Colorado Flu Report

Back to influenza data

Reporting through the week of May 26, 2018.

All data are provisional and may change as more reports are received.


Surveillance for the 2017-2018 influenza season officially began on Oct. 1, 2017, and will run through May 26, 2018. The Colorado Flu Report is published weekly to provide a concise and up-to-date summary of influenza activity in Colorado. Current surveillance activities include:

  • Reporting of hospitalizations due to influenza.
  • Reporting of influenza-like illness (ILI) visits by selected sites.
  • Reporting of influenza testing activity by sentinel hospital labs.
  • Monitoring circulating influenza viruses through molecular typing, reporting outbreaks of influenza in long-term care facilities (LTCF).
  • Reporting of pediatric deaths due to influenza.

Case reporting

As of Oct. 1, all hospitalized flu cases with any positive flu test will be considered confirmed.  

Synopsis for the Week Ending May 26th:

  • During the week ending May 26, 2018, 1 additional hospitalized influenza case was reported. The total number of hospitalizations since the beginning of the 2017-18 season is now 4,650. This is higher than any end of season cumulative count of influenza-associated hospitalizations in Colorado.
  • Influenza-like illness reported by Kaiser Permanente (which includes providers from the North Central, Northeast, Northwest, South and South Central regions of the state) decreased from 0.39% to 0.28%. Influenza-like illness reported by Primary Care Partners (Northwest region) increased from 0.63% to 0.64%.
  • Sentinel hospital labs (24 of 24 reporting) tested 411 specimens and 2 (0.5%) were positive for influenza.
  • Since the week ending February 24, 2018, influenza B has predominated over influenza A in individuals with influenza-associated hospitalizations.
  • There have been a total of 183 outbreaks associated with influenza for the 2017-18 influenza season. This is the highest number of influenza-associated outbreaks recorded in Colorado.  
  • Mortality due to pneumonia and influenza is below the epidemic threshold and decreased from 5.5% to 4.9%. This is below the national level of 5.8%.
  • One novel influenza A variant (H1N2v) was detected during the 2017-18 influenza season. An individual hospitalized with influenza in the San Luis Valley was found to be infected with a novel influenza A virus, H1N2v. This individual had exposure to swine at an agricultural event in the week preceding illness onset.
  • One pediatric death associated with influenza was reported for the 2017-18 influenza season. This was associated with influenza A (not subtyped) and had an onset of the week ending December 30, 2017.


Due to reporting delays, the numbers of hospitalizations by week are recalculated each week.

Figure 1. Due to delays in reporting numbers are incomplete, especially for the most recent week

Figure 2. This map displays crude rates of reported influenza-associated hospitalizations, by county. Due to delays in reporting, rates on this map may
not include data from the most recent week. N= 4,675

To see county-specific data, click here.

Table 1. Influenza-Associated Hospitalizations by Age Group, Colorado| October 1, 2017 - May 26, 2018




CO Population

Rate per 100,000

<6 mo





6-23 mo





2-4 yr





5-18 yr





19-24 yr





25-49 yr





50-64 yr





65+ yr





Virologic surveillance

Table 2. Virologic Surveillance, Colorado and National Data, 2017-2018


 CO Week 20

CO Cumulative Data

National Cumulative Data*

(May 20, 2018- May 26, 2018)

(October 1, 2017- May 26, 2018)

(October 1, 2017- May 19, 2018)

Total Positive Specimens




Influenza A

1 (100%)

 3,286 (71%)

38,198 (71%)


0 (0%)

124 (4%)

5,670 (15%)


0 (0%)

1 (<1%)

4 (<1%)


0 (0%)

1003 (30%)

31,908 (84%)

Subtyping not performed

1 (100%)

2,158 (66%)

620 (2%)

Influenza B

0 (0%)

1,363 (29%)

15,383 (29%)

Yamagata lineage

0 (0%)

123 (9%)

10,498 (68%)

Victoria lineage

0 (0%)

1 (<1%)

1,326 (9%)

Lineage not performed

0 (0%)

1,240 (91%)

1,326 (9%)

A/B Not Distinguished

0 (0%)

1 (<1%)


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

Table 3 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 3 and Figure 4 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 from Kaiser Permanente Colorado (which includes providers from the North Central, Northeast, Northwest, South and South Central regions of the state) and Primary Care Partners, P.C. in the Northwest region. Baseline levels are calculated using an average of non-influenza weeks for the past three seasons 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 24 sentinel clinical laboratories across the state of Colorado. This includes the number of influenza tests performed Sunday through Saturday of each week. Figure 5 shows the number of specimens positive by virus type and the overall percent positivity of influenza testing across all sentinel laboratories.

Rate of influenza-associated hospitalizations by season
The rates of influenza-associated hospitalizations by season is Figure 6.
​For seasons 2013-14 and 2014-2015,​ ​the case definition did not include positive rapid influenza tests until influenza was widely circulating. ​For seasons 2016-17 and 2017-18, ​the ​case​ ​definition includes all positive influenza tests, including rapid​ tests​​,​ throughout the entire season.

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

Pneumonia and Influenza (P&I) Mortality Surveillance

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 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 2013-14 through 2017-18. More information regarding P & I data can be found: 

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