“Taking the red pill” is a term used by some to describe when you stop believing “mainstream opinion” and start believing “the secret truth”. Widely used by conspiracy theorists.
However in relation to the above post it wouldn’t be applicable, as it is mainstream scientific opinion that vaccines provide safety from viral illnesses. So wanting to be vaccinated to protect against Covid would be taking the “blue pill” (remaining with mainstream thought), whereas anti vaxxers will say they took the “red pill” when they realised the “truth” about vaccines.