2406: Viral Vector Immunity

Explain xkcd: It's 'cause you're dumb.
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Viral Vector Immunity
We've secretly replaced this customer's instant coffee with our patented substitute. Let's see what she ... uh oh, I think she spotted us through the window. Now she's getting something from the closet ... oh jeez, she has a sword! Run!!
Title text: We've secretly replaced this customer's instant coffee with our patented substitute. Let's see what she ... uh oh, I think she spotted us through the window. Now she's getting something from the closet ... oh jeez, she has a sword! Run!!


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The comic attempts to explain a virus vector vaccine, and one way it can fail, using the story of the Trojan Horse as an analogy.

A vaccine is a way to familiarize a host's immune system with a pathogen without actually causing the host to fall ill. There are many types of vaccines that have been developed, all of which are ways to present a significant segment of a molecular structure to the host body, so that the immune system recognizes the pathogen and mounts an immune response faster when a real infection happens.

A viral vector vaccine uses a modified virus, different from the pathogen being immunized against, as a carrier to deliver a molecular payload into the host body. This modified virus is called the vector because it is the method of delivery of the molecular piece of the pathogen. If the recipient has a strong immune response to the vector itself, the immunization may be less effective.

For example, a modified (to be harmless) cold virus can be used to deliver an (harmless) outer segment of another virus into the body. The ability of the vector virus to sneak into the host body and inject itself into the cell's reproductive system is used to trick the body into producing numerous copies of the recognizable piece of the other virus. That way the immune system recognizes the molecular segment of the other virus without actually being infected with that virus, which tends to significantly decrease recognition time after a real infection, leading to a less severe infection speeding up recovery.

The comic represents this idea with the Trojan horse being the (in this case) harmless vector virus, carrying, contrary to the traditional story, a beneficial payload into the cell.

In the comic the warriors recognize the shape of the delivery vehicle (the Trojan horse) as being similar to an animal that trampled one of their own earlier and therefore refuse it entry. An amusing point here is that they are not as such surprised at the arrival of a wooden vehicle at their doorstep, rather that its shape resembling an animal they have found threatening before, which is similar to how simple in its judgements the immune system can be.

The title text is a further riff on this theme, playing on an advertising campaign for freeze dried coffee. In the advertisements a narrator would claim to have secretly replaced fresh brewed coffee with that made from freeze dried to see if subjects could tell the difference. The contents of the coffee cup being the payload and the narrator the virus vector.


[A large wooden horse statue on wheels stands before a city wall, upon which are standing several warriors who are shouting and brandishing spears.]
Warrior 1: Look! It's a statue of that horrible animal that trampled Steve!
Warrior 2: Burn it!
Warrior 3: Smash it!
Warrior 4: Push it into the gorge!
[Caption below the panel:]
How vaccine failure due to viral vector immunity works
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Mentioning explainxkcd on Randall's tweet https://twitter.com/xkcd/status/1345061851424501761 started off some explanations 18:09, 1 January 2021 (UTC)

Is anyone else reminded of the "Boil 'em, mash 'em, stick 'em in a stew" rhythm? "Burn it, smash it, push it into the gorge". Into breaks the rhythm a bit, but perhaps it could be a The Two Towers (Lord of the Rings) movie reference? 19:59, 1 January 2021 (UTC)

Couldn't a similar comic be used to explain how immunity works in general? Instead of the horse being a vaccine vector, it would be a pathogen, and the immune cells recognize it from a previous encounter and attack it. Vaccine vector failure occurs when the vector resembles something you've developed immunity to. Barmar (talk) 20:33, 1 January 2021 (UTC)

Yes, I agree with this — the current explanation for the comic (Trojan horse = immunity vector, Steve-trampling horse = common pathogen) doesn't explain how viral vector immunity works, it explains how it fails to work. I think a more appropriate explanation for the comic would have the Trojan horse be the pathogen against which immunity was desired, and the Steve-trampling horse be the DNA carried by the immunity vector. This would also be consistent with the traditional use of the Trojan horse to signify an unexpected threat (as opposed to the current interpretation's, whcih has the Trojan horse be beneficial). 08:05, 2 January 2021 (UTC)
I think you're absolutely right that the current explanation is interpreting the comic as an explanation of how viral vector immunity fails to work, while your suggested explanation interprets the comic as an explanation of how viral vector immunity works. The caption of the cartoon, "how vaccine failure due to viral vector immunity works" shows that the existing interpretation is the intended one.Yp17 (talk) 14:24, 3 January 2021 (UTC)
I think you are misunderstood about the workings of a viral vector vaccine. The whole premise of this approach to vaccination is to use the ability of a virus to sneak into a human body undetected, later releasing its payload into the cells, but for a beneficial gain, rather than harm. The vector virus is perfectly represented by a trojan horse - it is supposed to enter the gates unrestricted. In the case of a real infection, the virus RNA injects itself into the cell and takes over its live processes, much like the soldiers took over the city after coming out of the Trojan horse. In the case of a vector vaccine, the trojan horse bears a beneficial payload inside. The trampling-horse is an incidental "immunity" to everything that looks like horses, i.e. immunity to the vector virus, not the payload virus (which they never get to experience since the horse never makes it in). 02:21, 3 January 2021 (UTC)
I think the issue is that it's an imperfect, flawed analogy to begin with where the details and their relationships don't quite match those of the subject it is being compared to, so any attempt to accurately explain the analogy can't be perfect either.-- 05:28, 4 January 2021 (UTC)
Yes, and Randall has now done just that: vaccine-triggered immunity is now explained (via Star Wars analogy) in 2425: mRNA Vaccine. DKMell (talk) 09:07, 16 February 2021 (UTC)

I would disagree with the title text explanation, at least to a degree. The narrator is the person being recognised and threatened with the sword, but the narrator is not the vehicle of delivery of the modified payload (the coffee), that would still be the cup. I think either the metaphor or the explanation breaks at this point, which is not uncharacteristic of the title text often deviating from the stricter rules of the comic. 21:30, 1 January 2021 (UTC)

Right. The common theme is that the victim of a trick has seen through the ruse. In the title text, the narrator is the perpetrator of the coffee replacement trick, and the victim has detected the difference (or already knows about it by hearing from someone else -- similar to the way the immune system is forewarned by vaccines) and is now coming after the narrator with a sword. Barmar (talk) 06:12, 2 January 2021 (UTC)
I don't think it's so much the victim seeing through the ruse, it's that the victim has other reasons for attacking the narrator, before even getting to the point where she would drink the coffee and possibly notice any difference, removing the whole point of the ruse without the victim realising that there was a ruse. In the case of the Trojan Horse, this other reason is Steve's previous encounter with a horse, leading them to destroy the horse statue without the Greeks inside it ever coming into play. In the case of the viral vector, the other reason is the previous immunity to the carrier virus, destroying it before the payload can be delivered into the cells. In both these cases, the "ruses" fail because of unconnected reasons the "perpetrators" didn't know about. In the case of the title text, even the reader doesn't know this unconnected reason. 11:31, 3 January 2021 (UTC)

The narrator in the alt text/title text is the scientist/researcher performing the experiment. Except that the researcher doesn't usually get threatened with attack from the research subject. In some cases perhaps they should though, such as the Tuskegee experiments.

In the other comics mentioning Steve no-one liked him too much...--Gunterkoenigsmann (talk) 06:06, 4 January 2021 (UTC)

There is a huge gorge several kilometers southeast of Troy at Kemer Creek. 22:46, 10 January 2021 (UTC)