Why This Year’s Flu Season Could Be Severe and What the CDC Recommends

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A parent whose toddler develops flu symptoms faces a question that a seasonal forecast cannot answer: how quickly should the child be assessed for treatment? The Centers for Disease Control and Prevention (CDC) considers children younger than 5 at higher risk of flu complications. It recommends prompt antiviral treatment for higher-risk people with flu or suspected flu.

CDC expects moderate overall severity for the coming season, with low to moderate confidence. Flu antiviral medicines are prescription drugs that fight flu viruses in the body. Antiviral medicines work best when started within two days of getting sick. Whatever the season turns out to be, CDC’s advice for a higher-risk person with flu symptoms is the same: contact a clinician promptly.

What CDC can tell us about the coming season

CDC’s seasonal outlook uses the peak of influenza hospitalizations to assess severity and draws on historical trends and expert opinion. CDC says there is no evidence that a severe 2026-2027 season is more likely than in past seasons, although severity is difficult to predict before a season begins. The most recent season, 2025-2026, was classified as moderate overall but high severity for children ages 0-17.

In its report for the week ending October 3, 2026, CDC said seasonal influenza activity was increasing in many parts of the country but remained low. Clinical laboratories reported that 3.6 percent of the respiratory specimens they tested that week were positive for influenza. That is a share of the specimens tested, not of all Americans, and an early rise does not show how high the season will peak.

CDC says season-to-season severity depends partly on the virus subtypes, or forms, in circulation, the population’s immunity to them and how well the vaccine works.

Why the season that peaked in 2025 was so severe

CDC classified the 2024-25 season as high severity overall and for children, adults and older adults, the first high-severity season since 2017-18. Its assessment compares influenza-like illness, hospitalization rates and influenza deaths with past seasons. CDC researchers said the high hospitalization rates in 2024-2025 could have been driven by lower flu vaccination coverage and virus characteristics.

Among specimens tested by clinical laboratories nationwide, 31.6 percent were positive for flu in the week ending February 1, 2025, the season’s peak. That was the highest peak CDC had reported in nine seasons, going back to 2015-2016.

Influenza A accounted for 88.8 percent of the positive tests in clinical laboratories that season, and influenza B for the rest. H1N1 and H3N2 are subtypes of influenza A that routinely circulate in people. Among the seasonal influenza A viruses that public health laboratories classified by subtype that season, 53.1 percent were H1N1 and 46.9 percent were H3N2.

Flu viruses also change over time, in two ways. Antigenic drift means small genetic changes in flu viruses that can change their surface proteins. Antigenic drift can change a flu virus enough that existing antibodies have more difficulty recognizing and fighting it, sometimes leaving a person susceptible to infection again.

Antigenic shift is a different process: an abrupt major change in an influenza A virus can produce a new subtype against which people have little immunity. Antigenic shift can lead to a pandemic, as happened when a new H1N1 virus emerged in spring 2009. A severe flu season differs from a pandemic: CDC says flu epidemics are not uncommon, while flu pandemics are quite rare. The virus mix and these mechanisms help explain why seasons differ; they do not show how much of the 2024-2025 severity each one caused.

How 2024 and 2025 compare

The 2025-2026 row shows preliminary minimum estimates through May 23, 2026, rather than final full-season totals.

Flu is counted by season rather than by calendar year. Flu viruses circulate year-round in the United States, but the usual season is fall and winter, and activity most often peaks between December and February. By all three measures in the table, the 2024-2025 season was heavier than the 2023-2024 season.

The figures come from CDC’s end-of-season burden summary for 2023-2024 and its 2026 summary for 2024-2025. The earlier estimate used information on influenza-confirmed hospitalizations and deaths from October 1, 2023 through April 30, 2024. CDC’s estimate of 28,000 flu-related deaths therefore covers the 2023-2024 season, which spans parts of two years; it is not a total for calendar year 2024.

The 2024-2025 summary says its estimates remain preliminary until all contributing data are available, expected in 2027. The earlier 2023-2024 summary also labels its estimates preliminary and says that report will not be updated.

CDC explains that a death can be counted as flu-related when flu contributes to the cause of death, even if flu is not the primary cause. Not everyone who dies with an influenza-like illness is tested, and deaths involving complications can be difficult to classify.

Who faces the greatest risk, including from H1N1

Seasonal A(H1N1)pdm09 viruses descend from the virus that caused the 2009 pandemic and have circulated seasonally since then. The name points back to that pandemic; it does not mean a current infection is part of a new one. During the 2009 H1N1 pandemic, severe outcomes were most common among children, young adults and high-risk groups such as pregnant women, according to CDC. CDC’s list of people at higher risk, which follows, is for flu complications in general rather than for H1N1 alone.

CDC lists adults 65 and older among those at increased risk of serious flu complications. All children younger than 5 are at higher risk, with the greatest risk under age 2 and the highest hospitalization and death rates among infants younger than 6 months.

Pregnancy increases the risk of flu complications, including for up to two weeks after pregnancy ends. CDC also identifies nursing-home and other long-term-care residents as higher-risk groups.

CDC’s list includes people with asthma or other chronic lung disease, neurologic conditions and blood disorders such as sickle cell disease. It also includes people with diabetes, heart disease, kidney or liver disorders, metabolic disorders and weakened immune systems. Neurologic conditions are disorders of the nervous system: the brain, spinal cord and nerves. Metabolic disorders disrupt the body’s process of getting or making energy from food.

