The emergency entrance of a brick city hospital on a clear autumn morning, with an empty ambulance bay

In the Chicago Health Department’s ER Record, Flu Was 8.5 Percent of Visits at Last Season’s Peak, the Most in Nine Seasons, and 22 Percent for School-Age Children

Flu accounted for 8.5 percent of Chicago emergency-department visits at last season's peak, the most in nine seasons, and set the same record in every age group, 22.3 percent for school-age children. RSV usually peaks first among the youngest; COVID-19 has stayed under 3 percent since 2022.

Flu accounted for 8.5 percent of emergency-department visits in the Chicago Department of Public Health’s file in the week ending Dec. 27, 2025, the largest weekly share in the nine seasons the file covers on one consistent basis. Every age group set the same record that winter, from 22.3 percent of visits by children 5 to 17 and 21.3 percent by children under 5 to 5.5 percent by adults 65 and older. COVID-19, which reached 18.0 percent of visits in the week ending Jan. 1, 2022, has peaked below 3 percent in every season since.

CDPH emergency-department and admissions file (7ce8-bpr6), seasons 2017–18 through 2025–26 · read October 1, 2026 · weekly shares are the file’s own; peaks and comparisons are ours

What the file measures

Illinois hospitals report their emergency-department visits to the state’s syndromic surveillance system, and the city health department counts the visits whose discharge diagnoses match federal definitions of flu, RSV, COVID-19 and acute respiratory illness, then divides by all emergency-department visits, as its 2024–25 season summary describes. The file gives those shares weekly, for all ages and by age group, and from the 2019–20 season the same shares of inpatient admissions. A share rises when a virus sends more people to the emergency department and also when fewer people come in for anything else, so it measures a virus’s weight in the emergency room, not a count of the sick. Neither the file nor the department’s summary says whether the visits are by Chicago residents or to hospitals in Chicago.

The file goes back to 2015–16, but its own description says the seasons before 2017–18 come from a different system and cautions against comparing the two periods. In those two seasons the flu and RSV shares never rise above two-hundredths of a percent, so this page starts with 2017–18. The description also says the numbers may differ from other public sources, and they do: the department’s 2024–25 summary put that season’s flu peak at 7 percent of visits and the previous season’s at 5.3, while the file now shows 6.4 and 4.6 for the same seasons. This page uses the file throughout, so every season is measured the same way.

Was last season’s flu peak unusual?

Flu’s largest weekly share of emergency-department visits
Age groupFlu’s peak share, 2025–26Highest share, 2017–18 to 2024–25
All ages8.5%7.9%
Children under 521.3%16.2%
Children 5 to 1722.3%21.1%
Adults 18 to 446.2%5.9%
Adults 45 to 644.3%4.1%
Adults 65 and older5.5%4.8%

Yes, by this record as it stood with its latest week ending Sept. 19, 2026. The 2025–26 peak was the highest of the nine seasons in every age group. The next highest for all ages was 7.9 percent, in the week ending Nov. 26, 2022. Last season’s peak came in the same week for every group but the oldest, whose share peaked a week later, in the week ending Jan. 3, 2026. The laboratory record peaked the same week: our page on flu’s arrival found flu was 24.7 percent of reported tests in the week ending Dec. 27, 2025. Our reading: two separate records, one counting tests and one counting emergency visits, put the top of last winter’s flu season in the same week, which makes the timing more than a quirk of either file.

Which virus peaks first?

