Debunking help: Leaky COVID vaccines and Marek's Disease in chickens

There’s a new argument burgeoning among anti-vaxxers: That it’s a big problem and a giant red flag that there are so many breakthrough cases of Delta among the vaccinated. The fact that the COVID vaccines don’t approach 100% effectiveness against virus spread among the vaccinated has led to no-vaxxer tagging the COVID vaccines with the “leaky vaccine” label – and then to comparisons with Marek’s Disease in chickens.

The idea is that as more people get vaccinated for COVID, the more vaccinated get and transmit COVID. Furthermore, COVID continues to mutate even in vaccinated people. Eventually the mutations get to the point where the vaccines can only mitigate against sickness and worst effects – and no longer have an effect on community transmission at all.

This is where Marek’s Disease is cited. The Wikipedia article has a good quick overview on Marek’s mitigation in chickens. In brief:

Because vaccination does not prevent infection with the virus, Marek’s is still transmissible from vaccinated flocks to other birds, including the wild bird population. The first Marek’s disease vaccine was introduced in 1970. The disease would cause mild paralysis, with the only identifiable lesions being in neural tissue. Mortality of chickens infected with Marek’s disease was quite low.

Current strains of Marek virus, decades after the first vaccine was introduced, cause lymphoma formation throughout the chicken’s body and mortality rates have reached 100% in unvaccinated chickens. The Marek’s disease vaccine is a “leaky vaccine”, which means that only the symptoms of the disease are prevented. Infection of the host and the transmission of the virus are not inhibited by the vaccine. This contrasts with most other vaccines, where infection of the host is prevented.

Under normal conditions, highly virulent strains of the virus are not selected. A highly virulent strain would kill the host before the virus would have an opportunity to transmit to other potential hosts and replicate. Thus, less virulent strains are selected. These strains are virulent enough to induce symptoms but not enough to kill the host, allowing further transmission. However, the leaky vaccine changes this evolutionary pressure and permits the evolution of highly virulent strains. The vaccine’s inability to prevent infection and transmission allows the spread of highly virulent strains among vaccinated chickens. The fitness of the more virulent strains is increased by the vaccine.

Anyway, you can see where this goes when used in anti-vax arguments: Delta breakthroughs = leaky vaccine = COVID vaccines will make future variants worse just like what happened with Marek’s in chickens.

My spidey sense is that this is off the mark, but I struggle to formulate a good, grounded response. I can’t answer why other human vaccines haven’t done what the Marek’s vaccine did in chickens – I only know (?) that this hasn’t yet happened with a human vaccine (?). The best I can offer is human’s aren’t chickens, COVID is not Marek’s, and the COVID vaccines aren’t Marek’s vaccines.

Do we have a better counter to the “leaky vaccines are dangerous” argument?

I don’t know… If an anti-vaxxer gave me this argument, I would tend to simply agree with them, because there is nothing whatsoever anyone can present to them that would convince them out of their insane beliefs.

As the saying goes " you cannot use facts and logic to argue someone out of a position that they did not use facts and logic to get into in the first place."

Plus, if you just agree with them and cough at them, they will go far away from you, which is exactly where I want anti-vaxxers to go.

The Covid vaccines do not confer total protection against transmission of the delta variant. They do, however still confer a significant amount of protection.

And even if this were true, the reason why the virulent variant of Marek’s was selected for was because the vaccine prevented it from killing hosts. So the way to combat such a situation would be to kill and incinerate anyone who contracts the vaccine-resistant disease. This presents some, um, issues, when applied to a human disease, rather than a chicken one.

I’d say the main debunking is simply that the delta virus started before the vaccines, and was outcompeting the original strain before the vaccines. There is no sign that it evolved in response to vaccines at all.

The above is probably all I’d say to the person you talked to. But the sad fact is that that viruses evolving to evade vaccines is definitely a thing, and something that we have to concern ourselves with. The thing is, the best way to prevent that is to beat it to the punch. We have to get enough people vaccinated or otherwise immune before the mutation has time to occur and spread.

But the alternatives are worse. Isolation could in theory work, if people would do it, but they won’t and/or can’t completely isolate. So we’re left with natural immunity, which would take even longer to happen, and would kill a lot more people in the meantime.

The main thing that gives rise to mutations is just time. The sooner we can end the pandemic, the lower the risk of mutations.

I have no relevant expertise whatsoever, but my first reaction was “if this is a phenomenon likely to occur in humans, why is the only real-world example of it a chicken disease that I’ve never heard of before?” I mean, we vaccinate humans against lots of diseases and have been doing so, in many cases, for decades; none of these vaccines work 100% of the time; and yet … we don’t have mutant vaccine-enhanced super-strains of measles or mumps, do we? So why would mutant vaccine-enhanced COVID be especially likely?

The most accurate response I can think of is that there’s inevitably an arms race between pathogens and our pathogen-fighting technologies. A lot of antibiotics that have worked quite well in the past are increasingly ineffective, which will eventually force us to develop newer classes of antibiotics or other alternatives like phages.

