Data Methods

Learn more about the data that informs AHEAD.

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HIV incidence indicator icon HIV incidence

HIV incidence measures the number of all new infections, whether diagnosed or undiagnosed, in a given year. Estimates of annual HIV infections (incidence) are based on NHSS data for persons aged ≥ 13 years. Incidence estimates are useful for planning and for allocating of funds, as well as evaluating the impact of prevention programs. For example, these estimates can be used to assess changes in characteristics of persons most at risk for acquiring HIV infection.

Formula for the Incidence indicator


CD4 Depletion Model

HIV incidence estimates were produced using the result of the first CD4+ T-lymphocyte [CD4] test after HIV diagnosis and an estimation method based on a CD4 depletion model (referred to hereafter as the “CD4 model”). The first CD4 test results after HIV diagnosis are routinely collected by all jurisdictions as part of the National HIV Surveillance System (NHSS).

CD4 cells, a type of white blood cell, aid in fighting infections. HIV targets CD4 cells: without treatment, HIV reduces the number of CD4 cells in a person’s body over time. A person’s CD4 cell count is used to determine the stage of disease. Assuming that no treatment has been received, the CD4 cell count can be used to estimate the time from infection to the date of the CD4 test. The CD4 model was applied to NHSS data to estimate the distribution of delay from infection to diagnosis and then to produce estimates of HIV incidence and prevalence among adults and adolescents.

Reporting of the first CD4 test result after diagnosis of HIV infection is a required data element on the HIV case report form; however, completeness of reporting varies among states and local jurisdictions. Estimates should be interpreted with caution for EHE areas (including Phase 1 EHE states, EHE jurisdictions, or states that contain EHE jurisdictions) that do not have laws requiring complete reporting of laboratory data or have incomplete reporting.

  • EHE areas without laws requiring complete reporting:
    • Idaho
    • New Jersey
  • EHE areas with incomplete reporting:
    • Kansas
    • Kentucky
    • Pennsylvania (excluding Philadelphia)
    • Vermont
    • Puerto Rico

HIV incidence among adults and adolescents were obtained via the following:

  1. Among all HIV diagnoses reported by states to the CDC, the date of HIV infection was estimated for each person with a CD4 test by using a CD4 depletion model.
  2. The distribution of delay (from HIV infection to diagnosis) was used to estimate the annual number of HIV infections (new infections in a given year), which includes persons with diagnosed infections and persons with undiagnosed infection.

To reflect model uncertainty, all estimates were rounded to the nearest 100 for estimates of more than 1,000 and to the nearest 10 for estimates of less than 1,000.

The relative standard error (RSE) is a measure that shows how large the standard error is, relative to the size of the estimated value. It is calculated by dividing the standard error of an estimated value by the estimated value itself, and then multiplied by 100 and expressed as a percent. Smaller RSEs are indicative of more reliable results, and larger RSEs indicative of less reliable results.

Estimates with a relative standard error (RSE) of ≥30% do not meet the standard of reliability and are represented in the following way:

  • Estimates with an RSE of 30% - 50% are marked with an asterisk (*), indicating that they should be used with caution.
  • Estimates with an RSE>50 percent are not shown, and are replaced with the phrase “Data N/A due to high relative standard error.”

More information can be found at: HIV Surveillance Report Supplemental Report Volume 26, Number 1.

For more details, please refer to the following source:

  1. Centers for Disease Control and Prevention. Terms, Definitions, and Calculations Used in CDC HIV Surveillance Publications. Available at: Terms, Definitions, and Calculations | Surveillance Overview | Statistics Center | HIV/AIDS | CDC (Accessed: May 14, 2021).