Colorado Flu Report
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2018-2019 Influenza Season
All data are provisional and may change as more reports are received.


Surveillance for the 2018-2019 influenza season officially began on September 30, 2018, and will run through May 18, 2019. The Colorado Flu Report is published weekly on Tuesdays to provide a concise and up-to-date summary of influenza activity in Colorado. Current surveillance activities include monitoring the following:

  • Hospitalizations due to influenza
  • Influenza-like illness (ILI) patient visits at outpatient clinics and emergency departments
  • Positivity trends of influenza testing activity by sentinel hospital labs
  • Outbreaks of influenza in long-term care facilities (LTCF)
  • Influenza-associated pediatric deaths
  • Pneumonia and influenza mortality

Summary for the Week Ending May 18, 2019

  • The geographic spread of influenza activity in the state of Colorado is sporadic.
  • 14 additional influenza-associated hospitalizations were reported during the week ending May 18, 2019. The total number of hospitalizations since the beginning of the 2018-19 season is now 3,825.  
  • The highest hospitalization rates are among 0-4 year olds and individuals 65 and older.
  • The predominating strain during the 2018-19 season has been influenza A, H1N1, but as of the week ending February 16, H3N2 viruses are predominating over H1N1. Influenza B viruses are also co-circulating.
  • Influenza vaccine effectiveness is generally lower against influenza A, H3N2 viruses than against A, H1N1 or B viruses.
  • One genetic clade of A, H3N2 viruses, the 3C.3a clade, has recently become predominant among circulating A, H3N2 viruses. This virus is circulating in Colorado.
  • According to laboratory testing, these viruses are antigenically distinct from the A, H3N2 virus included in this season’s vaccine.
  • CDC recommends antiviral medications for treatment of influenza, regardless of a patient’s influenza vaccination status
  • Early empiric treatment with influenza antiviral medications is recommended for hospitalized and high-risk patients, especially those 65 years and older. Antiviral treatment should be started as soon as possible after illness onset and should not wait for laboratory confirmation.
  •  More information about antiviral guidance can be found at 
  • Influenza-like illness patient visits reported by outpatient clinics has increased from 3.8% to 3.9%. This is below the baseline level of 5.05%. This includes data from Kaiser Permanente and Primary Care Partners clinics located in the North Central, Northeast, Northwest, South and South Central regions of the state.
  • Syndromic surveillance of influenza-like illness patient visits in emergency departments in the Denver-metro area is at 1.0%, which is the same as the previous week. This is below the seasonal baseline level of 1.78%.
  • Sentinel hospital labs (20 of 23 reporting) tested 734 specimens and 22 (3.0%) were positive for influenza.
  • There have been 84 outbreaks associated with influenza reported for the 2018-19 influenza season.
  • Mortality due to pneumonia and influenza in Colorado for the week ending April 27th was at 6.3%. This is higher the national level of 6.1%.
  • Three pediatric deaths associated with influenza have been reported for the 2018-19 influenza season. Two cases were associated with influenza A, H1N1; and one case was associated with influenza A, H3N2. This occurred during the weeks ending January 12, 2019, February 16, 2019, and March 16, 2019.

Flu Hospitalizations

Due to delays in reporting numbers are incomplete, especially for the most recent week.

As of September 30, 2018, all hospitalized patients with a positive influenza test are considered a confirmed case.

County-specific flu hospitalization data

For seasons 2014-15 and 2015-16, the case definition did not include positive rapid influenza tests until influenza was widely circulating. For seasons 2016-17, 2017-18, and 2018-19 the case definition includes all positive influenza tests, including rapid tests, throughout the entire season.

Table 1.

Influenza-Associated Hospitalizations by Age Group





Colorado Population

Rate per 100,000 People

0-4 years





5-17 years





18-49 years





50-64 years





65+ years





Table 2.

Virologic Surveillance, Colorado and National Data



 CO Week 20

CO Cumulative Data

National Cumulative Data*


(May, 12 2019- May 18, 2019)

(September 30, 2018- May  18, 2019)

(September 30, 2018- May 11, 2019)

Total Positive Specimens




Influenza A

8 (57.1%)


39,908 (96.3%)


0 (0%)

741 (20.1%)

21,855 (54.8%)


5 (62.5%)

649 (17.6%)

16,490 (41.3%)

Subtyping not performed

3 (37.5%)

2,297 (62.3%)

1,563 (3.9%)

Influenza B

6 (42.9%)

136 (3.6%)

1,544 (3.7%)

Yamagata lineage

0 (0%)

1 (0.7%)

381 (24.7%)

Victoria lineage

0 (0%)

1 (0.7%)

617 (40.0%)

Lineage not performed

6 (100.0%)

135 (98.5%)

546 (35.4%)

A/B Not Distinguished

0 (0%)

2 (0.05%)


*Due to reporting delays, national cumulative data shown is 1 week delayed from the state-level data.

Table 1 reflects both PCR and rapid antigen testing performed, by current week and cumulative totals for Colorado and the United States. The Colorado weekly and cumulative data includes virologic information on influenza specimens from hospitalized patients tested by hospital laboratories and the Colorado Department of Public Health and Environment laboratory. The national cumulative

data includes virology information reported to the CDC from public health laboratories located in all 50 states, Puerto Rico and the District of Columbia.

Influenza-like Illness (ILI) Surveillance

The percentages shown in Figure 5 are based on the number of primary care office visits assigned a diagnosis consistent with influenza-like illness, divided by the total number of clinic visits for the week. This data comes from Kaiser Permanente and Primary Care Partners. This includes clinics in the North Central, Northeast, Northwest, South and South Central regions of the state.

Percentages shown in Figure 6 are based on the number of emergency department visits of patients with flu influenza like
symptoms or patients with diagnosis codes consistent with influenza-like illness, divided by the total number of visits
for the week in the emergency departments. This information is provided through syndromic surveillance, an
integrated electronic health information system which collects information from hospital emergency departments and
standalone emergency departments in the following counties: Adams, Arapahoe, Boulder, Denver, Douglas, and Jefferson.

Baseline levels for outpatient and emergency department visits are calculated using an average of non-influenza weeks for
the past season plus two standard deviations. A non-influenza week is defined as periods of two or more consecutive weeks
in which each week accounted for less than 2% of the season’s total number of influenza-associated hospitalizations.

Positivity Data from Sentinel Laboratories

Influenza positivity data is collected from 23 sentinel clinical laboratories across the state of Colorado. This includes the number of influenza tests performed Sunday through Saturday of each week. Figure 7 shows the number of specimens positive by virus type and the overall percent positivity of influenza testing across all sentinel laboratories.

Pneumonia and Influenza Mortality

Pneumonia and influenza mortality data comes from the National Center for Health Statistics mortality surveillance. This information comes from death certificate data provided by state and vital statistics offices. Data is released two weeks after the week of death. The seasonal baseline is calculated by using data applied from the previous 5 years. The epidemic threshold represents the point at

which the observed proportion of deaths attributed to pneumonia or influenza was significantly higher than what is expected for that time of year in the absence of substantial influenza-related mortality. Figure 8 shows P & I mortality for Colorado compared to the national level. Figure 9 shows the trends of P & I data for influenza seasons 2014-15 through 2018-19. More information regarding P & I data can be found: 

Previous Seasons’ Flu Reports

To view the weekly CDC flu surveillance reports, go to: