Planning your Colorado flu shot this fall? Start with what happened last winter. Colorado set a flu record that no one wanted. In the week ending December 27, 846 Coloradans were hospitalized with influenza, the most in the 20 years the state has tracked it (Colorado Sun). The previous record, set in December 2014, was 582.
If you were in our clinics in December and January, you saw it. Waiting rooms full of coughing kids. Phone lines jammed. Patients who told us, sometimes through a fever, that they meant to get the shot and never did.
This post is the one we wish we could hand every patient before that happened. It covers what last season looked like in Colorado, why the timing of your Colorado flu shot matters more than most people realize, what the 2026-27 vaccine does and does not do, and who needs it most. It also includes a short tool to help you pick the best week for your own shot.
The short version: October through mid-November is a window that balances how fast protection builds against how long it lasts for your Colorado flu shot. This post explains why. For background on last year’s virus, see our earlier post on the 2025-26 flu shot and the new subclade K virus.
What last season taught us
The 2025-26 season was not just big. It was early, and it hit a state that was under-vaccinated going in.
The main driver was a new H3N2 variant called subclade K, first identified in August 2025. It had drifted away from the strain in last year’s vaccine, and it was found too late to change the formula (CDC). It spread quickly around the world and pushed flu season earlier in many countries (WHO).
In Colorado, the state health department said the surge began about four weeks earlier than usual. In mid-November, 65 people were hospitalized with flu statewide. By the end of December, it was 782 in a single week (CPR), and the final count for the week ending December 27 came in at 846.
| Colorado, 2025-26 | What we saw |
|---|---|
| Peak weekly flu hospitalizations | 846 (week ending Dec 27), a 20-year record |
| Prior record | 582 (December 2014) |
| Flu test positivity at the late-December surge | About 34% |
| ER visits that were flu | Almost 10% |
| Coloradans vaccinated by mid-January | 28.7% |
| Adults 65 and older vaccinated (late December) | 49% |
| Children ages 6 to 9 vaccinated (late December) | 29% |
Sources: Colorado Sun, CPR. Vaccination figures are from different dates and are not directly comparable.
Sources: Colorado Sun and CPR. The 846 is the final count for the week ending December 27.
Two lessons stand out. First, the vaccine still worked even though it was a partial mismatch. Early data suggested it prevented roughly three quarters of pediatric hospitalizations and about a third of adult hospitalizations. Second, fewer than three in ten Coloradans were vaccinated when the surge arrived. Roughly half of adults 65 and older, the group most likely to end up in the hospital, were unprotected. That is a gap we can close.
When to get your Colorado flu shot: why October through mid-November is a common target
Flu shots are not a switch you flip. Two things happen after you are vaccinated, and timing is about balancing them.
Protection takes about two weeks to build. Get your shot the week the surge starts and you are exposed during the gap.
Protection also fades. Studies of vaccine effectiveness over a season find that protection declines gradually after vaccination, on the order of several percentage points per month, and the drop is more noticeable in adults 65 and older. A shot given in August can be running low by the time a January peak arrives. This is why the CDC advises that most people be vaccinated in September or October and recommends against getting the shot in July or August (CDC).
Colorado’s flu season typically builds through December and peaks around December to February. Last year showed the risk on the other side: a season that starts early can leave late vaccinators unprotected. Putting those together:
| When you get your Colorado flu shot | What it means |
|---|---|
| July to August | Too early for most adults. Protection may fade before the peak. Exceptions: children needing two doses, and the third trimester of pregnancy |
| September | The CDC lists September and October as its preferred window, but September is often earlier than necessary in Colorado. Protection can fade before a December to February peak, especially in adults 65 and older |
| October to mid-November | A commonly suggested window. Protection is fully built before even an early surge, and is still strong at the typical peak |
| Late November to December | Better than nothing and still worthwhile, but you may be exposed if the season comes early again |
| January or later | Still useful. Flu circulates into spring, so do not skip it because you missed the window |
How we think about timing for your Colorado flu shot: for many people, October to mid-November balances building protection with keeping it through the peak. September can be earlier than necessary here, since protection can wane before the December to February peak. For adults 65 or older, or people with a health condition that makes flu dangerous, earlier in that window is generally favored. Your own clinician can help you decide.
Timing figures come from the guidance above. Illustration only; each season is different.
