In eight of the 16 seasons in Chicago’s laboratory records, flu first reached 10 percent of tests in December, and the earliest it got there was the week ending Nov. 12, 2022. Last season flu reached that share in the week ending Dec. 13, 2025. By two weeks before it, 77 percent of the residents who would end the season with a recorded flu shot already had one, and 69 percent of the children.
Chicago Department of Public Health laboratory, intensive care and vaccination files, seasons 2010–11 to 2025–26 · read September 30, 2026The laboratory record is a weekly count of tests for respiratory viruses and of how many were positive, reported to the Chicago Department of Public Health by several hospital laboratories in the city and two commercial laboratories. The reporting is voluntary and the tests include people who live outside Chicago, by the file’s own description. For flu it goes back to the 2010–11 season. What it measures is the share of tests that found flu, which rises and falls with the virus. It does not count people who were sick.
To date each season’s arrival, this page uses one marker: the first week in which at least 10 percent of the reported tests were positive for flu. The line is ours, chosen because 14 of the 16 seasons cross it. The table below also gives the first week at 5 percent and the peak week, so the choice of line can be checked against them. A season in these files runs from early October through the following September, and a week is named here by the Saturday it ends on.
The dates bear on a practical question: whether a flu shot is in place before the season arrives. So this page sets them beside two other records: the weeks Chicago residents were admitted to intensive care with flu, and the weeks the state immunization registry dates their flu shots to.
Flu reached 10 percent of tests in December in eight seasons, and never before Nov. 12
| Season | Tests | First week at 5% | First week at 10% | Peak week | Share at peak |
|---|---|---|---|---|---|
| 2025–26 | 130,134 | Dec. 13 | Dec. 13 | Dec. 27 | 24.7% |
| 2024–25 | 163,519 | Dec. 14 | Dec. 21 | Jan. 25 | 23.4% |
| 2023–24 | 219,530 | Dec. 9 | Dec. 16 | Dec. 30 | 14.5% |
| 2022–23 | 219,543 | Nov. 5 | Nov. 12 | Nov. 26 | 28.7% |
| 2021–22 | 197,858 | April 2 | — | May 7 | 7.6% |
| 2020–21 | 37,447 | — | — | Sept. 4 | 0.4% |
| 2019–20 | 49,954 | Dec. 14 | Dec. 21 | Feb. 8 | 36.9% |
| 2018–19 | 40,845 | Dec. 29 | Jan. 12 | March 30 | 21.3% |
| 2017–18 | 37,418 | Dec. 16 | Dec. 23 | Jan. 27 | 32.8% |
| 2016–17 | 28,184 | Dec. 31 | Jan. 21 | Feb. 25 | 24.6% |
| 2015–16 | 25,892 | Jan. 30 | Feb. 6 | Feb. 20 | 21.0% |
| 2014–15 | 25,179 | Nov. 22 | Nov. 29 | Dec. 27 | 42.6% |
| 2013–14 | 19,480 | Dec. 14 | Dec. 21 | Jan. 4 | 22.3% |
| 2012–13 | 16,320 | Nov. 17 | Dec. 1 | Dec. 29 | 31.4% |
| 2011–12 | 10,251 | Feb. 18 | March 3 | March 17 | 23.5% |
| 2010–11 | 10,311 | Dec. 25 | Dec. 25 | Feb. 5 | 24.0% |
Of the 14 seasons that crossed the line, two did so in November (the weeks ending Nov. 29, 2014 and Nov. 12, 2022), eight in December, two in January, one in February and one in March. The last three seasons crossed it in the weeks ending Dec. 16, 2023, Dec. 21, 2024 and Dec. 13, 2025.
Two seasons never crossed it. In 2020–21 the laboratories reported 12 positive flu tests out of 37,447, and the weekly share never rose above 0.4 percent. In 2021–22 the share peaked at 7.6 percent in the week ending May 7, 2022, later than the peak of any season that reached 10 percent.
The number of tests behind these shares grew more than twentyfold between 2010–11 and 2022–23, from 10,311 to 219,543, and stood at 130,134 last season. The department’s weekly report cautions that “changes in testing patterns also influence yearly trends” when years are compared. So the height of one season’s peak is a poor guide to another’s, and this page compares dates. Does the choice of 10 percent drive them? In each of the 14 seasons that reached 10 percent, the 5 percent line was crossed in the same week or one to three weeks earlier, and the earliest it has been crossed is the week ending Nov. 5, 2022.
