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How to Compare AI Flu Forecasts With CDC Surveillance and Local Hospital Data

By Freedom251 Team · · 8 min read

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Compare a flu forecast only with an observation that measures the same outcome, place, and time period. For CDC’s current FluSight hospital target, that means comparing a forecast of weekly influenza hospital admissions with the matching NHSN admissions data—not with outpatient flu-like illness, positive-test share, a FluSurv-NET rate, or a hospital’s current census. Then account for forecast lead time, data revisions, and uncertainty.

Start by identifying what the forecast predicts

“Flu activity” can refer to several different measures. CDC surveillance describes where, when, and what influenza viruses are circulating; it does not directly count every influenza illness. Forecasting estimates future activity to support planning. As CDC explains in U.S. Influenza Surveillance: Purpose and Methods (2025) and About Flu Forecasting (2024), these are related but distinct jobs.

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FluSight’s hospital forecast target is weekly influenza hospital admissions reported through the National Healthcare Safety Network (NHSN). CDC’s 2025–2026 evaluation covered the current week and up to three weeks ahead for the United States, states, Puerto Rico, and Washington, D.C. Since the 2021–2022 season, CDC has based its flu hospitalization forecasts on NHSN data.

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For NHSN, a new influenza admission is an inpatient admission with a positive flu test at admission or within the preceding 14 days, according to CDC’s U.S. Influenza Surveillance: Purpose and Methods (2025). It is not the same measure as the number of patients currently in beds, the number of people seeking outpatient care, or all infections in a community.

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Choose the observation that matches the target

Observation What it measures Geography and appropriate use What not to treat it as
NHSN hospital admissions Weekly new influenza hospital admissions under the NHSN definition FluView Interactive describes national and HHS-region series; state and jurisdiction target data are used in FluSight evaluation. Match a national forecast to the national series, or a state forecast to that state’s target. Not outpatient ILI, test positivity, a hospitalization rate, or a single hospital’s census. Facility-level NHSN submissions do not mean every facility’s data are publicly exposed.
FluSurv-NET hospitalizations Laboratory-confirmed influenza-associated hospitalizations among residents of defined surveillance areas, expressed as rates using the area population A population-based lens for hospitalization rates and patient characteristics in its covered areas. CDC’s FluSurv-NET methods page describes coverage of more than 90 counties or county equivalents in 14 states, more than 34 million people, an estimated 10% of the U.S. population, and year-round collection beginning in 2025–2026. Not an NHSN admission count or a complete national census. CDC cautions that FluSurv-NET results may not generalize to the whole country.
Outpatient influenza-like illness (ILI) Visits meeting an influenza-like illness definition; ILI can be caused by viruses other than influenza FluView Interactive offers national, regional, and select-state outpatient ILI data. Use it to compare a forecast for ILI or to understand outpatient activity, not to score a hospital-admission forecast. Not laboratory-confirmed influenza admissions or a direct count of flu infections.
Laboratory data Influenza testing results, including measures such as the share of tests positive FluView Interactive provides national, regional, and select-state laboratory data. These can help describe viral activity when the forecast predicts a matching laboratory measure. Not interchangeable with admission counts, ILI visits, or population hospitalization rates.
Local hospital or health-system data Whatever the institution records: for example, new admissions, current inpatient census, or positive tests among its patients Potentially useful for local operations if the facility or system has lawful access and can define the catchment, event, and time period. Compare only with a forecast built for a genuinely corresponding population or facility. Not automatically equivalent to a state or national NHSN series; local testing, transfers, referral patterns, and catchment can change what the count represents.

CDC’s FluView Interactive description presents these signals as complementary views of influenza activity, not different labels for a single outcome. Its public description does not establish a universal hospital-by-hospital NHSN dataset or a standard route for obtaining local facility data.

Align geography, dates, and forecast lead time

A forecast and observation can look mismatched even when both are accurate descriptions of their own data. Before comparing them, write down the place, target week, and forecast issue date. Keep the lead time attached to the result: a one-week-ahead prediction is not the same test as a three-week-ahead prediction.

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  • Geography: Pair national with national, state with the same state, and a local forecast only with an observation whose population or facility definition matches it. An HHS-region series, FluSurv-NET catchment, and hospital system are different geographic units.
  • Event week: Most influenza surveillance uses Sunday–Saturday weeks. Long-term-care facility data are an exception and use Monday–Sunday weeks. Do not shift one series to fit another without documenting the change.
  • Forecast vintage: Preserve the date the forecast was issued as well as the target week. The lead is determined by how far ahead the forecast was made, not by when someone later downloads it.
  • Observation vintage: Record the download date and whether counts were preliminary or final. Recent surveillance data can arrive with delay and be revised, so a forecast should be judged against a clearly identified data vintage.

