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Comment by cephaslr | Hacker News Reader

Something I have been wondering about, wouldn't a virus naturally evolve to be less deadly and more contagious over time? I.e. virus's tend to achieve a steady state like the cold and flu in the long run. If you are too deadly it hinders spread so I would expect Covid to get less deadly and more contagious over time, which kind of fits the data?

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You would think – but viruses don't necessarily become less deadly.

The virus wants to maximize transmissibility, and that might require trading off further against the host's health and increasing its death rate.

An example is Myxoma virus. It was intentionally introduced to pest Australian rabbit populations (to cull them) and studied.

After ~30 years of evolution, they found the dominant strain had a 70-95% death rate and left long-lasting lesions. Other strains with higher (~99%) and lower (~50%) death rates weren't as stable & prevalent.

Once a virus is transmitted (enough), what happens to the health of its host is irrelevant.

[1] https://www.sciencedirect.com/topics/immunology-and-microbio...

It basically needs to be extremely contagious without symptoms that stop the spread - like killing the host.

Which in part this virus has, asymptomatic and pre-symptomatic are as contagious as symptomatic. If it's enough to spread, doesn't matter much if the host dies after a few days or not.

>Which in part this virus has, asymptomatic and pre-symptomatic are as contagious as symptomatic.

Do we really know that yet?

>Infectiousness may peak before symptom onset (7). Viral loads appear to be similar between asymptomatic and symptomatic patients (8), although the implications for infectiousness are unclear. People experiencing symptoms may self-isolate or seek medical care, but those with no or mild symptoms may continue to circulate in the community. Because of this, those without severe symptoms have the potential to be “superspreaders” and may have an outsized influence on maintaining the epidemic.

Source: https://science.sciencemag.org/content/370/6515/406

While it's not confirmed they are more infectious, they walk among the community without being aware, therefore more prone for spreading.

Generally, yes. But also we’re a bit unlucky that it isn’t very deadly: SARS was way deadlier To start with but as a result spread several orders of magnitude less (both naturally and because humans had a stronger reaction to “shut it down at all costs“).

The H1N1 virus from the 1918 pandemic grew less deadly over time, and essentially evolved into a seasonal flu that is still affecting people around the world today. Personally, I expect COVID19 to go the same way.

The h1n1 is a bit of a strange case though because it jumps species boundary very often and mutates very quickly. COVID-19 appears to evolve much, much slower. Based on my limited understanding of virology, I think this means it's unlikely to become a seasonal problem unless the general populations immune system does not retain immunity to the same strain for a significant period of time.

We already know CoV2 rarely causes any symptoms in children. I wonder if there was a time in prehistory where common Rhinoviruses and Coronaviruses wiped out larger populations of elderly.

Even if our immune system memory doesn't last very long for these types of viruses, wouldn't young people getting exposed to it now likely reduce its impact long term?

It also seems like >95% of people who die from this are over 55 years old. When you get into your 70s~80s, aren't common colds one of the things that result in natural deaths, due to an aging immune system allowing pneumonia to set in where it wouldn't in a younger person?

> We already know CoV2 rarely causes any symptoms in children. I wonder if there was a time in prehistory where common Rhinoviruses and Coronaviruses wiped out larger populations of elderly.

Didn't pneumonia, influenza (among small pox and others) have terrible consequences on American native Indians ?

I believe small pox was the major one, I've heard among the Incas for example, it wiped out between 60 and 90 percent of the population. The main reason the colonists were able to spread so easily in the two continents is because disease wiped out huge swaths of the native population before many settlers even arrived. It's not like muskets are all that superior to bow and arrow (if at all, due to reload speed). Much of the land was just left vacant and unoccupied, and former cities abandoned.

And then came Colt's Revolver.

> A Texas Ranger, Captain Samuel Walker, wrote Colt a testimonial that read, in part:"Your pistols...[are] the most perfect weapon in the World... to keep the various warlike tribes of Indians and marauding Mexicans in subjection."

https://www.pbs.org/wgbh/theymadeamerica/whomade/colt_hi.htm...

