The Center for Disease Control states the timing of the annual flu season can vary from season to season. During most seasons, activity begins to increase in October, and peaks between December and February and can remain elevated into May. The flu season is said to start after consecutive weeks of elevated flu activity has been registered in the various CDC influenza surveillance systems.

The Influenza Division at the CDC collects, complies, and analyzes information on influenza activity year-round in the US FLU-VIEW which is a weekly influenza surveillance report and Flu View interactive is an online application that gives more in depth data that is updated on a weekly basis.

The US Influenza surveillance system is a collaborative effort of not just the CDC but many state, local, territorial health departments, public health, clinical labs, hospitals, and so on. There are 5 categories from 9 different sources in order that all look to see where influenza is occurring, where it is circulating at, detecting changes in the viruses, and finally ensuring the impact it is having on illnesses, hospitalization, and deaths.

The data includes total number of hospitalized patients with lab confirmed influenza virus infection, the previous day number of admissions, as well as total number of hospitalized ICU patients with lab confirmed influenza.

A full description of this system data is available at Influenza Hospitalization Surveillance Network including hospitalization rates for multiple season and different age groups and data on patient characteristics are available at FluView Interactive.

https://www.cdc.gov/flu/weekly/fluactivitysurv.htm