Answer:

boosters

Answer	Crossword Clue
BOOSTERS	Fans
BOOSTERS	Second shots
BOOSTERS	Immunization shots, e.g.

BOOSTERS	Promoters.
BOOSTERS	Amplifiers, in radio.
BOOSTERS	Devices for rocket propulsion.
BOOSTERS	Promoters: Colloq.
BOOSTERS	Opposite of the knockers.
BOOSTERS	Stages of space exploration
BOOSTERS	Enthusiastic supporters
BOOSTERS	Some vaccine shots