09/09/2026
Your flu vaccine cannot give you the flu. It’s an inactivated (dead) virus or contains no live virus at all — biologically incapable of causing infection.
So why do some people feel achy or run a low fever after? That’s your immune system doing exactly what it’s supposed to: building antibodies. Common side effects after vaccination include soreness, mild fatigue, and low grade fever for a day or two — that’s immune activation, not illness.
Other reasons people blame the shot:
• They caught a different virus around the same time (flu season = cold/RSV season too)
• They got exposed to flu right before the shot, before protection kicked in (~2 weeks to full immunity)
• Coincidental timing — not causation