Try it: find your best week
Flu Shot Timing Planner
What is in this year’s Colorado flu shot
This is the encouraging part. Every component of the 2026-27 flu vaccine has been updated from last season, including a new H3N2 strain chosen specifically because subclade K spread so widely (CDC). Last year’s vaccine was built before anyone knew subclade K existed. This year’s was built with it in view.
All U.S. flu vaccines this season are trivalent, meaning they protect against three strains (CDC). The World Health Organization recommended these viruses for the Northern Hemisphere (WHO):
| Strain type | Egg-based vaccines | Cell-based and recombinant vaccines |
|---|---|---|
| Influenza A (H1N1) | A/Missouri/11/2025 | A/Missouri/11/2025 |
| Influenza A (H3N2) | A/Darwin/1454/2025 | A/Darwin/1415/2025 |
| Influenza B (Victoria) | B/Tokyo/EIS13-175/2025 | B/Pennsylvania/14/2025 |
You do not need to memorize any of that. What matters is that all three viruses are new picks, and that the H3N2 update targets the family of viruses that drove last winter’s record.
One honest caveat: H3N2 changes quickly, and a good match on paper does not guarantee a good match in February. Even in years when the match is imperfect, vaccination still reduces the chance of hospitalization and severe illness, as last season showed.
Does the Colorado flu shot work? Effectiveness in plain numbers
The most common thing we hear is some version of “it only works about half the time.” That number describes protection against getting any flu illness at all, and it varies by year. It leaves out the outcome that matters most: staying out of the hospital.
Last season was a hard test, because the dominant virus had drifted from the vaccine strain. Even so, the results held up where it counts:
| Outcome, 2025-26 | Estimated protection from vaccination |
|---|---|
| Hospitalization in children | About 70 to 75% |
| Hospitalization in adults | About 30 to 40% |
Sources: early estimates from England reported by the CDC, consistent with early Colorado data cited by CPR. These were early estimates in a mismatched year. Final U.S. figures may differ.
Three points to take from this:
- A mismatched year still helps. Adult protection against hospitalization was lower than in a good-match year, but a one-in-three reduction in hospital admission is a meaningful benefit across a population.
- Children benefit the most. Protection in kids was strong even against the drifted virus.
- Vaccination also tends to make illness milder in people who get sick anyway. That means fewer days in bed, and fewer complications such as pneumonia or worsening heart and lung disease.
Early 2025-26 estimates in a mismatched year. The lighter segment shows the range. Final U.S. figures may differ.
Five things we hear about the Colorado flu shot in the exam room
“The flu shot gave me the flu.” It cannot. Injected flu vaccines contain either inactivated virus or a single viral protein, neither of which can cause infection. A sore arm, mild aches, or a low fever for a day or two is your immune system doing its job. The more likely explanation is that you caught a cold, or flu itself, in the two weeks before the shot took effect.
“I never get sick.” Good, and that may be partly luck. Flu behaves differently from the common cold. It can put healthy adults in bed for a week and, occasionally, in the hospital. Vaccination is also about the people around you: infants too young to be vaccinated, grandparents, coworkers on chemotherapy.
“I got the shot and still got sick, so it doesn’t work.” Many respiratory viruses circulate in winter, and only one of them is influenza. If you did have flu despite vaccination, the shot often makes the illness shorter and milder, and lowers the chance of complications. It is protection, not a force field.
“I’d rather get natural immunity.” Flu is not a good virus to collect immunity from. It can trigger pneumonia, heart attacks, strokes, and flare-ups of asthma, COPD, and diabetes. Studies have found the risk of a heart attack rises sharply in the days right after a flu infection. Getting the vaccine gives your immune system a preview without that risk.
“It’s too early, or too late.” See the timing section above. Too early, meaning July or August, is a real consideration. Too late is rarely true. If it is January and you have not been vaccinated, getting your Colorado flu shot is generally still worthwhile.
