https://arstechnica.com/science/2025/10/believing-misinforma...
Basically conspiracy heads like to be in opposition to the "establishment", and will look for and spread anything fitting this theme. It gives them sense of own worth, and a sense of community - a small circle of "people who have the secret knowledge".
But the medical regulatory establishment of the US being taken over by anti-vaxxers and conspiracy theorists is worth at least a single article, I think, and it does directly and obviously harm society. Funding for research, education, trust in and access to vaccines are all going to be negatively affected, the destruction of medical infrastructure and knowledge will have long-term consequences beyond a single administration.