From the 10 percent week to the peak took two weeks in six seasons
How much warning does the 10 percent week give? In six of the 14 seasons the share peaked two weeks later, last season among them: 10 percent in the week ending Dec. 13, 2025, and the peak, 24.7 percent, in the week ending Dec. 27, 2025. The longest climb was 11 weeks, in 2018–19, and the median was four.
The department’s flu FAQ says: “It takes about two weeks for the vaccine to start preventing flu.” By that figure, a shot given in the week flu reached 10 percent of tests reached its two-week mark in the peak week in those six seasons and before the peak in the other eight. That is arithmetic on dates. The files say nothing about whether any shot prevented any illness.
Intensive care admissions have peaked between late November and mid-April
The second record is of the sickest patients. Illinois hospitals must report every patient admitted to intensive care who tests positive for flu, and the department’s file counts those admissions for Chicago residents by week since the 2015–16 season.
| Season | Flu admissions to intensive care | Peak week | Admissions that week |
|---|---|---|---|
| 2025–26 | 284 | Jan. 3, 2026 | 53 |
| 2024–25 | 541 | Feb. 1, 2025 | 60 |
| 2023–24 | 302 | Dec. 23, 2023 | 34 |
| 2022–23 | 206 | Nov. 26, 2022 | 34 |
| 2021–22 | 52 | April 16, 2022 | 8 |
| 2020–21 | 2 | — | — |
| 2019–20 | 480 | Feb. 15, 2020 | 56 |
| 2018–19 | 361 | March 9, 2019 | 30 |
| 2017–18 | 583 | Jan. 6, 2018 | 81 |
| 2016–17 | 280 | Feb. 25, 2017 | 33 |
| 2015–16 | 280 | March 12, 2016 | 37 |
The busiest week for intensive care has come as early as the week ending Nov. 26, 2022 and as late as the week ending April 16, 2022, two peaks in one calendar year. In each of the ten seasons other than 2020–21, which had two admissions, it fell within three weeks of the laboratory peak. The totals swing widely, from 2 in 2020–21 and 52 in 2021–22 to 583 in 2017–18.
Last season: 284 flu admissions to intensive care, almost half of them in three weeks
Hospitals reported 284 intensive care admissions of Chicago residents with flu in 2025–26, counted through Sept. 19, 2026, against 541 the season before. It is the sixth-highest total of the eleven seasons on file. The admissions were bunched: 130 of them, 46 percent, came in the three weeks ending Dec. 27, Jan. 3 and Jan. 10. Residents 65 and older account for 132 of the 284, a rate of 35.0 per 100,000 by the file’s own calculation, against 3.9 for ages 18 to 49.
Which flu was it? The laboratories sort positive tests into two types of the virus, influenza A and influenza B. Of the 7,222 positives they reported last season, 78 percent were influenza A and 22 percent influenza B. Most influenza A positives were not sorted further. Of the 660 that were, 606, or 92 percent, were the subtype called H3N2.
Flu was one of three viruses in the same file. It counts 262 intensive care admissions of residents with RSV, respiratory syncytial virus, last season, 175 of them children under 5, and 164 with COVID-19. The department’s last weekly report of the season, published May 29, 2026, gave 282, 257 and 157 through May 23 and said: “A report summarizing the 2025-26 respiratory virus season will be published in June.” On September 30, 2026 the department’s reports page listed its summary of the 2024–25 season and none yet for 2025–26.
How recorded flu shots lined up with flu’s arrival
A separate department file counts residents with a flu shot on record in the state immunization registry and places each in the week the shot is dated: Influenza Vaccination Coverage, Citywide. It counts a vaccination season from July through June, so its 2025–26 season closed on June 27, 2026, and the figures here are that season’s counts as the file stood on September 30, 2026. Set beside the laboratory record, they show how many of a season’s recorded shots had already been given when flu arrived. Our reading of the vaccination files is in Chicago’s Immunization Registry Shows 29,979 Fewer Children With a Recorded Flu Shot in 2025–26 Than in 2021–22, and 20,894 More Seniors. By ZIP Code, 17 to 40 Percent of Residents Had One on Record.