Who needs the Colorado flu shot most, and which vaccine fits
The CDC recommends an annual flu vaccine for everyone 6 months and older (CDC). Some groups have more to lose from flu than others, and early vaccination is especially worth discussing with your clinician:
| Group | Why it matters | What to know |
|---|---|---|
| Adults 65 and older | Highest risk of hospitalization and complications. Immune response to standard vaccines is weaker | The CDC prefers a high-dose, adjuvanted, or recombinant vaccine for this group. We carry the high-dose vaccine, one of the preferred options |
| Children under 5, especially under 2 | Higher risk of hospitalization. Only 29% of Colorado children ages 6 to 9 were vaccinated last December, and an elementary school-aged child from metro Denver died of flu | Children 6 months to 8 years who have not had at least two prior doses need two doses, four weeks apart. Starting early allows the second dose to be done before the surge |
| Pregnant women | Flu is more dangerous in pregnancy. Vaccination also protects the baby for the first months of life | Any recommended injectable flu vaccine, any trimester. Third-trimester women can be vaccinated a little earlier if needed |
| Adults with chronic conditions | Asthma, COPD, diabetes, heart disease, kidney disease, and weakened immune systems all raise the stakes | Ask us or your clinician which formulation fits. Waiting until late fall is generally not advised |
| Healthy children and adults | Lower individual risk, but they spread flu in homes, schools, and workplaces | A standard-dose flu shot is the usual option |
At our clinics we offer a standard-dose flu shot for everyone 6 months and older, and a high-dose flu shot for adults 65 and older.
Not sure where you fit? Tell us at your visit or when you book, and we will talk through the options we carry.
Vaccine fatigue, and what we say about it
We understand why some patients are done with vaccine conversations. Health news has been relentless for years, advice has changed, and trust has been worn down. If you are tired of being told what to do, that is a reasonable place to start.
Here is how we approach it in the exam room.
We keep it clear. The CDC recommends an annual flu vaccine for everyone 6 months and older, and we are glad to talk through your questions about your Colorado flu shot. We do not bury it in caveats or turn it into a lecture.
We answer the real question. Usually it is not about the flu at all. It is about a past bad reaction, a worry about ingredients, or doubt that it will do anything. Those deserve a real answer, and you are welcome to ask us anything.
We keep it about you. For some people the reason is their own risk. For others it is a newborn at home, a parent in memory care, or a job where missing a week is costly. We ask what matters to you and start there.
We respect the decision. If you decide against it, we will not think less of you, and we will still be here if you change your mind. We will also make sure you know what to do if you get sick, including when to call us for antiviral treatment, which works best when started within the first 48 hours.
Last winter’s record came in a state where fewer than three in ten people had been vaccinated. Small changes in that number matter. If this post helps you or someone in your family make an informed choice, it has done its job.
Get your Colorado flu shot with us
We have flu vaccine on hand at both our Littleton and Arvada clinics, and getting in is easy. Flu shots are available by appointment, at our planned Tuesday flu clinics, as a walk-in, or during a visit for another reason. There is no need to wait weeks for an appointment.
- Book a quick visit. Schedule online through our online scheduling page or call (303) 730-2167. A flu shot visit takes only a few minutes.
- Come to a flu clinic. We are planning dedicated flu clinics on Tuesdays. Call (303) 730-2167 to confirm. Bring the whole family.
- Add it to a visit you already have. If you are coming in for anything else this fall, tell us when you book and we will give you the shot then.
- Ask about the right vaccine. If you are 65 or older, pregnant, or have a chronic condition, we will talk through the options we carry.
What it costs. Most insurance plans cover the flu vaccine. If yours does not, our self-pay price is set to cover our costs and nothing more. Cost does not have to be a barrier, so call us if you have questions before you come in.
Walk-ins are also welcome, though there may be a short wait.
Many people find it easiest to schedule their Colorado flu shot between October and mid-November. A quick visit or one of our planned Tuesday flu clinics makes it simple.
Last winter set a record we would rather not see again. Vaccination is one of the most effective steps people can take to help make sure it does not.
Sources
- Colorado Sun: Did Colorado just have its worst week for flu hospitalizations?
- CPR: Hundreds of Coloradans hospitalized with flu as numbers explode
- CDC: 2025-2026 Flu Season
- CDC: 2026-2027 Flu Season
- CDC: Interim clinical considerations for seasonal influenza vaccines
- WHO: Recommendations for influenza vaccine composition for the 2026-2027 Northern Hemisphere season
Disclaimer: This post is for general informational purposes only and is not medical advice. It is not a substitute for care from your own healthcare provider, and it does not create a patient relationship. Individual needs vary, so talk with us or your own clinician about what is right for you.