It also includes people with a body mass index of 40 or higher and people younger than 19 taking long-term aspirin or medicines containing salicylates. Body mass index measures weight relative to height. Salicylates are a group of medicines that includes aspirin. If either category is unclear, ask a clinician whether CDC's higher-risk criteria apply. Risk is not only a matter of age: a younger adult with one of these conditions may still need prompt treatment.

CDC also identifies increased flu hospitalization risk among non-Hispanic Black, Hispanic or Latino, and American Indian or Alaska Native people.

What vaccination changes, and when to get it

CDC recommends annual flu vaccination for people 6 months and older who have no medical reason to avoid it. CDC says vaccination reduces the risk of flu illness, hospitalization and death, and can reduce illness severity in people who are vaccinated but still become sick.

Outpatient medical visits are visits for care without being admitted to a hospital.

CDC’s burden summary reported these effectiveness estimates:

The estimates cover different age groups and outcomes, so they cannot be combined into one figure for the vaccine’s protection.

CDC explains that a vaccine-effectiveness estimate of 60 percent means an average 60 percent reduction in the risk of the particular flu-related outcome being studied. It also reports a confidence interval, a range expressing uncertainty around that estimate. The percentage compares risk with and without the vaccine; it does not mean that share of vaccinated people is completely immune.

Protection varies with age, health and the match between vaccine viruses and the viruses circulating that season. Antibodies take about two weeks to develop after vaccination. Some vaccinated people can also become infected with a flu virus outside the vaccine's coverage, or with a covered virus despite vaccination.

For 2026-2027, all three virus components in the flu vaccine were updated compared with 2025-2026. The updates include a change targeting the A(H3N2) subclade K virus that spread widely during the previous season. A subclade is a genetic subgroup within influenza viruses. The update does not show in advance how effective this season’s vaccine will be.

For most people needing one dose, CDC generally recommends vaccination in September or October, ideally by the end of October. Most adults, especially those 65 and older, should generally avoid vaccination in July or August because protection may decrease over time, unless a later opportunity is unlikely. Vaccination should continue after October while flu viruses remain a threat to the community.

Some children ages 6 months through 8 years need two doses if they are being vaccinated for the first time, have received only one prior dose, or have an unknown vaccination history. Those doses must be at least 4 weeks apart, so CDC recommends starting as soon as vaccine is available.

For adults 65 and older, CDC prefers high-dose inactivated vaccines, recombinant vaccines or inactivated vaccines with an adjuvant over standard-dose vaccines without an adjuvant. If a preferred vaccine is unavailable at the time of vaccination, CDC advises a standard-dose inactivated vaccine without an adjuvant instead.

Inactivated flu vaccines use killed, noninfectious vaccine viruses, while recombinant flu vaccines use proteins from a flu virus. An adjuvant is an added ingredient that helps create a stronger immune response. Ask the vaccinator for an age-appropriate vaccine from one of CDC's preferred categories. People 65 and older should not receive a nasal-spray flu vaccine.

Infants younger than 6 months are too young for a flu shot. CDC says flu vaccination during pregnancy helps protect babies from flu illness and hospitalization during their first several months. It also advises annual flu vaccination for caregivers and household members aged 6 months and older.

People with a severe, life-threatening allergy to a non-egg ingredient in a flu vaccine should not receive that vaccine. After a severe allergic reaction to a flu vaccine, CDC advises avoiding that same vaccine and consulting a clinician about whether other options are appropriate. Egg allergy alone does not prevent receipt of a vaccine otherwise appropriate for age and health.

What to do before and after symptoms start

CDC recommends washing hands, avoiding touching the eyes, nose and mouth, and improving air quality by bringing in outside air, purifying indoor air or gathering outdoors.

For respiratory symptoms not better explained by another cause, CDC advises staying home and away from others, including household members who are not sick. Normal activities can resume after symptoms have been improving overall for at least 24 hours and there has been no fever for at least 24 hours without fever-reducing medicine. A temperature lowered by medicine does not satisfy that fever-free condition.

For the next 5 days after returning, CDC recommends extra precautions such as cleaner air, good hygiene, a well-fitted mask, distancing or testing when around others. Feeling better and being unable to pass the virus on are not the same thing.

CDC advises antiviral treatment as soon as possible for people hospitalized with flu, those with severe, complicated or worsening illness, and people at higher risk of complications. For these priority groups, treatment decisions should not wait for laboratory confirmation of influenza.

Later antiviral treatment can still help, especially for higher-risk people or those hospitalized with more severe illness. CDC advises contacting a clinician promptly when a higher-risk person develops flu symptoms. The two-day window is a reason to act quickly, not a reason to assume treatment is pointless once it has passed.

CDC says adults should obtain medical care right away for warning signs such as difficulty breathing, persistent chest or abdominal pain, confusion, seizures, not urinating, or severe weakness. For children, warning signs include trouble breathing, bluish lips or face, dehydration, lack of alertness, seizures and any fever in a baby younger than 12 weeks. CDC also lists fever or cough that improves and then returns or worsens, and worsening chronic conditions as warning signs for both children and adults.

How to follow flu activity where it matters

FluView, CDC's influenza surveillance report, and FluView Interactive are updated weekly, with preliminary data that can change as more information arrives. Flu activity and timing in a state or county can differ from the national pattern. FluView Interactive provides national, regional and state laboratory data for current and past seasons.

Start with the reporting week, then check whether the measure is positive laboratory tests, illness visits, hospitalizations or deaths. Compare the same measure from week to week, because different measures do not describe one trend.