The 20th century decolonization wars showed that natives can adapt to modern weaponry fairly well.

But you need the numbers to pull this off. Sioux are not numerous enough to threaten American dominance of their territory. If they were as numerous as, say, Punjabis, that would be a different story.

Punjabis acquitted themselves fairly well in 1849.

Yes that probably happened several times in human history. There are four other endemic coronaviruses. Most people catch them as children and build up some immunity, so even if they're reinfected later the symptoms are usually less severe. But they can be deadly to immunocompromised patients.

There is some strong circumstantial evidence that the 1889 "Russian flu" pandemic wasn't caused by influenza at all but rather by the emergence of HCoV-OC43. It killed about a million people worldwide.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7252012/

It's already clear that the vaccine, when it gets released, won't be a one time thing and we'll need more in the future to counter the mutations. Unfortunately, just in anecdotally talking to friends about it, it doesn't seem that this is widely known. It will be interesting to see how things play out as more people start to realize this. I suspect a lot of anti-vaxxers will be even more emboldened by it.

I do not think mutations are the main problem, just the fading of immune system memory.

But it does seem very likely the vaccine will not be a one time shot and you’re good forever.

I don’t think anything is “clear” - there are even indications that people exposed to the original SARS virus have their immune systems primed to more quickly respond to SARS-CoV-2.

We're unlucky in that Covid takes such a long time to kill. It's a little under a week until symptoms onset, another week until serious illness [1]. Hospital patients aren't dying of Covid until almost 2 weeks after admission [2]. So we're talking about victims dying maybe a month after exposure.

With other illnesses there's evolutionary pressure to be less deadly so that the host doesn't die before infecting others, but that's not a problem for Covid where people are most contagious before symptom onset.

[1] https://www.cdc.gov/coronavirus/2019-ncov/hcp/clinical-guida...

[2] https://www.bmj.com/content/369/bmj.m1923.full

Influenza isn't in a steady state, there are hundreds of strains[0] that compete with each other and recombine to form novel versions. Each year's flu vaccine targets four of them based on modeling as to which will be most prevalent in the upcoming flu season[1/2].

0: https://www.cdc.gov/flu/about/viruses/types.htm

1: https://www.cdc.gov/flu/prevent/quadrivalent.htm

2: https://www.fda.gov/vaccines-blood-biologics/lot-release/inf...

If you look at the death rate compared to March, it is dramatically lower, even though we're having 2x the cases. This could be because of better treatment, or because the virus itself is getting milder.

Or because more people (with mild symptoms) get traced and tested. We had a shortage of tests and masks on start of pandemic.

There are a lot of factors contributing, in particular the age distribution of those people getting infected.

One thing is "viral load", which has to do with how much of the virus is present at initial infection. Because immune system response takes time to ramp up, e.g. 10x more virus in the beginning means that it can do much more damage before the immune response. It's likely that social distancing and masks have reduced the average viral load at infection time.

That is mostly due to the way they are spread as a less impacted infected person would be more outgoing and spread that virus more. When somebody more impacted by a virus would be more isolated due to the impact. So the less impacting one gets more exposure and spread.

So that in itself would be a factor, however virus can mutate in various ways and it is the mutations that effect the incubation period and that window of being infectious but not showing any symptoms - that is always going to impact things and certainly a large factor in why COVID managed to spread better than expected.

But much hindsight and data analysis will play out for years and years, after all - we are still looking at the Spanish flu data and seeing different aspects to this day.

Look for counter examples. Have we seen this in HIV? Smallpox? It's a plausibility. It's far from a guarantee.

And the opposite, /could/ happen, too. The virus could mutate to have a longer incubation period, or be more deadly, or both.

I have seen a statemen somewhere tha for viruses it goes "transimissability, mortality, stealthiness: pick two". Not sure how true it is, but looks plausible.

At least for natural viruses. Engineered ones could be extremely terrifying.

I keep wondering what unintended consequences our nearly-unprecedented efforts on non-pharmaceutical interventions ("lockdown", social distancing, masks, etc.) might end up having on the virus - for example, could all our isolation and distancing be accidentally selecting for strains of the virus that transmit more readily over further distances?

