Need truth about Spanish Flu factoid

No. It’s not that the less-lethal form is more contagious. It’s that a live host with mild to no symptoms is able to pass the infection to more people than a severely ill host who’s first bedridden and then dead. And someone who has been infected with the less-deadly strain is likely to have immunity to the deadlier strain. More people get infected with the less-deadly strain not because it transmits more easily, but because it doesn’t sabotage itself by immobilizing or killing the host.

One complicating factor with SARS-CoV-2 is that it’s asymptomatic in so many people. It seems to me that would reduce the evolutionary pressure to evolve to a less-lethal form. That is, it’s already less-lethal for a lot of people.

I’m very much aware of all that. Please reread my posts more carefully, starting with #11.

You may be aware of it, but I don’t believe you’ve incorporated it into your thinking. As I said, a less-lethal virus will spread more quickly because it doesn’t immobilize the host. This will create immunity ahead of the spread of the more-lethal strain. This is sufficient for a strong evolutionary advantage in favor of the less-lethal strain.

What is there about this that fails to address the points you’ve brought up?

I’m sorry, it does not appear that you’ve paid attention to what I’ve been saying. I suggested reading more carefully but in any event, but I’m not looking to explain it again.


As I think about it now, I think it’s very possible that the whole premise is incorrect and that it’s not accurate to say that “the virus is mutating to a less lethal form” with the implication that the less lethal form is supplanting the more lethal form, but rather that there is also, in certain places, a less lethal form, which coexists with the original version.

I suspect the people who have been trying to explain it to you feel the same. I certainly do. :wink:

What happens is that a mutation occurs in one individual virus particle that makes it less lethal. The virus population as a whole isn’t mutating. The mutated form of the virus reproduces itself more rapidly because it spreads from host to host more rapidly. Yes, the two forms co-occur for a time but as people die or recover and become immune to both forms the less lethal form will eventually predominate.

Yeah, I’m not sure about you either. More below.

Of course it will eventually predominate. That’s obvious. I acknowledged this in my first post on the subject.

What I’ve been discussing since I brought this up is whether it would supplant the other version. That’s not the same thing as to predominate.

Did you miss this? Perhaps you weren’t paying attention either.

To put it in simple terms with some random numbers, in case you did, imagine that in the absence of mutated form, the original version would infect 10% of given population. So let’s say the mutated form is more efficient at spreading and will therefore infect 20% of the population. Question is what that does to the original version. If you assume that the 20% who caught the new version can’t get the old version, then that would reduce the number of potential hosts by 20%. OK. That’s not enough for the old version to be supplanted. There are still plenty of potential hosts to go around.

While there have been instances of pathogens seeming to become less virulent over time, there is nothing inevitable about a less lethal form predominating. There are causes apart from pathogen evolution towards lower virulence that result in less consequential infections. Mammals, man included can experience selection towards better host defenses; in turn the pathogen evolves again to infect and harm more efficiently.

The Great Influenza of 1918-19 is one example of a virus that managed to spread very widely despite killing a relatively high proportion of those infected.

Thankfully, the current coronavirus has (so far) not demonstrated a mutation pattern interpreted as conferring greater virulence.

Some reading I found interesting:

http://virology.ws/2009/06/10/virulence-a-positive-or-negative-trait-for-evolution/