2026 ICD-10-CM Diagnosis Code T50.B15AAdverse effect of smallpox vaccines, initial encounter
T50.B15A is a billable ICD-10-CM diagnosis code for adverse effect of smallpox vaccines, initial encounter. The 7th character A marks it as an initial encounter code, used while the patient is receiving active treatment. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 917 through 918. The code is not accepted as a principal diagnosis by the Medicare Code Editor. Coders also document this condition as adverse reaction to smallpox vaccine. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Adverse effects of drugs and medicaments, initial encounter.
Code Identity
Code Classification
Code EditsBilling
Medicare Code Editor checks that affect claim validity for T50.B15A.
Approximate SynonymsGuidance
Alternate terms and clinical phrases that map to this code.
- Adverse reaction to smallpox vaccine
- Encephalitis caused by smallpox vaccine
Coding GuidelinesGuidance
When coding an adverse effect of a drug that has been correctly prescribed and properly administered, assign the appropriate code for the nature of the adverse effect followed by the appropriate code for the adverse effect of the drug.
The appropriate 7th character is to be added to each code from block Poisoning by, adverse effect of and underdosing of diuretics and other and unspecified drugs, medicaments and biological substances (T50). Use the following options for the applicable episode of care:
- A - initial encounter
- D - subsequent encounter
- S - sequela
Source: ICD-10-CM Official Guidelines for Coding and Reporting, FY 2026, published by CMS and the National Center for Health Statistics.
Clinical ClassificationClinical
AHRQ’s CCSR groups this code into broader clinical categories.
Clinical InformationClinical
Smallpox Vaccine
a vaccinia virus vaccine used for immunization against smallpox. it is now recommended only for laboratory workers exposed to smallpox virus. certain countries continue to vaccinate those in the military service. rare complications most often associated with older generation smallpox vaccines include vaccinia, secondary bacterial infections, and encephalomyelitis. because monkeypox virus and smallpox virus are both orthopoxvirus and are closely related smallpox vaccines provide effective cross-protection against mpox, monkeypox (https://www.cdc.gov/poxvirus/monkeypox/clinicians/smallpox-vaccine.html).
Table of Drugs and ChemicalsClinical
Patient EducationClinical
Vaccine Safety
Vaccines play an important role in keeping us healthy. They protect us from serious and sometimes deadly diseases. Vaccines are injections (shots), liquids, pills, or nasal sprays that you take to teach your body's immune system to recognize and defend against harmful germs. The germs could be viruses or bacteria.
The full article covers:
- What are vaccines?
- Do vaccines cause side effects?
- How are vaccines tested for safety?
Read the full article at MedlinePlus
Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.
Convert T50.B15A to ICD-9-CMHistory
The closest ICD-9-CM equivalents under the General Equivalence Mappings.
Code HistoryHistory
Questions About T50.B15AOverview
Is T50.B15A (Adverse effect of smallpox vaccines) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report adverse effect of smallpox vaccines, initial encounter on HIPAA-covered claims from October 1, 2025 through September 30, 2026.
What does the 7th character A in T50.B15A mean?
The final character A marks the initial encounter: use it while the patient is receiving active treatment for adverse effect of smallpox vaccines, such as an emergency visit or first evaluation.
What MS-DRG does T50.B15A group to?
On inpatient claims, adverse effect of smallpox vaccines, initial encounter maps to MS-DRG 917, 918, with relative weights from 0.8571 to 1.5684 depending on complications. Higher weights mean higher Medicare reimbursement.
Can T50.B15A be a principal diagnosis?
No. The Medicare Code Editor rejects this code as a principal diagnosis because adverse effect of smallpox vaccines, initial encounter describes a circumstance that influences health status rather than a current illness. Report it as a secondary diagnosis.
What is the ICD-9 equivalent of T50.B15A?
Under the General Equivalence Mappings, adverse effect of smallpox vaccines, initial encounter converts to ICD-9-CM 995.29 (adv eff med/biol NEC/NOS) and E949.0 (adv eff smallpox vaccine). The mapping is approximate, so confirm the match fits the documentation.
Footnotes
[1] Not chronic - A diagnosis code that does not fit the criteria for chronic condition (duration, ongoing medical treatment, and limitations) is considered not chronic. Some codes designated as not chronic are acute conditions. Other diagnosis codes that indicate a possible chronic condition, but for which the duration of the illness is not specified in the code description (i.e., we do not know the condition has lasted 12 months or longer) also are considered not chronic.
