(like) a shot in the arm

A sudden stimulus, something vitalising, uplifting or encouraging

e.g. The Covid restrictions have had a terrible impact on the arts sector. Fresh government investment in the arts would provide the much needed shot in the arm to the arts.

The idiom refers to the quick and dramatic effect that a shot (UK - injection) can give.
The idiom was first used in the US in the early 1900s. The Lewiston Evening Journal, January 1916:
“The vets (army veterans - former soldiers) can give politics a shot in the arm and the political leaders realize it.”