Dengue has multiple strains. The initial infection with one strain tends to be not so bad and you may acquire immunity to it. However, it triggers a much worse, often hemorrhagic outcome if you acquire one of the other strains.

Vaccines, afaik mirror that property, changing the risk calculation by removing the less risky first infection.

First a WHO link.:

https://www.who.int/news-room/questions-and-answers/item/den...

>There is an increased risk of hospitalized and severe dengue in seronegative individuals starting about 30 months after the first dose.

TL;DR vaxx good if you've been infected before, but if you're dengue naive, it might raise the risk of severe side effects

Now, on the topic of an actual immunological phenomenon with a name out of /pol/'s fever dreams: An In-Depth Analysis of Original Antigenic Sin in Dengue Virus Infection

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3014204/

TL;DR: Original antigenic sin describes a situation where the body's first exposure to an antigen shapes it's future response. This ends tragically in the case of dengue, where the response trained on a serotype is unfortunately overfit. Upon exposure to a different dengue serotype, the immune system defaults the response fit to the original infection, resulting in a response dominated by an inferior antibody. The outcomes are thus worse.

From what I'm seeing in more recent publications this model of dengue infection still seems to be the general consensus.

As to why I dug into this area in the first place, I live somewhere where dengue was quite the bugbear, and the "second time kills you" story was mentioned quite a bit. Turns out it's a pretty interesting area, since it clashes with the layman concept of immunity. I'm no expert in this domain though, so if I've missed out some progression on the science here, I'd really like to know.

Edit:

As others have also pointed out, there's also the case where the antibodies even enhance the infection process.

I’m not sure how mRNA vaccines get around that, but I imagine they have a plan, it would be interesting to hear it.

The issue with dengue in particular is a bit of a special case. Infection with a single serotype weakens the body against infection with other serotypes. Now. This poses a challenge to vaccination, as you need to find a way to get all four.

Currently, we have dengvaxia, which does do that. However, an interesting effect is that the vaccination is only useful if you already have a history of dengue. If you're dengue naive, the vaxx increases the risk of severe disease instead.