People go through stages as they fall into the conspiracy theory rabbit hole. Early in the decent they are still engaging in healthy reasoning patterns that I won't go so far as to say are "logical" or "rational," but they are still flexible enough to be diverted from the conspiracies. There's always a reason they start down that path: maybe someone close to them got badly sick, maybe they just had a child and are seeking out the best ways to protect them. If you can sit down with them and engage with them on this underlying cause for concern in an empathetic way, that's when you can change their mind and keep them in the zone of legitimate science and medicine. If they react to every discussion as a confrontation, they are beyond the point that bringing scientific evidence to them will change their mind.