I’d say the bolded part is the weak link in their argument. Were there to be a large population of wild humans out there, that was functionally impossible to vaccinate, then we might have a problem.

Now, if the anti-vaxxers want to admit that they’re as wild, stupid, and uncontrollable as a bunch of pigeons, then maybe this argument is valid.

Maybe there’s hope in the fact that the Marek’s vaccine is not an mRNA vaccine. What I hope has happened is that humanity has built a better mousetrap – that the mRNA technology is superior to previous vaccine technology and can thus make up for whatever shortcomings traditional vaccines are presumed to have against COVID.

Heck, maybe with an mRNA vaccine, Marek’s can finally be laid low.

I tend to think - and I could be wrong - that beating COVID will probably be somewhat similar to beating HIV. We might need several tools in the toolbox to use in tandem. We might need an mRNA vaccine as well as a vaccine other types of vaccines. We’ll probably figure out a regimen that involves multiple treatment options, depending on severity.

Just to throw out some other things:

Various cocktails of monoclonal antibodies apparently do work against COVID – and we have to think the development of the various “-mab” medicines for COVID is still in its infant stages. Perhaps better is yet to come.

I’m with you on the likelihood of a multi-pronged COVID vaccine approach in the future. There’s no doubt now that no one or two shot(s) of any of the current vaccines makes you good for life. Eventually, I think a better vaccine mousetrap will be built – and it could well be a series.

One reason I brought up HIV was that COVID behaves in some ways like the retrovirus, slipping its genetic material into host cells. In some cases it can hide in different parts of the body such as the brain. I’ve read that this explains why “reinfections” may occur or why the body seems to immediately beat the initial infection, leading to few if any symptoms initially but that the hidden virus particles can later replicate and then cause an infection that’s symptomatic. Even when the immune system brings COVID under control, it doesn’t give up easily.

Right now, COVID seems to behave more like the common cold or a bad flu in the sense that once the body brings it under control, it is pretty much beaten in most cases, leaving not much behind but antibodies - in most cases. But if it evolves to be an infection that can learn to hide in the brain or in other parts of the body, then this is going to require treatment that spans not just a week or two but possibly much longer. It might turn into something that’s more like a chronic infection.

Leaky vaccines are only dangerous for the unvaccinated. The vaccine for Marek’s disease has been around for over 50 years and vaccinated chickens are still protected from serious disease. It’s the unvaccinated ones that die even sooner now than decades ago.

Regarding natural selection, if most chickens are vaccinated, there will be selection for viruses with high transmissibility but less deadly to the vaccinated hosts. The unvaccinated are simply collateral damage to the virus.

The same could be true for the SARS-CoV-2 variants. The ideal situation for the virus is to be highly contagious, but not deadly like the coronaviruses that cause the common cold.

Note that the delta variant arose in India, a country that has low vaccination rates. Variant evolution will be much faster in unvaccinated populations because viral evolution tends to take place in people who fight an infection for a long period of time.

Here’s a scary thought. I’m not an expert, but what if the virus continues to circulate while people get vaccinated such that everyone is immunized against disease except newborns? Mom can provide some immunity in early months. But I guess that means that the covid vaccine should be early on in the childhood vaccination schedule.

@asahi, sorry if I’m misunderstanding your post, but SARS-CoV-2 is not in any way like a retrovirus. A retrovirus has an enzyme that allows it to convert its RNA genome into DNA which then needs another enzyme for that DNA to be integrated into our chromosomes. SARS-CoV-2 is not even in the same kingdom as retroviruses.

If SARS-CoV-2 may go into a latent phase, but only by hiding in immune privileged sites like the nervous system. I suppose it could hide in those cells, but I don’t know of any mechanism in which it stops its own reproduction and remains dormant until it reactivates for some reason.

I was under the impression that long covid is caused by tissue damage or a persistent and dysfunctional immune response. There could be viral particles hanging around, but not an active infection. Or that long covid is due to a low-level chronic infection where virus levels are too low to be detected by diagnostic methods, but they’re still there.

Regardless of the mechanisms, the virus definitely does not integrate into the genome of human cells unless this is an amazing new virus that does something that no other coronavirus does.

I tried to be careful not to say that it was a retrovirus - I know it’s not. I was only trying to point out that it apparently has some of the same behaviors that a virus like HIV does, knowing they’re not the same of course. I’m admittedly reaching the boundaries of what I can confidently discuss medically, scientifically as a layman. I read a lot about viruses, about the immune system, but that don’t make me a scientist, and I know that.

You’re forgiven. :wink:

There have been people saying that the mRNA vaccine will enter your DNA and alter your genes. I saw what you wrote and I was like, “No, Asahi, not the altering DNA thing!”

No, never meant that, lol. I read a lot about this stuff, but there does come a point when I realize that I’m playing with live wires.

Well, to be fair, some of the CTs I’ve seen who’ve been saying that, my reaction was: “Friend, your DNA needs altering!”

Who cares if a variant evolves resistance to existent vaccines? mRNA vaccines are modified in a matter of days or weeks. It’s just the testing & production that take time, and that will become more streamlined. Swap the Alpha spike protein out of the vaccine for the modified Theta or Lambda or whatever spike protein.