A chart caption should state the season, surveillance week, geography, target definition, forecast issue date or lead, and observation download date. Without these labels, readers cannot tell whether an apparent miss is a forecasting error or a mismatch in the comparison.

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Compare uncertainty as well as the central estimate

A forecast’s median or central estimate is only one part of its prediction. If intervals are available, show them alongside the observation and assess whether they are both informative and calibrated: an interval that is very wide may contain the outcome often but offer little planning value, while a narrow interval can be useful only if it captures outcomes at the expected rate.

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CDC’s 2025–2026 FluSight evaluation primarily used relative weighted interval score (relative WIS). A relative WIS below 1 indicates better performance than the carry-forward baseline; lower is better relative to that baseline. CDC also reported prediction-interval coverage—the share of observed outcomes that fell inside forecast intervals. A model’s rank alone omits how it compared with the baseline and whether its uncertainty ranges were reliable.

Use the published baseline when evaluating FluSight results. For a local forecast without a directly comparable published baseline, define a simple local naïve baseline in advance—for example, a documented rule that carries forward the latest observed value—and apply it consistently. Do not choose a baseline after seeing which one makes a model look strongest.

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What CDC’s 2025–2026 evaluation shows—and does not show

In CDC’s FluSight 2025–2026 Evaluation (published September 30, 2026), the FluSight ensemble ranked seventh of 39 included models by average relative WIS across jurisdictions, excluding the national forecast. It was one of 12 models that consistently beat the baseline in all jurisdictions; 33 of the 39 evaluated models performed better than the carry-forward baseline.

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That season’s results also show why an overall score is not a guarantee at turning points. For the ensemble’s two-week horizon across jurisdictions, fewer than one-quarter of prediction intervals contained the observed outcome in the week ending December 27, 2025, which coincided with the national and most common jurisdictional peak. Coverage stabilized near 95% starting in February 2026. Those figures describe that horizon and those periods, not whole-season coverage for every forecast or every model.

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CDC notes that ensemble forecasts have often been among the more accurate approaches in infectious-disease forecasting, but may miss rapid changes such as season onset or a peak. Its evaluation’s result belongs to the evaluated FluSight ensemble; it should not be attributed to every model described as “AI.” CDC groups submissions using teams’ method descriptions, with AI/machine learning among several methodology classes that also include statistical, mechanistic, and ensemble approaches.

A practical comparison workflow

  1. Write the target in plain language. Specify admissions, admission rates, ILI visits, test positivity, or another exact measure. Avoid the catch-all label “flu cases.”
  2. Select the matching observation. Use the same outcome definition and geography as the forecast. For a FluSight NHSN admission forecast, use the corresponding NHSN admission target—not FluSurv-NET rates, outpatient ILI, or local census.
  3. Fix the dates and horizon. Record the forecast issue date, event week, lead time, and week convention. Keep Sunday–Saturday and Monday–Sunday series distinct unless a documented transformation is necessary.
  4. Freeze the data vintage. Save the observation download date and note whether values are preliminary or final. If later revisions change the observation, report that revision rather than silently replacing the original comparison.
  5. Plot the distribution against the outcome. Show the central estimate and prediction intervals where supplied, together with the observed value. Evaluate both relative WIS or another suitable score and interval coverage; compare against a baseline.
  6. Break out place and epidemiologic phase. Report jurisdiction-level variation and examine onset, peak, and decline separately. A season-wide average can hide a sharp miss in the period when staffing or bed decisions mattered most.
  7. Judge usefulness for the decision. Ask whether the target and lead time support the intended action, such as bed planning, staffing, antiviral supply, or public messaging. A forecast can be directionally useful without being a reliable facility-level prediction.

How to handle a local hospital comparison

First determine whether the local number is a count of new admissions during a week, the number of current inpatients on a particular day, or a count of positive tests. These answer different questions. Then document the hospital or system’s catchment, transfers, testing practices, and any changes in reporting. A hospital can receive patients from outside its immediate community, and a change in who is tested can affect the observed series even when underlying flu activity has not changed in the same way.

Only compare a facility series with a forecast whose population and target definition align. A state-level forecast can provide context for a hospital, but it is not evidence that the model predicts that hospital’s workload. If no local forecast or matching observation is available, describe the comparison as contextual rather than as a local accuracy test.

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What to include when reporting a result

  • Season, forecast source and model or ensemble name, and target definition.
  • Geography and population or facility represented by the observation.
  • Forecast issue date, target week, and lead time.
  • Observation source, download date, and preliminary or final status.
  • Central estimate and intervals, plus the score, baseline, and interval coverage used.
  • Any mismatch, revision, or local-data limitation that affects interpretation.

CDC’s 2025–2026 page says evaluation of FluSight’s emergency-department visit percentages due to flu is forthcoming. The performance figures above concern hospital admissions and should not be applied to that separate forecast target.

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CDC surveillanceflu forecastingFluSightFluSurv-NETHealth dataNHSN

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