| Season | Flu reached 10% of tests | All ages | Under 18 | 65 and older |
|---|---|---|---|---|
| 2025–26 | Dec. 13, 2025 | 77% | 69% | 85% |
| 2024–25 | Dec. 21, 2024 | 82% | 74% | 88% |
| 2023–24 | Dec. 16, 2023 | 79% | 70% | 87% |
| 2022–23 | Nov. 12, 2022 | 56% | 46% | 69% |
Last season, 77 percent of the residents who would end the season with a recorded shot had it by Nov. 29, two weeks before flu reached 10 percent of tests. Among children the share was 69 percent, and among residents 65 and older 85 percent. The shares were lowest in 2022–23, when flu crossed the line in mid-November, four to six weeks earlier in the calendar than in the three seasons since. Two weeks before that, on Oct. 29, 2022, 56 percent had their shot, and 46 percent of the children.
What does that comparison leave out? Every resident without a recorded shot. The registry holds one for 28 to 30 percent of the city in a season, so these shares describe when residents with a recorded shot got it and nothing else. The department’s flu page says: “September and October are the best times to get vaccinated. However, as long as flu viruses are circulating, you can get the flu vaccine even in January or later.”
What these files cannot tell you
- How many people had flu. The laboratory file counts tests and the intensive care file counts admissions. Neither counts illness.
- Whether 10 percent means the same thing every season. The number and mix of tests changed over 16 seasons, which is why this page compares weeks and leaves the levels alone.
- Whether a shot prevented an illness. The vaccination comparison is between dates in two files. No record links a person’s shot to a person’s test or hospital stay.
- When flu will arrive this season. Past seasons give a range. In the newest week on file, the one ending Sept. 19, 2026, 19 of 2,023 reported tests were positive for flu.
Where to follow this season
- Week by week. The department’s Respiratory Illness Data page carries its dashboard. The page says: “The dashboard will be updated every Friday at 1 p.m.” It adds: “The full respiratory illness surveillance report will resume in October, or sooner if respiratory activity begins to increase in the summer months.”
- The files behind this page. Laboratory tests, intensive care admissions and flu types, on the city’s data portal.
- A flu level for each ZIP code. The city file Influenza Risk Level by ZIP Code rates each ZIP code each week on a scale of 1 to 10.
- Flu shots. The department’s immunization clinics page lists its four clinics, their hours and a calendar of vaccine events.
How we counted
- The files. Three Chicago Department of Public Health files on the city data portal, each read whole on September 30, 2026: Influenza, COVID-19, RSV, and Other Respiratory Virus Laboratory Surveillance (3,421 rows, updated Sept. 25, 2026), Influenza, COVID-19, and RSV Intensive Care Unit Admissions (10,257 rows, updated Sept. 26) and Influenza Positive Laboratory Tests by Type and Subtype (4,998 rows, updated Sept. 25). A season runs from the federal Centers for Disease Control and Prevention’s week 40, in early October, through week 39.
- Shares of tests. The share positive is the week’s positive flu tests divided by its flu tests, computed from the counts and checked against the file’s own percentage. The peak week is the week with the highest share. The laboratories report by choice, and their tests include non-residents.
- The 10 percent line. A marker chosen for this page. It is not a threshold the department uses. The 5 percent weeks and the peak weeks are in the same table so the choice can be checked.
- Intensive care. Season totals are the sum of the file’s weekly counts of Chicago residents. For 2025–26 the file’s own running total reads 282 in its newest weeks while the weekly counts add up to 284; in the ten earlier seasons the two agree. The department’s summary of 2024–25 counted October through August and reported 539 flu admissions; the file now shows 541 for that season. Rates per 100,000 are the file’s.
- The vaccination comparison. From the department’s file Influenza Vaccination Coverage, Citywide, read whole on September 30, 2026 after that morning’s update: for each season, residents with a recorded shot dated by the Saturday two weeks before the 10 percent week, divided by the residents with a recorded shot in the season’s last week, which ends in late June or the first days of July. The department calls the file provisional and an undercount. It begins with 2021–22, a season that never reached 10 percent.
- What we did not do. We did not estimate illness, forecast the coming season, or relate any shot to any outcome.
Computed by KCM Desk from Chicago Department of Public Health files and pages read September 30, 2026. Human-guided and edited — about this desk. If you spot an error, corrections come first.
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