I think it goes beyond mercury but more generally about the always evolving state of brain research:
- on one side you have old, relatively low-quality studies proving vaccines to be safe.
- on the other side, recent studies forcing us to re-think what we thought we knew. For instance, aluminium was previously thought to be eliminated via urine but has been proven to be able to migrate to the brain via the immune system where it can create chronic inflammation.
I think that we will quietly move away from aluminium-based vaccine for safety reasons. So this is terrific news if it offers another path for vaccination.
Vaccines are obviously "statistically" safe, but our knowledge on the matter is still evolving. And it's best to keep an open, scientific mind on the subject.
Some recent research papers on aluminium and the brain: https://www.sciencedirect.com/search/advanced?qs=aluminium%2...
A blog post from a parent that really helped me understand the POV of some "smart" anti-vaxxers: https://jbhandleyblog.com/home/2018/4/1/international2018