Increase in the IgG avidity index due to herpes simplex virus type 1 reactivation and its relationship with cognitive function in amnestic mild cognitive impairment and Alzheimer's disease

► The anti-HSV-1 Ab avidity index was higher in the aMCI than in the AD or controls. ► HSV-1 reactivation had occurred in the aMCI than in the AD or controls. ► The anti-HSV-1 IgG antibody avidity index was higher in subjects with higher MMSE. ► No significant difference was observed in the HSV IgG titer between the MCI and AD. ► The results suggest the avidity index may be a biomarker for the early diagnosis.