Vacunada literally means "vaccinated". It's a powerful and unexpected word to find in a fiery breakup anthem!
In the song, Shakira uses it as a brilliant metaphor. When she sings "estoy de ti vacunada" (I am vaccinated against you), she means she has become immune to her ex's lies and hurtful ways, a very creative way to declare her emotional strength and freedom.
In this pop-tango crossover, Colombian star Shakira draws a hard line against a messy love triangle. The core problem is simple: her partner is juggling two women ("Es un día ella y otro día yo" [One day it's her and another day it's me]). She calls the whole dynamic out with rapid-fire insults, labeling the mess "patético, neurótico, satírico, psicótico" [pathetic, neurotic, satirical, psychotic]. To sum up why this cannot work, she drops a great truth: "el tango no es de tres" [the tango is not for three]. Tango is a dance strictly built for two, so three people on the floor only guarantees chaos.
Her reaction to the drama is blunt, theatrical, and funny. She announces her immediate exit from his "negocios sucios" [dirty business], explaining that she has become totally numb: "Por ti me quedé como Monalisa / Sin llanto y sin sonrisa" [Because of you I ended up like the Mona Lisa / Without tears and without a smile]. Comparing her blank, exhausted expression to Leonardo da Vinci's famously unreadable portrait says everything. She even declares herself "vacunada" [vaccinated] and "A prueba de patadas" [kick-proof] against his nonsense before telling him that only "el cielo y tu madre" [heaven and your mother] can look after him now.