A second dengue infection can be more dangerous if it is caused by a different dengue serotype. There are four serotypes: DENV-1, 2, 3, and 4.

After the first infection, your immune system makes antibodies that protect well against that same serotype. If a different serotype infects you later, some old antibodies may attach to the new virus but do not neutralize it fully. Instead, they can help the virus enter certain immune cells more efficiently. This is called antibody-dependent enhancement (ADE).

That can lead to:

This is why secondary infection with a different serotype is a recognized risk factor for severe dengue, including dengue shock. The mechanism is supported in CDC's review of severe dengue pathogenesis, and is described in Park's Textbook of Preventive and Social Medicine as an immune-mediated severe response in secondary heterologous infection.

But it is not inevitable: most people with a second dengue infection do not develop severe dengue. Risk also depends on age, viral strain, pre-existing illnesses, pregnancy, and how quickly medical care is received.

If someone with dengue develops warning signs, especially around the time fever begins to settle, they need urgent assessment: severe abdominal pain, repeated vomiting, bleeding, unusual drowsiness/restlessness, cold clammy skin, breathing difficulty, or very little urine.