Optimal Lethality

I had this argument with my aunt, who said that one of the reasons we shouldn’t worry as much about COVID-19, is that viruses evolve to be less lethal over time, and she said that this is what happened with H1N1.

I’ve only been studying evolution for a few months, and in bacteria, not viruses. But this short timeframe gave me a hunch that this idea is just too elegant to be true. In expansion, this theory would be that viruses have an optimal level of fatality, after which they kill their host too quickly to spread efficiently through the population.

This idea sounds elegant but the reality of genetics is just a lot more messy. First of all, it’s really hard to make a statement that generalizes to all viruses’ evolution. There are just too many and they’re too different. Second, genes don’t translate directly to human concepts. Viruses don’t really have a gene for severity and a gene for transmissibility, it’s all a lot more jumbled up. A gene can affect transmissibility AND severity, or a whole lot of other things.

So I looked into H1N1 and what I found was very supportive of the claim. H1N1, it seems, became more lethal after the WHO declared it pandemic. When researchers infected ferrets (who for some reason have a similar system to humans) the additional mutations seemed to increase the rate of replication in the respiratory tract, which increased transmissibility, but also severity!

So my aunt was wrong about H1N1 (which is both encouraging and discouraging at once), but I wanted to see if there’s still any pattern that can generalize for other viruses. If that was the case, 1) it’d make me even more scared about COVID-19and 2) it’d be very evolutionairly beautiful and elegant.

But luckily I found a really cool counter-example: HIV. Apparently, following infection it adapts over time towards a less virulent form. There are a few theories as to why exactly (with antiretrovial therapy and adaptation to avoid the immune system being two), but the article also suggests my aunt’s exact theory, that evolution favours milder symptoms. So I can tell my aunt we were both right.

All this comes to show that our understanding of evolution and genetics is still too limited to paint with a broad brush. It’s very difficult to predict the evolution or traits of a virus, especially one that just now emerged.