I don’t know the risk of brain damage per se but if we run the numbers we get this.
On average 3.5 million births.
90% immunization rate = 3.1 million vaccinations a year
Divided by the number of reported SAE
That’s about 1/22000 for ALL SAE (pain in joints, rash, temporary low platelets, seizures, brain damage, etc)
Brain damage is considered very rare meaning much less than the others so I’d guess a power of ten less. Therefore 1/200,000. There’s a lot more dangerous things in life that we all participate in regularly. Regularly.