Flu season in Mountain: Ski season accelerates early spread; extreme cold drives the rest
HHS Region 8 covers more geographic area than any other HHS region, but most of it is empty. The flu story here is really about two things: Denver and Colorado's ski resorts on one end, and the extreme, isolating cold of Montana, North Dakota, South Dakota, and Wyoming on the other. The Denver metro drives the regional numbers; ski season creates a unique early-spread dynamic as resorts concentrate people from across the country in tight spaces from Thanksgiving through March; and Utah's unusually young, large-family demographics produce more pediatric flu than the state's population alone would suggest.
Current flu activity — HHS Region 8
This data is pulled live from the Delphi CMU Epidata API, which mirrors CDC FluView ILINet data for HHS Region 8. It reflects the most recent week available — typically data through the prior Saturday, published by the CDC the following Thursday.
States in HHS Region 8
HHS Region 8 covers Colorado, Montana, North Dakota, South Dakota, Utah, and Wyoming. These states are grouped together by the Department of Health and Human Services for federal health program administration, and the CDC uses the same regional boundaries for flu surveillance reporting. ILI activity data is aggregated across all ILINet providers in the region, so the number reflects the regional average — individual states can vary significantly.
When flu typically peaks here
HHS Region 8 peaks in January to early February, typically within a week of the national average. Colorado's ski corridor often shows early activity in December that can look like a premature peak — but it's usually a ski-season cluster before the broader community spread develops in January.
Utah often peaks slightly later than Colorado, in late January or early February. The state's younger median age (the lowest in the country, partly driven by LDS family size) means flu spreads efficiently through school-age populations. Pediatric cases peak earlier than adult cases, so Utah's school surveillance data is worth watching as a leading indicator.
Montana, North Dakota, South Dakota, and Wyoming are among the most sparsely populated states in the country. Flu arrives later, peaks at lower absolute levels, but can spread intensely through small communities once it arrives. Bismarck, Rapid City, and Billings function as regional hubs that distribute flu to surrounding areas along highway corridors.
What drives Mountain flu patterns
Ski resort effect. Colorado, Utah, and Wyoming's major ski resorts (Vail, Breckenridge, Park City, Jackson Hole, Aspen) concentrate visitors from across the country — and internationally — in gondolas, lodges, condominiums, and restaurants from Thanksgiving through March. Resorts are ideal flu transmission environments: cold outdoor air drives people inside, spaces are crowded, and visitor populations are constantly turning over. Flu researchers have identified ski resort clusters as early-season amplifiers in multiple seasons.
Denver as the regional hub. The Denver metro (3.5 million) is the region's only major population center and drives the regional ILI numbers. Denver International Airport, the fifth-busiest in the US, connects the Mountain region to national and international flu strains. When DEN shows activity rising, the regional numbers follow within two weeks.
Utah's pediatric skew. Utah has the youngest median age in the US and the highest birthrate. School-age children are the primary flu transmission vector, and Utah's large school enrollment relative to its population makes pediatric surveillance data particularly useful here. When schools show absences rising, community spread typically follows by one to two weeks.
Extreme cold in the northern states. Montana, North Dakota, and Wyoming experience some of the coldest winters in the contiguous US. While extreme cold itself doesn't cause flu, it drives prolonged, deep indoor crowding — the kind where the same people share the same spaces for months. Small-town gymnasiums, churches, and community centers become high-transmission environments for the entire winter season.
Recent seasons in HHS Region 8
Regional peak timing and severity can vary substantially from the national picture. The table below shows Region 8-specific peak months and severity for recent seasons, based on CDC FluView regional ILI data.
| Season | Regional peak | Dominant strain | Severity | Notable |
|---|---|---|---|---|
| 2024–25 | January | H3N2 / H1N1 | High | Above-average; Colorado showed early ski-resort clusters in December |
| 2023–24 | January–February | H1N1 | Moderate | Moderate; Utah peaked a week after Colorado; northern states lagged |
| 2022–23 | December–January | H3N2 | High | Early season; ski corridor showed clusters before Christmas |
| 2021–22 | February | H3N2 | High | Late season; Utah and Colorado peaked close together in February |
| 2019–20 | January | H1N1 | High | Average timing; northern states lagged by 2–3 weeks |
If you're heading to a ski resort in Region 8 between December and February, assume flu is circulating. Resorts concentrate visitors from high-activity regions in enclosed spaces — it's one of the few settings where you're reliably exposed to out-of-region strains at the height of season. Get vaccinated before you go. See current Mountain region activity. →
How to use this data
The live activity level above reflects the most recent week of CDC ILINet data for Region 8. There is always roughly a one-week lag between real-world conditions and published numbers — providers report weekly, the CDC publishes Thursdays, and this page reflects those numbers. During a rapidly rising season, treat the current level as a floor.
The ILI percentage is the share of outpatient visits attributed to influenza-like illness across all ILINet reporting providers in the region. It is not a case count and does not capture people who don't seek care. In regions with lower healthcare utilization rates (rural areas, communities with limited access), ILI percentages tend to understate true community activity.
For the most complete picture of the current season — including strain typing, lab positivity trends, and hospitalization data — the IsItFluSeasonYet homepage shows all of this in context. The regional activity shown here is the same data source as the homepage's region breakdown.