Chicago Flu Update
About this report
The Chicago Department of Public Health utilizes various surveillance indicators to monitor influenza activity in Chicago. This includes surveillance for influenza-associated intensive care unit (ICU) hospitalizations, monitoring circulating influenza viruses, as well outpatient and emergency department visits that are due to influenza-like illness (ILI). Influenza surveillance data are typically aggregated by week. This report is updated on Fridays for the previous Saturday through Sunday. On all graphs, the week ending date is displayed. Ending dates are accurate for the 2020-2021 season, but are approximations for all other years. All data presented here are preliminary and may change as more reports are received. Reported percentages for previous seasons represent final, end of season data and may differ from previously published reports.
This will be the last Chicago Flu Update issued for the 2020-2021 influenza season. The next update will be issued in October 2021. The Chicago Department of Public Health will continue to monitor surveillance indicators throughout the summer months. Influenza viruses are detected throughout the year at low levels and can cause disease. Vaccination is the best way to protect against influenza infection and all Chicagoans six months and older are encouraged to get vaccinated annually.
What is the risk?
Currently, the risk of influenza infection is low.
Are severe cases of influenza occurring?
Influenza-associated ICU hospitalizations are reportable in Illinois. All hospitals in Chicago must report cases to CDPH via Illinois National Electronic Disease Surveillance System (I-NEDSS).
For the week of May 16 to May 22, 2021, no influenza-associated ICU hospitalizations were reported (Figure 1).
Since September 27, 2020, one influenza-associated ICU hospitalization has been reported.
Figure 1. Number of influenza-associated ICU hospitalizations reported for Chicago residents, for the current season (2020-2021) and previous season (2019-2020), October-May.
Which influenza strains are circulating?
Data on influenza virus test results are reported by several Chicago hospital laboratories performing influenza RT-PCR. Rapid influenza test results are not included in these reports.
For the week of May 16 to May 22, 2021, 0 of the 398 (<1%) reported specimens that were tested for influenza were positive (Figure 2)
Since September 27, 2020, 5 of the 25,655 (<1%) reported specimens that were tested for influenza have been positive. The cumulative percent of specimens testing positive for influenza is lower than previous seasons for the same time period. The number of reported specimens tested for influenza is 41% compared to the same time period last season (Figure 3).
Figure 2. Percent of specimens testing positive (by RT-PCR) for influenza by subtype as reported by local laboratories serving Chicago hospitals, for the current season (2020-2021) and previous two seasons, October-May.
Figure 3. Cumulative number of specimens tested for influenza and cumulative percent of specimens testing positive (by RT-PCR) as reported by local laboratories serving Chicago hospitals, for the current season (2020-2021), and previous two seasons, October-May.
How much influenza-like illness is occurring?
Several outpatient clinics throughout Chicago participate in CDC’s Influenza-like Illness Surveillance Network (ILINet) by reporting on a weekly basis the total number of outpatient clinic visits, and of those visits, the number with influenza-like illness (ILI). For the week of of May 16 to May 22, 2021, 50 of the 5,691 (<1%) reported outpatient clinic visits were due to influenza-like illness (Figure 4).
In addition to ILINet, ESSENCE is an electronic syndromic surveillance system that utilizes the chief complaints of patients visiting emergency departments to monitor for influenza-like illness. Currently, ESSENCE captures nearly every emergency department visit in the city on a daily basis. For the week of May 16 to May 22, 2021, 171 of the 20,929 (<1%) total emergency department visits were due to influenza-like illness (Figure 5).
Figure 6 represents the percentage of emergency department visits due to influenza-like illness aggregated by Chicago patient zip codes and categorized into activity levels.* For the week of of May 16 to May 22, 2021, 0 of 59 zip codes had moderate to high ILI activity levels (levels 6 to 10).
*ILI Activity Level: The ILI percentage for each zip code for the current week is compared to the mean ILI percentage during the non-influenza months (May 17, 2020-September 26, 2020); level 1 corresponds to an ILI percentage below the mean, level 2 to an ILI percentage less than one standard deviation (SD) above the mean, level 3 to an ILI percentage more than one, but less than two SDs above mean, and so on, with level 10 corresponding to an ILI percentage more than eight SDs above the mean. Variations in ILI activity levels should not be interpreted as an influenza cluster or outbreak as other illnesses can cause similar symptoms (including COVID-19).
Figure 4. Percent of medically-attended outpatient visits attributed to influenza-like illness as reported by ILINet facilities, Chicago, by week for the current season (2020-2021) and previous two seasons, October-May.
Figure 5. Percent of emergency department visits attributed to influenza-like illness for Chicago zip codes based on chief complaint data submitted to ESSENCE, Chicago, by week, for the current season (2020-2021) and previous two seasons, October -May.
Figure 6. Influenza-like Illness (ILI) activity level by patient zip code determined by chief complaint data submitted to ESSENCE, Chicago, for the week of May 16 to May 22, 2021 (Week 20).
Where can I get more information?
The Centers for Disease Control and Prevention’s FluView report provides national updates and trends related to the intensity of influenza activity across the United States, as well as detailed information on antiviral resistance, severity of illness, and other topics. Updates specific to Illinois and Suburban Cook County are also available online. Current and archived issues of the Chicago Flu Update can be found on the CDPH website section Current Flu Situation in Chicago.
Healthcare facilities can report influenza-associate ICU hospitalizations to the Chicago Department of Public Health via the Illinois National Electronic Disease Surveillance System (INEDSS). For more information contact: SyndromicSurveillance@cityofchicago.org