Flu season in Pacific Southwest: The West Coast peak comes last — but California makes it count
The West Coast lag is one of the most consistent patterns in US flu surveillance. HHS Region 9 peaks roughly four to six weeks after the Southeast and two to three weeks after the national peak, almost every season. The explanation is partly geographic isolation — fewer direct surface transmission routes from the East — and partly California's milder winter temperatures, which mean people spend more time outdoors. Hawaii is in its own category: subtropical climate, year-round flu circulation, and travel patterns that link it more to Pacific Rim countries than to the continental US.
Current flu activity — HHS Region 9
This data is pulled live from the Delphi CMU Epidata API, which mirrors CDC FluView ILINet data for HHS Region 9. It reflects the most recent week available — typically data through the prior Saturday, published by the CDC the following Thursday.
States in HHS Region 9
HHS Region 9 covers Arizona, California, Hawaii, and Nevada. 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 9 typically peaks in February, making it the latest-peaking region in most seasons. In early-season H3N2 years, the peak can come as late as mid-February or even early March for some California subregions. In late-season years, when the national peak is already in February, Region 9 may show barely any additional lag.
California's internal variation is dramatic. The San Francisco Bay Area — dense, with significant transit ridership on BART and Muni — typically peaks a week or two ahead of Los Angeles. The Central Valley, with its large agricultural workforce and limited healthcare access, shows activity in ILI data later because fewer people seek outpatient care when sick. San Diego, close to the Mexican border, sometimes shows early activity when strains are already circulating across the border.
Hawaii is effectively a separate seasonal pattern. Flu circulates year-round at low to moderate levels, without the sharp winter spike seen on the mainland. The seasonal peak, when it occurs, is typically in February or March — the last of any US state. In some years, Hawaii doesn't show a traditional "peak" at all, just sustained low-level circulation with a modest winter elevation.
What drives Pacific Southwest flu patterns
The West Coast lag. The geographic and social structure of the West Coast creates a consistent buffer against early-season spread. Fewer direct surface transit routes from the East, mild winters that keep people outdoors longer, and a population that is generally younger and more health-conscious contribute to delayed onset. The lag is real and measurable across decades of FluView data.
California's size and internal variation. California's population (40 million) means its ILI numbers weigh heavily in the regional calculation. The Bay Area and LA metro together account for the majority of the regional ILI signal. The Central Valley and rural Northern California are underrepresented in ILI data because primary care access is limited and fewer people report to ILINet providers.
Nevada's Las Vegas concentration. Las Vegas is one of the world's highest-density short-stay tourist destinations. The hotel and casino workforce — which provides services to 40 million visitors per year — experiences concentrated exposure. When flu is circulating nationally at High levels, Las Vegas shows accelerated spread through its service industry before the broader Nevada population is affected.
Arizona's dual demographic. Arizona has one of the country's largest retiree populations (Scottsdale, Tucson, Sun City) alongside a large, relatively young immigrant workforce in agriculture. These two populations have very different healthcare access and vaccination rates, and flu affects them differently. Retiree communities can show significant hospitalizations even in moderate-severity seasons.
Recent seasons in HHS Region 9
Regional peak timing and severity can vary substantially from the national picture. The table below shows Region 9-specific peak months and severity for recent seasons, based on CDC FluView regional ILI data.
| Season | Regional peak | Dominant strain | Severity | Notable |
|---|---|---|---|---|
| 2024–25 | February | H3N2 / H1N1 | High | Above-average; Bay Area led LA by ~10 days; Hawaii had modest elevation only |
| 2023–24 | February | H1N1 | Moderate | Moderate; Region 9 lag fully intact — peaked 5 weeks after Southeast |
| 2022–23 | January–February | H3N2 | High | Early severe season nationally; CA still peaked in January rather than December |
| 2021–22 | March | H3N2 | High | Unusually late; CA peaked in March, the latest of any major state |
| 2019–20 | February | H1N1 | High | Typical timing; ended abruptly with COVID-19 in March |
If you're in California and the national news is talking about peak flu season, you're probably 2–4 weeks away from your own peak. The West Coast lag is real and reliable. That window is your best remaining opportunity to get vaccinated — it takes two weeks for the vaccine to reach full effectiveness, which lines you up right at the California peak. Track current Region 9 activity. →
How to use this data
The live activity level above reflects the most recent week of CDC ILINet data for Region 9. 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.