ICD-10-CM > Z00-Z99 > Z20-Z29 > Z28 > Z28.3 > Z28.31 Z28.31 Underimmunization for COVID-19 status Non-Billable/Non-Specific Notes These codes should not be used for individuals who are not eligible for the COVID-19 vaccines, as determined by the healthcare provider. Codes Z28.310Unvaccinated for COVID-19Z28.311Partially vaccinated for COVID-19