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. Repeat dosing will be evaluated at the mid dose level. Subjects taking intravenous AAT augmentation therapy are not required to wash out from IV AAT in the low and mid dose cohorts. In the repeat dose and the high dose cohorts, subjects must wash out from IV AAT for at least 10 days, as applicable.