How CDC Classifies Flu Severity
In 2017, CDC adopted and outlined a new methodology for classifying flu season severity. Based on data from past flu seasons, CDC researchers used key flu indicator data (e.g. influenza-like illness and pneumonia and influenza death) 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 have classified seasonal severity from 2003-2004 through the 2021-2022 flu seasons (the severity of the 2020-2021 season will not be assessed because of very low levels of flu activity. Overall, five seasons were classified as low severity, 9 as moderate, three 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.
|
Season |
Child |
Adults |
Older Adults |
Overall |
|---|---|---|---|---|
| 2003-04 | Very High | Moderate | High | High |
| 2004-05 | Low | Moderate | Moderate | Moderate |
| 2005-06 | Low | Low | Low | Low |
| 2006-07 | Low | Low | Low | Low |
| 2007-08 | Moderate | Moderate | Moderate | Moderate |
| 2008-09 | Low | Low | Low | Low |
| 2009-10 | Very High | Moderate | Low | Moderate |
| 2010-11 | Moderate | Moderate | Moderate | Moderate |
| 2011-12 | Low | Low | Low | Low |
| 2012-13 | Moderate | Moderate | High | Moderate |
| 2013-14 | Moderate | Moderate | Moderate | Moderate |
| 2014-15 | Moderate | Moderate | High | High |
| 2015-16 | Low | Moderate | Low | Moderate |
| 2016-17 | Moderate | Moderate | Moderate | Moderate |
| 2017-18 | High | High | High | High |
| 2018-19 | Moderate | Moderate | Moderate | Moderate |
| 2019-20 | High | High | Moderate | Moderate |
| 2021-2022 | Low | Low | Low | Low |
*2020-2021 flu season was not estimated because of minimal flu activity.
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 health 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 the three key flu indicators using data from past flu seasons. CDC then classifies the severity of specific 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?
Flu severity is assessed using the following data :
- the percentage of visits to outpatient clinics for ILI,
- the rates of influenza-associated hospitalizations, and
- the percentage of deaths resulting from pneumonia or influenza that occurred during each season. For the 2021-2022 season, we used counts of flu deaths from 2016-2017 to 2021-2022, excluding the 2020-2021 season.
The Moving Epidemic Method is then used to translate the data into standardized intensity thresholds (ITs) for each indicator. Using this method, 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 help assess the chance that an influenza indicator, like influenza-like-illness (ILI) or influenza percent positive, 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. ITs were developed for the overall population and further separated into three age groups: children, adults, and older adults. The ITs used 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 2019–20, including the 2009 pandemic.
Does severity vary by age group?
Seasonal severity can vary by age group. Older adults (65 years and older) experienced the greatest number (4) of seasons that were classified as high severity (2003–04, 2012–13, 2014–15, and 2017-18) while children (0-17 years) were the only age group to experience any season that was classified as very high severity (they experienced two very high seasons, 2003–04 and the 2009 pandemic). Adults (18-64 years) experienced two seasons that were classified as high severity (2017–18 and 2019-20).
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. Intensity thresholds give researchers the ability to determine flu season severity more systematically.
How has the pandemic affected methodology of these severity calculations?
When COVID-19 mitigation measures, such as school closures, lockdowns, and mask wearing, were implemented in March 2020, influenza-like illness (ILI) activity decreased. Because of this, the period during the 2019-2020 flu season that was most impacted by the emergence of the virus that causes COVID-19 was excluded from the threshold calculation and severity assessment for that season. The 2020-21 season was also excluded in its entirety since there was minimal flu activity that year. Additionally, most pneumonia-coded deaths during this timeframe were related to COVID-19. The number of pneumonia and influenza (P&I) combined deaths increased during the pandemic, but this increase is unlikely to reflect flu activity. To avoid bias, counts of NCHS ICD-coded flu deaths were used instead of P&I deaths to calculate mortality thresholds and assess severity.
What were the results of applying the new methodology retroactively to 2003-2004?
Seasonal severity varied by age group. Older adults (65 years and older) experienced the greatest number (4) of seasons that were classified as high severity (2003–04, 2012–13, 2014–15, and 2017-18) 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) experienced two seasons that were classified as high severity (2017–18 and 2019-20).
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.
