CDC: Classifying Influenza Season Severity

ILI Visits - Week 5 2018 - CDC FluView

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By just about any metric you'd care to apply, last year's H3N2 flu season was unusually severe. Hospitals were slammed, doctor's office visits for ILI matched the levels seen during the 2009 H1N1 pandemic, and the number of flu-related pediatric deaths during a non-pandemic flu season was broken.

A few (of many) blogs on last winter's flu season include:

CDC Statement On Record Pediatric Flu-Related Deaths

MMWR: Reviewing Our High Severity 2017-2018 Flu Season

MMWR: Interim Estimates of 2017–18 Seasonal Influenza Vaccine Effectiveness (VE) — US Feb. 2018

HHS/CDC Press Conference On Active Flu Season

While we see a winter flu season every year, it's timing, longevity, and even the cohort (kids, adults, elderly) most affected can vary from year-to-year.   H3N2 seasons tend to be more severe than H1N1, but H1N1 often hits younger adults harder.

Most of people, I suspect, gauge a flu season's severity by whether they were laid up with the virus, and how bad that was, but luckily the CDC has a more scientific method.

One that has continued to evolve over the years.

Flu seasons prior to 2003-2004 season are difficult to compare because of substantial differences in the way surveillance was conducted before then. Pediatric flu-related deaths, for example, were not a `reportable' event prior to that year.

In 2017, the CDC unveiled a new methodology for classifying the severity of flu seasons, and they've gone back and applied those methods to flu seasons going back to 2003-2004.

Yesterday the CDC published an updated list, adding the last two flu seasons, along with an explainer on how this new system works.

Note: Although much of the included FAQ addresses the first 13 years of data, the chart includes the last 2 seasons as well.

How CDC Classifies Flu Severity


In 2017, CDC adopted and outlined a new methodology for determining flu season severity.  Based on data from past flu seasons, CDC researchers used key flu indicator data to develop intensity thresholds (ITs) to classify the severity of flu seasons. This methodology was published in the American Journal of Epidemiology in October 2017.

Based on the intensity thresholds, CDC researchers classified seasonal severity from 2003-2004 through the 2015-2016 flu seasons. Overall, four seasons were classified as low severity, seven as moderate, two as high, and none as very high.

In addition to assessing the severity of seasonal flu seasons, this methodology can also be used to determine severity during a flu pandemic. 

FAQs

How does CDC track flu activity?

CDC tracks flu activity through key flu indicators, including the percentage of influenza-like illness (ILI) visits to outpatient clinics, the rates of influenza-associated hospitalizations, and the percentage of deaths resulting from pneumonia or flu.

How does CDC define “severity”?

CDC began using a new approach published in 2017 to define flu season severity. CDC first calculates intensity thresholds [ITs] for three key flu indicators using data from past flu seasons. CDC then classifies the severity of individual flu seasons by determining whether the indicator values during the peak of each flu season crossed their respective ITs.

What did researchers do to classify the severity of flu seasons in this study?

Flu severity is assessed using the following data :

1. the percentage of visits to outpatient clinics for ILI,
2. the rates of influenza-associated hospitalizations, and
3. the percentage of deaths resulting from pneumonia or influenza that occurred during each season.

The Moving Epidemic Method is then used to translate the data into standardized intensity thresholds (ITs) for each indicator. The peak value of an indicator is compared to the respective IT for that indicator to arrive at a severity classification.

• Severity is classified as “low” if at least 2 of 3 indicators for the overall population peak below the respective systems’ lowest ITs.
• Severity is classified as “moderate” if at least 2 indicators for the overall population peak between the respective systems’ lowest and mid-range ITs.
• Severity is classified as “high” if at least 2 indicators for the overall population peak between the respective systems’ mid-range and high ITs.
• Severity is classified as “very high” if at least 2 indicators for the overall population peak above the respective systems’ highest ITs.
• Severity was also assessed by age group.

What are intensity thresholds and why are they important?

An intensity threshold is a value developed using data from past flu seasons that helps assess the chance that a system will go above a certain threshold. In this instance, intensity thresholds (ITs) are important because they help researchers classify flu severity based on the level of flu activity at the peak of the season. For this study, ITs were developed for the overall population and further separated into three age groups: children, adults, and older adults. The ITs used in the paper correspond to a 50% (1 in 2), 10% (1 in 10) and 2% (1 in 50) chance of exceedance during an influenza season. These threshold values were used to classify flu severity into low, moderate, high, and very high categories and then applied to U.S. seasons from 2003–04 through the 2015–16, including the 2009 pandemic.

Has the method used to define “severity” changed?

Before this methodology was adopted in 2017, CDC used various criteria to subjectively determine severity, including visits to outpatient clinics, the number of flu-positive respiratory specimens, hospitalizations, and deaths. By contrast, intensity thresholds give researchers the ability to determine flu season severity systematically.

What were the results of applying the new methodology retroactively to 2003-2004?

Of the 13 influenza seasons overall, four were classified as low severity, seven as moderate severity, and two as high severity (2003–04 and 2014–15). No seasons were classified as very high severity.

Seasonal severity also varied by age group. Older adults (65 years and older) experienced the greatest number (3) of seasons that were classified as high severity (2003–04, 2012–13, and 2014–15) while children (0-17 years) were the only age group to experience two seasons that were classified as very high severity (2003–04 and the 2009 pandemic). Adults (18-64 years) did not experience any seasons that were classified as high or very high severity.

Why is it important to assess flu season severity?

By measuring the severity of influenza seasons, CDC can use key flu indicator data to guide public health actions, such as targeting prevention and treatment messages to appropriate audiences and make recommendations designed to prevent flu illnesses and death. See systematic assessment