This is the 1176th most frequent Catalan word.
vacuna
"Vacuna" in Catalan means "vaccine" in English.
Here, 'vacuna' is used as a noun referring to the medical substance designed to trigger immunity against a disease (the flu in this case).
La vacuna contra la grip es recomana cada any.
The flu vaccine is recommended every year.
In this sentence, 'vacuna' is the imperative form of the verb 'vacunar', used to instruct someone to administer a vaccine to prevent illness (rabies here).
Vacuna el teu gos per evitar la ràbia.
Vaccinate your dog to prevent rabies.
Here, 'vacuna' appears again as a noun, referring generically to a newly developed immunization product within the context of clinical testing.