Grading a Flu Season State by State: Minnesota and Utah Try CDC's Severity Method, 2017–18

The 2017–18 U.S. flu season was high-severity: the most outpatient visits for influenza-like illness (ILI) since the 2009–10 pandemic, and the highest rate of flu hospitalizations since adult hospitalizations were added to surveillance in 2005–06. Emergency departments and hospitals were strained, and there were spot shortages of flu antivirals.

Flu activity varies widely by region, and local severity assessments could better guide public health action, health care planning and tailored messages. CDC grades severity nationally — using outpatient ILI visits, flu hospitalization rates, and the share of deaths from pneumonia or influenza — but the method hadn't been tested at the state level.

The pilot

In February 2018, field teams in Minnesota and Utah looked for state indicators and tried grading severity in real time. Indicators had to have data for 2017–18 and at least five earlier seasons, reflect flu seasonality completely and representatively, and be timely.

Using the 2012–13 to 2016–17 seasons, each indicator got intensity thresholds: values with a 50% (IT50), 10% (IT90) and 2% (IT98) chance of being exceeded in 2017–18. The season was low, moderate, high or very high if at least two of three indicators peaked below IT50, between IT50 and IT90, between IT90 and IT98, or above IT98.

The result

Both states' interim grade, in mid-February 2018, was high. Flu hospitalizations, for example, peaked above IT90 in Minnesota and above IT98 in Utah by season's end in May 2018. The end-of-season grades stayed high — matching the nation.

Flu hospitalization indicators and intensity thresholds, Minnesota (thresholds 255, 511 and 778) and Utah (3.66, 5.99 and 8.06), 2012–18 seasons. CDC

What's next

The national method adapted successfully to both states. Utah began piloting weekly severity reports in 2018–19. Other states may find it useful for local messaging, preparedness and response — during ordinary seasons and in a pandemic — and CDC's Influenza Division offers help to jurisdictions that want to try it.

ILI means fever of at least 100°F (37.8°C) with cough and/or sore throat, without another known cause.

Sources

Based on Hughes MM, Doyle JD, McCaffrey K, et al., "Notes from the Field: Assessment of State-Level Influenza Season Severity — Minnesota and Utah, 2017–18 Influenza Season," MMWR volume 68, number 6, Centers for Disease Control and Prevention; a work of the United States government in the public domain.