How common is gbs, and how common is it among people who have been vaccinated against seasonal influenza?

Rare: In the USA Guillain-Barre syndrome (GBS) occurs in about 1 to 2 per 100, 000 regardless of immunization status. GBS can be triggered by viral & bacterial infections. Modern influenza vaccine is thought to increase the incidence of GBS by 1 in a million, or less. Remember that between 3,000 to 49,000 people die each year from influenza. The risk is much less for vaccine than influenza disease.

Gbs: Is not affected by the flu shot, I am not sure of the prevalence.