The International Classification of Diseases, 10th Revision, Clinical Modification (ICD-10-CM) has revised billing codes related to COVID-19 vaccination. Beginning April 1, 2022, new categories will be introduced pertaining to underimmunization for COVID-19. Patients who are unvaccinated — which is defined as having not received any dose of a COVID-19 vaccination — should be assigned the new Code Z18.310 for reimbursement purposes. Partially vaccinated individuals — defined as those who have received at least one dose of a multidose COVID-19 vaccine regimen but do not yet meet the Centers for Disease Control and Prevention criteria for full vaccination — may be assigned Code Z28.311.