Did you let Astarion take a nibble? Yeah, it doesn't come without consequences. Thankfully, removing it is somewhat easy.
Meeting Astarion for the first time in Baldur's Gate 3 already held hints that the pale elf is hiding something, especially because you get to see his face up close, with oddly long fangs.
Just in case you didn't figure it out immediately, the vampire will not leave anything to doubt as he will attempt to feed on your player character during the first night in the camp.
Should you allow him to get fed, you will be afflicted with a debuff called Bloodless while Astarion will gain a buff called Happy.
Bloodless stat changes
Bloodless will reduce your Attack, Ability Checks and Saving Throws by one, leaving you more susceptible to enemy status effects while you will miss more of your own attacks.
Conversely, Astarion's Happy buff will move those values up by one for him. In other words, if Astarion is doing more damage in combat than your PC, this may be a good tradeoff but for those who walk around bonking people with two-handed swords, you might want to get rid of the status effect.
How to remove Bloodless
Removing your debuff will take one Long Rest with supplies. Simply taking a short rest or even a long rest without supplies will not do anything but waste time.
Therefore, you will need to loot some camp supplies around the world in case you don't have enough for your party and then take a long rest. Remember to either auto-fill when choosing the type of rest in camp or pick which supplies will be used yourself.
In case you don't want to take another long rest immediately after getting bit, Bless is the perfect way to counter Bloodless if you have a Cleric around, which you should, as it will completely negate the effects of the debuff at the very least. In the best-case scenario, you will also get buffed with better Attack and Saving Throw rolls.
For Ability Checks, a Cleric can also give you Guidance to either negate the effects or buff you, depending on the roll. Since Shadowheart is available from the earliest point in the game, you will do well to bring her along whenever possible.
Whether you want Astarion to be happy or want to keep your blood where it's supposed to be is up to you but I know my Great Weapon Fighter could definitely use that additional Attack.