Doubt is a double-edge sword. On the one hand, uncertainty is essential for epistemic progress, and yet, doubt can also make us vulnerable to deception, confused to the point of no longer knowing what is true. What distinguishes a doubt that is epistemologically beneficial from one which is deceptive, or even manufactured in the context of a conspiracy theory? In this chapter, we explore doubt, its role, and the way it is being handled in the context of the public controversy about the COVID-19 vaccine. We approach conspiracy theories as argumentative discourses and reconstruct the generic structure of a conspiracy theory macro argument. Through the structure, we look into the discourse of the twelve prominent anti-vaxxers known as the “Disinformation Dozen”, focusing on the argumentative potential that doubt can have in the public controversy about the COVID-19 vaccine. We suggest to distinguish ambivalence from scepticism and denialism as three argumentative potentials that a motivated doubt can have. We argue that ambivalent doubt ought to be acknowledged, addressed and incorporated into the public health narrative, in order to avoid that an unnecessarily broad interpretation of conspiracy theory dominates the public debate and leaves an uncertain public a prey to it.