OctNovDecJanFebMarchAprilMayRSVRSV, under 5fluflu, all agesCOVID-19COVID-19, all ages2017–182017–18: Flu peaked at 4.6 percent of visits in the week ending Feb. 3, 20182017–18: RSV among children under 5 peaked at 4.8 percent of visits in the week ending Jan. 6, 20182018–192018–19: Flu peaked at 1.7 percent of visits in the week ending Feb. 2, 20192018–19: RSV among children under 5 peaked at 6.5 percent of visits in the week ending Dec. 29, 20182019–202019–20: COVID-19 peaked at 9.8 percent of visits in the week ending May 9, 20202019–20: Flu peaked at 5.0 percent of visits in the week ending Feb. 8, 20202019–20: RSV among children under 5 peaked at 6.4 percent of visits in the week ending Jan. 11, 20202020–212020–21: COVID-19 peaked at 7.9 percent of visits in the week ending Nov. 28, 20202020–21: Flu peaked at 0.2 percent of visits in the week ending Dec. 5, 2020RSV peak: Sept. 20212021–222021–22: COVID-19 peaked at 18.0 percent of visits in the week ending Jan. 1, 20222021–22: Flu peaked at 1.2 percent of visits in the week ending May 7, 2022RSV peak: Oct. 20222022–232022–23: COVID-19 peaked at 2.8 percent of visits in the week ending Dec. 24, 20222022–23: Flu peaked at 7.9 percent of visits in the week ending Nov. 26, 20222022–23: RSV among children under 5 peaked at 9.8 percent of visits in the week ending Oct. 22, 20222023–242023–24: COVID-19 peaked at 2.6 percent of visits in the week ending Dec. 30, 20232023–24: Flu peaked at 4.6 percent of visits in the week ending Dec. 30, 20232023–24: RSV among children under 5 peaked at 7.7 percent of visits in the week ending Dec. 2, 20232024–252024–25: COVID-19 peaked at 1.2 percent of visits in the week ending Jan. 11, 20252024–25: Flu peaked at 6.4 percent of visits in the week ending Jan. 25, 20252024–25: RSV among children under 5 peaked at 9.1 percent of visits in the week ending Dec. 21, 20242025–262025–26: COVID-19 peaked at 1.7 percent of visits in the week ending Jan. 3, 20262025–26: Flu peaked at 8.5 percent of visits in the week ending Dec. 27, 20252025–26: RSV among children under 5 peaked at 6.5 percent of visits in the week ending Feb. 14, 2026
Nine seasons in the emergency-department record One row per season, October to May. Each mark is the week a virus took its largest share of visits: RSV among children under 5, flu and COVID-19 among all ages. COVID-19 has no mark before 2019–20, and a note in the row gives any peak that fell outside October to May.
Each virus’s largest weekly share of emergency-department visits, and the week it came
SeasonRSV, children under 5Flu, all agesCOVID-19, all ages
2017–184.8% (Jan. 6, 2018)4.6% (Feb. 3, 2018)none
2018–196.5% (Dec. 29, 2018)1.7% (Feb. 2, 2019)none
2019–206.4% (Jan. 11, 2020)5.0% (Feb. 8, 2020)9.8% (May 9, 2020)
2020–213.7% (Sept. 11, 2021)0.2% (Dec. 5, 2020)7.9% (Nov. 28, 2020)
2021–224.9% (Oct. 1, 2022)1.2% (May 7, 2022)18.0% (Jan. 1, 2022)
2022–239.8% (Oct. 22, 2022)7.9% (Nov. 26, 2022)2.8% (Dec. 24, 2022)
2023–247.7% (Dec. 2, 2023)4.6% (Dec. 30, 2023)2.6% (Dec. 30, 2023)
2024–259.1% (Dec. 21, 2024)6.4% (Jan. 25, 2025)1.2% (Jan. 11, 2025)
2025–266.5% (Feb. 14, 2026)8.5% (Dec. 27, 2025)1.7% (Jan. 3, 2026)

RSV among the youngest children, in most seasons. In six of the seven seasons other than 2020–21 and 2021–22, RSV’s share of visits by children under 5 peaked four or five weeks before flu’s share of all visits. Last season was the exception: flu peaked in the week ending Dec. 27, 2025, and RSV among young children not until the week ending Feb. 14, 2026. In 2020–21 and 2021–22 RSV peaked at the end of summer, in September 2021 and at the start of October 2022. RSV’s highest share in the file came in the week ending Oct. 22, 2022, at 9.8 percent of visits by children under 5.

What happened to COVID-19’s share?

It fell and stayed low. Its highest weekly share in the file is 18.0 percent of all emergency-department visits, in the week ending Jan. 1, 2022. Since then COVID-19 has peaked at 2.8, 2.6, 1.2 and 1.7 percent, season by season. In inpatient admissions its peak share went from 22.4 percent in the week ending Jan. 1, 2022, to 1.7 percent last season.

How much of the emergency room does the season fill?

A fifth or more at the winter peak in six of the nine seasons. Broad acute respiratory illness, a category that takes in colds, pneumonia and the three viruses, has accounted for between 15.2 and 27.5 percent of visits at each season’s peak, and 21.3 percent last season. Flu’s share of inpatient admissions peaked at 4.8 percent last season, the highest of the seven seasons with admissions in the file.

Where does the new season start?

Low. In the file’s latest week, ending Sept. 19, 2026, flu was 0.1 percent of emergency-department visits, COVID-19 0.2 percent and RSV under a tenth of a percent. The season the file counts begins with the first week of October. The health department updates its respiratory illness dashboard every Friday at 1 p.m., and its flu page says “September and October are the best times to get vaccinated.” Our page on recorded flu shots lists the city’s free clinics, and our page on flu’s arrival gives the weeks flu reached 10 percent of laboratory tests in each of 16 seasons.

Known limits

Every figure is a share of visits or admissions, not a count of people. A person can visit more than once, and a diagnosis-based count misses patients who were never tested or coded. The file’s figures are provisional and can change. The age groups are the file’s own. The 2020–21 and 2021–22 seasons are shown but set aside when comparing timing: flu nearly vanished in 2020–21 and peaked in May in 2021–22, and RSV peaked at the end of summer in both. The two seasons before 2017–18 are left out, as the file’s description advises. Nothing here says why any season was heavier than another.

  • Source CDPH, Inpatient, Emergency Department, and Outpatient Visits for Respiratory Illnesses (7ce8-bpr6), read in full October 1, 2026 (file updated Sept. 25, 2026): the weekly share of visits and admissions whose discharge diagnoses match the CDC’s definitions, which the file stores as fractions and this page prints as percents. Seasons run from week 40 of one year to week 39 of the next, as the file defines them.
  • Peaks A season’s peak is its largest weekly share; a week is named by the Saturday it ends on. Timing compares RSV among children under 5, the age group in which RSV’s share is largest, with flu among all ages.
  • Other sources CDPH’s 2024–25 season summary and the department’s respiratory and flu pages, read the same day; each sentence quoted or summarized was checked on the page.

Computed by KCM Desk from records read October 1, 2026. Human-guided and edited — about this desk. If you spot an error, corrections come first.

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