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It's general to all vaccines, not specific to just COVID. The search term you want is "leaky vaccine".

Basically, when you have a treatment (a 'leaky vaccine') that prevents symptoms but not transmission, then the normal evolutionary constraint for a virus to become less-harmful (to maximize transmission) goes out the window - because anyone that's been vaccinated could just as easily carry a more-harmful version of the virus.

It doesn't always happen, and it's hard to predict when it will, but Marek's disease is an example of a worst-case scenario actually happening: when a leaky vaccine scenario allows the virus to mutate and become highly lethal.

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> normal evolutionary constraint for a virus to become less-harmful (to maximize transmission)

I don't know why we're taking it for granted that maximizing transmission is the same as being less harmful. The delta variant seems both more harmful AND more transmissible (and developed in an area that didn't have vaccines yet last fall).

There seems to be a good case to be made that, say, ebola is too harmful to be super transmissible. I don't think we can say the same for Covid yet. The majority of cases have never been fatal, on the contrary, being transmissible is what's made it a public threat and mass scale killer already. "Transmitting it without symptoms" has been a thing since day 1.

> The delta variant seems both more harmful AND more transmissible (and developed in an area that didn't have vaccines yet last fall).

I left it out of the initial explanation for the sake of brevity, but there's several outcomes from a leaky vaccine. Two of them are: increased lethality, and adaptation to vaccines, requiring new vaccines to be developed.

So yeah, the delta variant makes it look like we might be dealing with a leaky vaccine scenario.

You have to look at the reasons why this behavior is occuring: Viruses get less harmful if it improves transmission. E.g. if you are more active and have a higher rate of contacts, even if infected and sick with a certain strain, that strain has an advantage. With Covid, most of the transmission occurs before symptoms appear, meaning that the severity of the following illness is irrelevant to transmission. In that case, the virus has no advantage from being less harmful.

Also, simpler changes are preferred over more complex ones. Delta probably just has a mutation that allows it to reproduce faster after an infection, meaning that both infectiousness as well as severity of illness go up at the same time. Mutations that increase infectiousness but lower severity are usually more complex, so less likely to occur, especially if not selected for.

Sorry I wasn't clear: I have read about Marek but what I didn't find is can it happen with COVID vaccines or not.

Oh, gotcha. Here's a source that might help.

> There is a theoretical expectation that some types of vaccines could prompt the evolution of more virulent (“hotter”) pathogens. This idea follows from the notion that natural selection removes pathogen strains that are so “hot” that they kill their hosts and, therefore, themselves. Vaccines that let the hosts survive but do not prevent the spread of the pathogen relax this selection, allowing the evolution of hotter pathogens to occur. This type of vaccine is often called a leaky vaccine.

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

Basically, it all comes down to whether or not the vaccines prevent transmission. If they do, we're all good. If they do not, we might be in trouble. It looks like they do not.

@raphlinus posted a link from the author of the Marek study, apparently he's not worried about it.

https://theconversation.com/vaccines-could-affect-how-the-co...

And there's others who are worried.

More to your point, though, I'm more interested in the peer-reviewed stuff than op-eds.

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