Flu Season 2026 to 2027

Flu activity usually climbs through the autumn and peaks in winter. Across the 10 seasons from 2015 to 2016 to 2025 to 2026, leaving out 2020 to 2021, the national share of visits for influenza-like illness crossed its season threshold around mid December at the median and peaked around late January. In the CDC's words: "In the United States, flu season usually occurs in the fall and winter. While influenza viruses spread year-round, most of the time flu activity peaks between December and February."

1.6% of outpatient visits in the United States were for influenza-like illness in the week ending Sep 12, 2026, between seasons and under the 2.7% level that marks the start of flu season there. ILINet counts visits for fever with a cough or sore throat, whatever the cause, so it tracks flu activity, not only laboratory confirmed flu.

Nationally, the median peak week across the 10 seasons counted falls in late January, and the 2025 to 2026 season peaked in the week ending Dec 27, 2025 at 8.3% of visits. Over a longer span, the CDC writes: "During this 40-year period, flu activity most often peaked in February (17 seasons), followed by December (7 seasons), January (6 seasons) and March (6 seasons)."

It is the symptom definition the CDC's outpatient network counts. "For this system, ILI is defined as fever (temperature of 100°F [37.8°C] or greater) and a cough and/or a sore throat." The CDC adds: "Since ILINet monitors visits for ILI, not laboratory-confirmed respiratory viruses, it can capture visits due to any respiratory pathogen that presents with the symptoms of fever plus cough or sore throat."

The CDC says: "For most people who need only one dose of influenza vaccine for the season, September and October are generally good times to be vaccinated against influenza. Ideally, everyone should be vaccinated by the end of October." It also notes: "It takes about two weeks after vaccination for antibodies to develop in the body and provide protection against influenza virus infection. That's why it's best to get vaccinated before influenza viruses start to spread in your community."

By the national share of visits for influenza-like illness, the 2025 to 2026 season, which peaked at 8.3% in the week ending Dec 27, 2025, the highest weekly figure since at least 2015 to 2016.

Hospitals in the United States reported 1,486 new laboratory confirmed flu admissions in the week ending Sep 12, 2026. Last season's busiest week, ending Jan 3, 2026, had 42,510. The count comes from the CDC's National Healthcare Safety Network, which every acute care hospital reports to weekly.

The CDC writes: "There was no discernible peak in flu activity during the 2020-2021 season due to the uncharacteristically low level of influenza virus circulation that season." Counting that season would drag every median down and pull the typical peak week to a date flu never kept.