Why Do Some People Get Fever After a Flu Shot

Added: 2026-06-03

[00:00:00]Why do some people get a fever after a flu shot while others feel completely fine?

[00:00:05]A mild fever [music] after a flu shot can happen and it's usually temporary.

[00:00:10]But interestingly, not everyone reacts the same [music] way. Some people feel nothing at all.

[00:00:15]Others may feel tired, achy, or slightly feverish for a day or two.

[00:00:20]Why? Because every immune system responds differently. [music] When your body detects the vaccine, it releases immune signals called cytokines.

[00:00:28]These signals help activate your immune defenses, but cytokines can also raise body temperature and trigger flu-like symptoms temporarily. In general, younger adults tend [music] to have stronger immune reactions than older adults.

[00:00:41]That's one reason why some people may notice more side effects after vaccination. The good news? A mild fever after a flu shot is usually a normal immune response, not the flu itself.