Very simplified 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 trigg

Very simplified 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.