How common is GBS? How common is it among people who have been vaccinated against flu? The background rate for GBS in the Unites States is about 80 to 160 cases of GBS each week, regardless of vaccination. The data on the association between GBS and seasonal flu vaccination are variable and inconsistent across flu seasons. If there is an increased risk of GBS following flu vaccination it is small, on the order of one to two additional GBS cases per million doses of flu vaccine administered.