Influenza season in the United States follows a predictable calendar: activity rises in October, peaks between December and February, and tapers through March and April. The 2026 to 2027 season is expected to follow that pattern, with flu shots for the updated formulation available by September.
Here is what the current data shows and what to watch for over the next several months.
Why Flu Season Starts in Fall
Influenza viruses circulate year-round at low levels, but transmission accelerates in fall and winter for several reasons. Lower humidity allows the virus to remain airborne longer. People spend more time indoors in closer contact. And school reopening in August and September creates concentrated exposure settings that seed community spread.
The World Health Organization estimates that influenza causes approximately one billion cases annually worldwide, including three to five million cases of severe illness and between 290,000 and 650,000 respiratory deaths. In the United States, seasonal flu results in tens of millions of illnesses and tens of thousands of deaths each year, with the elderly, very young children, and people with underlying conditions accounting for the majority of serious outcomes.
Who Is Most at Risk
Severe influenza outcomes are not evenly distributed. The groups at highest risk for hospitalization and death include:
- Adults 65 and older
- Children under 5, particularly those under 2
- Pregnant women, through all trimesters and for two weeks postpartum
- People with chronic conditions including heart disease, lung disease, diabetes, kidney disease, and obesity
- Immunocompromised individuals, including those on chemotherapy or long-term steroids
For healthy adults between 18 and 64, influenza is typically a miserable but self-limited illness lasting 7 to 10 days. For the groups above, it can progress to pneumonia, hospitalization, and death. This is the rationale behind annual vaccination recommendations that apply to everyone 6 months and older.
When to Get the Flu Shot
The 2026 to 2027 flu vaccine formulation is based on the strains that circulated most widely in the Southern Hemisphere during its 2026 winter season, which ends in September. Vaccine is typically available at pharmacies and medical offices by mid-September.
The timing of your shot matters. Vaccine-induced immunity takes about two weeks to fully develop and wanes over the course of the season. Getting vaccinated in September or October positions you with peak immunity during the highest-risk months of December through February.
If you missed the September to October window, vaccination in November or December still provides meaningful protection. Even late-season vaccination in January or February can protect against strains that circulate through March.
Getting vaccinated in July or August is generally not recommended for most adults because immunity may wane before the peak of the season. The exception is pregnant women, who should get vaccinated as soon as the current season's vaccine is available regardless of timing.
What to Expect This Season
The 2025 to 2026 flu season was moderate in severity and began slightly later than average, with activity peaking in January 2026. There is no reliable way to predict severity before a season begins, because it depends on how well the vaccine matches circulating strains and on which strains dominate.
What is consistent: seasons with high A(H3N2) activity tend to be more severe, particularly for adults 65 and older. Seasons dominated by A(H1N1) or B strains tend to hit children harder. The composition of the 2026 to 2027 season will only become clear as surveillance data accumulates through October and November.
Antiviral Treatment
Antiviral medications including oseltamivir (Tamiflu), zanamivir (Relenza), and baloxavir (Xofluza) can reduce the duration and severity of influenza illness when started within 48 hours of symptom onset. They are most important for high-risk individuals.
If you are in a high-risk group and develop fever with sudden onset of body aches, cough, and fatigue, contact your provider promptly. Waiting to see whether symptoms improve on their own before calling costs the window in which antivirals are most effective.
Antivirals are not a substitute for vaccination. They reduce illness duration by roughly one day and reduce the risk of complications, but they do not prevent infection the way vaccination does.
Telling Flu Apart from Other Fall Illnesses
Rhinovirus, RSV, COVID-19, and influenza all circulate in fall and winter and can present similarly. The features most characteristic of influenza are sudden onset, high fever above 101 degrees F, and significant muscle aches. A cold caused by rhinovirus typically builds gradually over one to two days and rarely produces high fever or severe body aches.
A rapid flu test at a pharmacy or clinic can confirm the diagnosis if you are unsure. Positive tests matter most when they would change treatment decisions, specifically when antiviral treatment is being considered for a high-risk patient.
Track Influenza Activity in Your State
Virus Watcher monitors influenza activity across all 50 states using integrated data from clinical, laboratory, and wastewater surveillance sources. You can set alerts for your state so you know when activity is rising before it makes the local news.
State-specific tracking is available at:
