How Do I Code a Flu Shot or Other Immunization Visit? Z23 for the Encounter, Z28 When the Vaccine Is Not Given, Z28.3 for Underimmunization Status

Z23, Encounter for immunization, is one code for every vaccine. The Guidelines define its job:

“Code Z23 is for encounters for inoculations and vaccinations. It indicates that a patient is being seen to receive a prophylactic inoculation against a disease. Procedure codes are required to identify the actual administration of the injection and the type(s) of immunizations given. Code Z23 may be used as a secondary code if the inoculation is given as a routine part of preventive health care, such as a well-baby visit.”

Two things follow. First, Z23 never identifies which vaccine: the tabular carries the note “procedure codes are required to identify the types of immunizations given,” and the vaccine product and its administration are reported outside ICD-10-CM. A claim for a flu shot, a tetanus booster and a shingles vaccine carries one Z23. Second, sequencing turns on the reason for the visit. When the patient came in for the shot, Z23 is first-listed — it is one of the encounter Z codes that can lead, as the Z-code-as-principal guide explains. When the shot was given during a routine examination, the examination leads: Z23 carries a Code First note for any routine childhood examination, so at a well-child visit Z00.129 (or Z00.121 with abnormal findings) is first and Z23 follows, exactly as the guideline’s well-baby example says. Z23 also carries a Code Also note for Z71.85, Encounter for immunization safety counseling — a code the Guidelines restrict to counseling “regarding the safety of a vaccine,” not the routine discussion of risks and side effects that accompanies an ordinary vaccination.

When the vaccine is not given. Category Z28, Immunization not carried out and underimmunization status (Includes: vaccination not carried out), records the reason: Z28.0- contraindication (acute illness Z28.01, chronic condition Z28.02, allergy to the vaccine Z28.04, other Z28.09), Z28.1 belief or group pressure, Z28.2- patient decision (Z28.21 refusal, Z28.20 unspecified reason, Z28.29 other reason), Z28.8- other reason (Z28.81 already had the disease, Z28.82 caregiver refusal, Z28.83 vaccine unavailable, Z28.89 other), and Z28.9 unspecified. These describe a decision or circumstance at this encounter; they are added to the code for whatever the visit was for.

Underimmunization as a status.Z28.3, Underimmunization status, with Z28.310 unvaccinated for COVID-19, Z28.311 partially vaccinated for COVID-19 and Z28.39 other underimmunization status, describes the patient rather than the visit. It carries a Use Additional Code note for the Z28.0-/Z28.1/Z28.2-/Z28.8- reason the immunization was not carried out, and the Guidelines place a sequencing limit on it in Section I.B.14: “codes for social determinants of health and underimmunization status should only be reported as secondary diagnoses.” The same section allows underimmunization status to be documented by clinicians other than the patient’s provider. The tabular adds, at the COVID-19 codes, that they “should not be used for individuals who are not eligible for the COVID-19 vaccines, as determined by the healthcare provider.”

A reaction to a vaccine is a different family: T88.0, Infection following immunization, and T88.1, Other complications following immunization, not elsewhere classified, are complication codes, reported when the provider documents a complication of the immunization itself.

1. Establish why the patient came. For the vaccination itself → Z23 first-listed. For a routine examination during which a vaccine was given → the examination code (Z00.12- for a child, Z00.0- for an adult) first, Z23 secondary.
2. Report the vaccine product and its administration with procedure codes; Z23 does not name the vaccine.
3. If a due vaccine was not given, add the Z28.0–Z28.9 code that matches the documented reason.
4. If the provider (or another clinician, per I.B.14) documents unvaccinated or partially vaccinated status, add Z28.3- as a secondary code, with the Z28.0-/.1/.2-/.8- reason code its Use Additional note asks for.
5. Use Z71.85 only for counseling about vaccine safety as such — not for the routine risk information given with a shot.
6. A documented complication of a vaccine is T88.0 or T88.1, not Z23.

1. Adult walk-in for a seasonal flu shot; nothing else addressed.Z23, first-listed. The vaccine and its administration are procedure codes.
2. Well-child visit at 12 months with scheduled vaccines, no abnormal findings.Z00.129 first, then Z23 — the guideline’s well-baby case, and Z23’s own Code First note.
3. Flu shot offered at a chronic-disease visit; the patient declines it. The reason for the visit leads; Z28.21, Immunization not carried out because of patient refusal, is added.
4. Visit for counseling about COVID-19 vaccine safety; no vaccine given; the provider documents that the patient is unvaccinated.Z71.85 for the counseling, with Z28.310 as a secondary status code and the Z28.2- reason code its note asks for.

• Expecting Z23 to tell the payer which vaccine was given — it does not; that is the procedure code’s job, and the tabular says so.
• Listing Z23 first at a well-child visit. Z23 carries Code First for the routine childhood examination.
• Reporting Z28.3- as the first-listed or principal diagnosis; the Guidelines make underimmunization status secondary only.
• Assigning Z71.85 for the routine risk-and-side-effects discussion at an ordinary vaccination visit.
• Coding a vaccine reaction to Z23 instead of the T88 complication codes.
• Reporting Z28.310 or Z28.311 for a patient the provider has determined is not eligible for the vaccine.

• The reason for the visit: the vaccination itself, or a routine examination during which it was given.
• Whether the vaccine was administered, and if not, the documented reason.
• Vaccine safety counseling as the subject of the encounter (Z71.85), as distinct from routine risk information.
• Unvaccinated or partially vaccinated status, which any clinician may document, and eligibility as determined by the provider for the COVID-19 status codes.
• A documented complication of the immunization.