Why do mRNA vaccines sometimes cause flu-like symptoms?

Sometimes, mRNA vaccines make people feel a bit like when you're playing with a new toy that’s super exciting, but your body needs a little time to adjust.

Imagine your body is like a busy kitchen, and the vaccine is like sending in a message telling the kitchen to start making a special dish. The message says, “Make some fighters to help protect you from sickness!” But when the kitchen starts working on this new recipe, it might get a little tired or confused at first, that’s why you might feel a bit like you have the flu.

When your body reads the message and starts making these fighters (called antibodies), it can cause some extra work. That extra work is what makes you feel tired, achy, or even a little hot, just like when you play hard all day and then go to bed feeling exhausted.

Some people might not notice these feelings at all, maybe they're used to being tired after playing with their favorite toy. Others might feel them more strongly, depending on how much work the kitchen had to do. But don’t worry, it’s just your body getting ready to fight off germs!