The Sponsor is developing KB408, a replication-defective, non-integrating herpes simplex virus type 1 (HSV-1)-derived vector engineered to deliver functional full-length human SERPINA1 to the airways of people with alpha-1 antitrypsin deficiency (AATD) via nebulization. This study is designed to evaluate safety and pharmacodynamics of KB408 in adults with AATD with a PI\*ZZ or PI\*ZNull genotype. Three planned dose levels of KB408 will be evaluated in single dose escalation cohorts. Subjects taking intravenous AAT augmentation therapy are not required to wash out from IV AAT in the low and mid dose cohorts. At the high dose, two cohorts will be conducted in parallel to evaluate patients on and off IV augmentation therapy.