<insert “Dodge This” gif from The Matrix here>

This is a long-ish piece I’ve just read on another board that addresses the “leaky vaccine”/Marek’s Disease anti-vax arguments:

The “leaky vaccine” concerns are so inherently flawed, it’s hard to know where to start. The argument is built upon a sequence of poor assumptions (in bold):

  1. Vaccines prevent all infections . Even the best vaccines aren’t 100% effective, and those targeting respiratory viruses (eg. flu) are especially vulnerable. Why? Because systemic vaccines (shots) stimulate antibodies in the blood, not the respiratory mucosa of the nose/throat/airways. There’s a mechanism where antibodies are converted between classes/locations, but that process takes several days after the virus is recognized by the immune system.

SARS-CoV-2 poses a particular dilemma, because it has the ability to spread asymptomatically early in the course of infection. Delta compounds the problem by multiplying far more rapidly than original recipe SARS-CoV-2, which leads to a lot more virus to potentially spread. These findings are seen in Delta infection independent of vaccination status.

  1. Vaccinated people have the same viral load as unvaccinated peeps, so they are spreading COVID in a similar fashion. Ignoring the bastardization of “viral load” to mean the amount of virus measured anywhere in the body (as opposed to only in the bloodstream) + limitations of PCR cycle threshold to identify intact virions, it’s not entirely clear how readily transmission occurs in vaccinated vs. unvaccinated hosts. Short of a perfect contact tracing system, we’ll never know definitively who’s responsible for every infection.

In general, people with symptoms are thought be better vectors of transmission, and those same people tend to have higher viral burden in their airways for a longer duration. So even if the average amount of virus in the nasopharynx measured at a single point in time is roughly equal for vaccinees/unvaccinated, because unvaccinated people are far more likely to be symptomatic, it stands to reason they also are more likely to spread infection. Moreover, it’s hard to imagine an asymptomatic super spreader (another peculiarity of covid, which tends to disproportionately occur in clusters), as a combination of high magnitude and extended duration of viral shedding (in a high risk environment) is almost certainly necessary to promote one person infecting so many others.

  1. "Leaky vaccines" cause viruses to become more deadly. We’ve learned a bunch of chickens are dying, and the virus responsible for their deaths has become more virulent after a vaccine was introduced decades ago. The implication is the “leaky” vaccine caused the mutation(s) which made the virus nastier.

Evolution doesn’t work that way. Mutations occur randomly, because viral replication is imperfect. Most mutations are inconsequential (don’t impact genes responsible for the structure/function of the virus), though some are harmful, and others beneficial to a virus’ reproductive fitness. The latter mutations are selected over multiple generations, with the viruses most proficient in spreading eventually winning out. But there’s nothing inherently more dangerous about a more contagious virus, or one that replicates more efficiently. On the contrary, some have argued a tendency for viruses to become less deadly over time, as dead hosts don’t promote viral propagation as effectively as living ones.

So there’s no guarantee a virus will become more potent. It’s just random chance. And even if a virus happens to mutate into something more deadly, there’s no reason to blame it on vaccines. If enough people were vaccinated, there would be a competitive advantage to a vaccine resistant strain, for sure, but being able to evade vaccine-induced immunity <> ability to harm (at least in comparison to the wild type virus, in someone who isn’t vaccinated).

Since the mRNA vaccines target the receptor binding region of the SARS-CoV-2 spike protein, any mutation resulting in vaccine resistance (ie. altering the spike so antibodies don’t recognize it) might also impair viral entry into cells. That’s one of the reasons vaccine developers chose the RBD as the vaccines’ target: it’s really important for viral function, and the structure must remain relatively conserved to allow infection in the first place.

  1. Marek’s disease is a model for the harms of “leaky” vaccinations, which can be applied to SARS-CoV-2/Covid vaccines.

Because the vaccine for Marek’s disease allow infection to propagate in asymptomatic birds, it creates a pool of virus with commensurate mutation risk. As mutation is ultimately a function of the total amount of replicating virus, random mutations can pile up, until eventually the disease is worse. Unvaccinated chickens shoulder this increased risk.

While all the points about viral evolution mentioned in #3 apply, there are other problems with this argument. Marek’s disease is caused by a type of herpesvirus. Herpesviruses have the ability to evade the immune response and remain latent, only to resurface decades later (think shingles, due to reactivation ( not reinfection ) of the varicella (herpes) zoster virus, which also causes chickenpox). Coronaviruses don’t share this trait, so there’s no viral reservoir beyond what exists during active infection.

So it all boils down to the total amount of virus in vaccinated versus unvaccinated populations, and the risk:benefit of vaccination. I won’t rehash all those arguments, but current symptomatic infections, hospitalizations and deaths all disproportionately occur in unvaccinated individuals. And nearly all the concerning variants have arisen in areas with poor vaccine coverage. Nothing suggests vaccination is creating worse outcomes, both in the short and long term.

That’s a great post. Did you see the dingbat, anti-vax reply to it? :roll_eyes: