The illness, and why a second bout is worse
Dengue is a virus spread by Aedes mosquitoes - the striped ones that bite in daylight, breed in a bottle cap of water and live comfortably in cities. It causes a sudden high fever, a severe headache behind the eyes, deep muscle and joint pain that earned it the name breakbone fever, and a rash. Most people recover in a week or two feeling wrung out.
There are four types of dengue virus. Recovering from one gives lasting protection against that type only, and a later infection with a different type carries a distinctly higher risk of severe dengue, where plasma leaks from the blood vessels. That asymmetry is the single most important fact about this disease, and it is why previous infection strengthens rather than weakens the case for vaccination.
Being straight about the limits
Qdenga is licensed for use whether or not you have had dengue before, and it does not require a blood test first. Protection is good but not complete, it varies between the four virus types, and the long-term picture is still being built - this vaccine has not been in use for decades the way hepatitis A has.
So it does not replace bite avoidance, and it is not a routine addition to a fortnight in Phuket. It is a live vaccine, which rules it out in pregnancy, while breastfeeding, and for people whose immune system is suppressed. We will go through all of that before anything is given, and we would rather you left without it than with it on a misunderstanding.
Daytime bites change the advice
The mosquito that carries dengue bites in the morning and late afternoon, not at night. Bed nets, which are the mainstay against malaria, do comparatively little here.
What works is repellent applied and reapplied during the day, loose covering clothing, and tipping out the standing water that collects around accommodation - plant pot trays, buckets, blocked gutters. If you are also in a malaria area you need both strategies at once, and we will set out what that looks like on your trip.