Comment by tunesmith | Hacker News Reader

Comment by tunesmith | Hacker News Reader

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"Selective pressure". I find this phrase really confusing, because I think people use it differently. I'm still trying to nail down my own knowledge about this, but here's the gist I have so far.

Viruses are not like bacteria. Bacteria can mutate in direct response to antibiotics. Viruses cannot mutate in direct response to vaccines. Some people use "selection pressure" to describe what happens with bacteria and antibiotics. Others use it in a looser sense.

Viruses mutate purely randomly. Now, if a virus happens to mutate in a way that escapes vaccines, and if vaccinated people are more likely to party because they are vaccinated, then yes, you could argue the mutation has an advantage against other variants the vaccine has protected against, in a social sense. Some people use "selection pressure" in this sense, which I think causes confusion.

So the more relevant question is, if a virus mutates in an unvaccinated person, and that mutation happens to have the ability to escape all current vaccines, then, can the virus spread faster in a 50% vaccinated community than a 0% vaccinated community?

Answers as far as I can tell:

- Arguably yes, if the 0% vaccinated community is so sick from other variants that they aren't out and about to get exposed to the new variant. But I think this answer is cheating and doesn't get to the heart of the question. So, control for population activity and assume that both the 0% and 50% populations have the same chance of being exposed.

- If the 0% vaccinated community is infected with variants that can "crowd out" the newer mutation, then it could grow faster in the 50% vaccinated community. But I don't know if this is actually possible - I do know it's possible to catch multiple variants at one time, and I'd expect that usually a mutation that is more fit in terms of escapability might also have fitness advantages with transmissibility.

- If the mutation responded to the vaccine, in a direct biological sense, then it would clearly grow faster in the 50% population since it wouldn't have existed otherwise. But this is the scenario that not possible as I understand it, since it is based on a misunderstanding of how viruses work.

I welcome any corrections from those who are more familiar with this.

Bacteria do not mutate directly, in fact, no living creature we know of mutates directly in response to anything. All evolution is random mutation in offspring randomly providing fitness to itself and it's progeny above its predecessors and peers. That is what Darwinian evolution is.

That makes sense. You could say that the "lack of vaccination" yields mutations, much in the same way that the "lack of antibiotic" (like people refusing to finish their prescription) can yield antibiotic-resistant bacteria. But in neither case does it "cause" the mutation.

It appears that I was improperly influenced by the second comment in this thread: https://www.reddit.com/r/askscience/comments/ozh9mi/is_the_d...

>Bacteria can mutate in direct response to antibiotics. Viruses cannot mutate in direct response to vaccines.

What do you mean by this? Are you a Lamarckian?

I don't know what that is. Put a bacteria and an antibiotic in a petri dish, and the bacteria can evolve in a way that has greater antibiotic resistance. Put a vaccine in a petri dish with a virus, and nothing happens. Bacteria are living organisms. Viruses are not.

Viruses are subject to selection, mutation, and heredity. These are the three necessary ingredients for Darwinian evolution.

Whether they're defined as "life" or not isn't really relevant.

(Personally, I have a pet definition of "life" that just equates it to being subject to Darwinian evolution. Viruses not being defined as life never sat well with me)

Vaccines can't do their thing in a petri dish, they need an immune system to work via. That's just a petri dish not being an adequate simulation of the environment viruses are normally in, and doesn't imply anything about whether viruses are subject to Darwinian forces.

I just keep seeing repeats of logic like this (this example from this random article: https://www.unitypoint.org/article.aspx?id=db428f77-6e61-497... )

"The creation of a vaccine for any new virus could also cause additional mutations.

“Let’s explain this concept a little further. Any virus will keep trying to change, so it can continue to spread. With all vaccines, the more quickly people get vaccinated the better. The slower vaccination happens, the higher the chance of having mutations in the virus and the appearance of more variants. And, as we are seeing with the delta variant, the more the virus can spread in the community.""

Isn't that just completely wrong? Saying that the vaccine "caused" that mutation when it was actually the lack of the vaccine that caused it?

Here's another one, a friggin NPR article (https://www.npr.org/2021/02/09/965703047/vaccines-could-driv...). It says "At the same time, vaccines can contribute to virus mutations" while the article itself is really only describing that it's the lack of vaccines that yield more time/opportunity for the virus to mutate. Why do people keep saying the virus causes the mutations? It's completely irresponsible.

> Any virus will keep trying to change

First, the "virus" in this sentence means "all the trillions of individual virions currently in our environment, including the trillions currently in living cells. This does not mean that individual virions are changing in response to any stimulus or lack thereof. What they are saying is that as those trillions beget many trillions more, there will be many random mutations. Most of those mutations will result in failure. Some will end up as successful as their antecedent. A tiny few will end up with mutations that make them more successful in the current environment. So the mutations happen regardless of anything we do or don't do.

But as we take more countermeasures, it becomes harder for the baseline virions to be successful (because the vaccines work, distancing works, etc.). Any mutations that are only as successful as baseline are going to have trouble spreading. So by default what we will see is more of the mutations that result in things like easier transmission. Because they are better suited to the current environment where people are taking countermeasures. And because baseline is not as successful, the proportion of the new variants will increase over time.

Putting this together, if there's no selection pressure, mutations don't confer any advantage and they will tend to disappear from the population. So through that lens, the new variants wouldn't necessarily have gained a foothold if people weren't using countermeasures. That's not specifically the vaccines, though. Delta is apparently more transmissible generally, and that could also be a response (via random mutation) to social distancing & masking measures.

So the vaccine didn't cause the mutation, but the presence of the vaccine and other human countermeasures shaped the evolution of the vaccine.

https://journals.plos.org/plosbiology/article?id=10.1371/jou...

> Why do people keep saying the virus causes the mutations? It's completely irresponsible.

The truly amazing thing is that they cite the Delta variant as an example for why vaccines cause dangerous mutations, without noting that Delta arose in December 2020, in India.

Not a lot of folks were vaccinated there around that time.

Think of viral particles as analogous to bacterial spores or plant seeds. A bacterial spore also wouldn't necessarily evolve in a petri dish with antibiotics, it needs a certain set of conditions to exit dormancy, for example a petri dish isn't implied to have agar in it nor do all bacteria thrive on agar.

The actual "life" of a virus starts once the viral load has co-opted the cellular machinery of the infected cell.

Viral infections can definitely evolve when a selective pressure is put on them.

My understanding of selection pressure in relation to viruses is something like viruses that escape the body's defences where those defences are strengthened. Eg if lots of people have antibodies triggered by a vaccine then it favours those viruses that avoid that immune response and infect the host.

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