If you think you’re coming down with a bug, and especially if you have a fever, we recommend waiting to get the flu vaccine until you’re feeling better. When you get the vaccine, your white blood cells quickly find and destroy the invaders and create a multitude of memory cells, which are capable of recognizing the same virus in the future. This lays the groundwork for a full-on attack when your body is faced with the real thing.
However, if you have a respiratory illness without a fever, or if you’re only mildly ill, it’s OK to get the flu shot.
Flu vaccines administered by a needle contain inactivated versions of the virus. It won’t cause the flu, but getting the vaccine once you’re very sick could make an existing infection temporarily worse, since your white blood cells are already working to fight off the germs your body has encountered.
The process of building immunity to the flu isn’t noticeably felt by healthy people. Once you’ve been on the mend for a few days, and once your fever has subsided, we recommend coming in for that flu shot.