His idea was elegant, what was good for the virus was good for us.

Except for one large problem — viruses do not care about whether they kill the host or not. The host only has to survive long enough for the virus to be passed along to a new host. Viruses do not have a grand plan in the world, they are random and chaotic.

And this is the major point that debunks Smith’s hypothesis — some viruses do become weaker, some stay the same, and some become more virulent. And it’s almost impossible to predict what they do because it’s so random and chaotic.

A virus’ whole life is to get into cells, hijack the cell’s machinery, and then reproduce. There’s no master control of whether the host (be it human or bacteria or bird or whatever) will live or die. Evolution is pushing the virus in the direction of quickly reproducing and evading the immune system, whatever else happens to the host is nearly irrelevant, as long as it lives long enough to allow the virus to spread to another host.

But the history of viruses in humans tells the real story of the evolution of viruses. Let’s take a look at a few viruses and how they may have changed over time:

  • Polio — the poliovirus probably first infected humans about 3000-4000 years ago, when first written records appeared (Daniels, 1997). Of course, it may have been around earlier than that. Until it has been nearly eradicated by vaccines, polio’s virulence has been unchanged over the the past few thousand years.
  • Smallpox — the smallpox virus probably first infected humans at least 5000 years ago, but it could be much longer. Smallpox kills up to 30% of victims, and it showed no decline in virulence until it was eradicated with vaccines.
  • Measles — recent research shows that the measles virus appeared in humans around 3000 years ago, evolving from a cattle virus called rinderpest. Again, there is little evidence that there has been a reduction in virulence over the three thousand years.
  • Rabiesrabies virus has been infecting humans and other mammals for at least 4000 years. And this is the one virus that completely debunks this virus evolution myth — it is 100% fatal, to any animal including humans, after initial symptoms appear. And this virulence hasn’t changed for as long as we know the virus was around.

There is really not much to support Smith’s ideas about virus evolution. People try to use the 1919 flu pandemic as evidence — that pandemic killed at least 50 million people worldwide, then claim that the flu isn’t dangerous anymore.

Well, the flu is certainly dangerous today, killing on average 290,000-600,000 people a year. And the last flu pandemic, the 2009 H1N1 pandemic, infected around 700 million to 1.4 billion people across the world and killed between 150 and 575 thousand people.

The evolutionary pressure on viruses, including its survival, spread, and virulence, results from multiple factors including the number of individual hosts it can infect, how long the hosts survive after infection, the quality and quantity of the immune system response, and time between infection and symptoms. In other words, virus evolution is complicated and unpredictable.

We do not know the track a virus will take, and anyone who says they know either deserves a Nobel Prize or hasn’t got a clue. There is no prediction and no pattern for virus pandemics like this one.

People like to point to the Omicron variant as proof that the virus is getting less virulent. However, Omicron appears to be more virulent, and the data isn’t in to tell how much. Given the hospitalization and death numbers, I’m scratching a hole in my head trying to figure out why people are saying this about Omicron.

However, this is what we do know about the future of COVID-19 — we don’t know much. The next variant could be much more infectious and deadly because, once again, virus evolution doesn’t mean it gets weaker.

There have been thousands of variants of SARS-CoV-2 — the vast majority die out because they don’t confer an advantage to the virus, so either the immune system deals with it quickly or other, stronger, variants displace it quickly.

The one advantage humans have over these viruses is vaccines. Despite the easily debunked claims of the anti-vaxxers, unvaccinated people are estimated to be 17 times more likely to be hospitalized and 20 times more likely to die of COVID-19 compared to people who are vaccinated. Those are facts, like evolution.

Vaccines worked in nearly eliminating polio and outright eliminating smallpox because it confers an advantage to humans — it primes the immune system to destroy the virus before it can replicate and infect the host and then transmit to other hosts. It stops the virus right there.

SARS-CoV-2 mutates a lot, like the flu, which means that new variants have evolved to be more infectious by avoiding the immune system (though they can’t completely avoid it, which is why vaccinated people have a fitness advantage over unvaccinated people).

Virus evolution is a fascinating field of study. But a thoroughly debunked 125-year-old idea that viruses become less virulent over time should not be a reason to avoid vaccines and pretend all will get better. Evolution doesn’t work that way. Viruses just want to reproduce, and unvaccinated people are much better at that than vaccinated ones.


Michael Simpson
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