That will be a tough one to measure.

However measurement of the impact of lock-downs/social distancing upon existing known virus's like colds and flu's would give an insight into that whole area.

The mitigations are effective in slowing the spread (to varying degrees, due to a multitude of factors), which in turn slow down the rate of mutation. In any case, I would expect the dispersion of the virus to follow a pattern similar to https://www.youtube.com/watch?v=plVk4NVIUh8

Don't know about your country/region but here the mitigations are put in effect because hospitals can't handle the pressure anymore.

It's not like the virus has a brain. Statistically most could become less deadly, but it doesn't mean all will follow the same pattern. In this case, the virus becomes less deadly as we know how to treat it better and most infected and tested now are younger people.

I can imagine that a single nucleotide polymorphism doesn't care about what seems natural.

No, but its effect on the virus population kind of does.

Are you saying that a virus population has a conscience or morality ?

No the person is speaking about the evolutionary nature of the virus

But the way viruses evolve to be less deadly is that the deadlier strains kill a bunch of people quickly, before they can spread too much. So that's not exactly reassuring for any given mutation.

In purely theoretical models virulence naturally evolved to a smaller value.

Only if they are so deadly that it's harming their transmissibility.

Conventional wisdom often claims that virus evolve to become less fatal - but I don't think that matches the historical record.

There's evidence for influenza virus existing thousands of years before 1918. As such, it seems to have evolved into a much more deadly strain at that point.

Reproducing more efficiently is a win for natural selection and will normally result in a dominant strain: whether that strain is more deadly or not is going to be random.

We might optimistically anticipate some regression to the mean fatality rate (where the mean is close to 0).

> Reproducing more efficiently is a win for natural selection and will normally result in a dominant strain: whether that strain is more deadly or not is going to be random.

The conventional wisdom is such because a virus that is swiftly fatal and/or has more dramatic health consequences would have less chance to propagate to other hosts. For instance, because the original host will be unable to move or will look threatening to others so they will know not to reside close to them. This should make it less than random.

Unless of course some unforeseen factor makes this reasoning untrue.

Taken to extremes, it's easy to see how a virus that is 50% fatal within 24 hours will not last long itself.

Is there really very much selection pressure between 1% and 2% fatality rates over 4 week timespans, though? Especially if immunity is conveyed by infection, I don't see any reason why a virus like that would evolve to be less fatal within the timescales that humans care about.

Generally if it's twice as fatal it'll probably makes people twice as sick so they're more likely to go out less, stay home more, and less likely to infect other people.

However if it did damage in a way that doesn't manifest for a long time this wouldn't necessarily be the case. But that would be the exception more than the rule.

I think what happened with the 1918 pandemic (and the 2019 one for that matter) is a virus hopping between species can "reset the clock" on that trend and start as very fatal for humans.

> There's evidence for influenza virus existing thousands of years before 1918. As such, it seems to have evolved into a much more deadly strain at that point.

Influenza refers to a class of viruses, not a single virus.

You seem to be suggesting that 1918 influenza occurred spontaneously and independently of other influenza? No.

"Occurred spontaneously" no. Independently, yes.

Just like SARS-CoV-2 wasn't a thing a year ago, but crossed over to humans late 2019, the 1918 Spanish flu virus did indeed only start infecting humans in 1918, independently of other viruses. 1918 just happens to be the year it (most likely) crossed over from an animal to a human.

Independently, no.

Assuming generously that you are right about this, why (for example) are viruses crossing over in the other direction - from humans to animals - never considered important?

We might only care about the part of viral evolution and epidemiology that we closely observe or are immediately impacted by, but that does not mean the other aspects of evolution are independent.

Evolution is a random and long-term process. What you're thinking of is natural selection. Even that does not guarantee things will get better. For example, there was a worse second wave of Spanish Flu. Natural selection only implies there's less chance these terrible things will last. And, to adapt an economic adage, "The disease can remain deadly longer than you can remain alive."

Natural selection is the basic mechanism that enables evolution, the one is part of the other.

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