How Long Does the Flu Shot Last? What Health Experts Want You to Know Before Peak Flu Season This Fall

The arrival of fall means peak flu season is not far behind, and health experts say getting vaccinated now will give people protection throughout the coming months. Here is what to know about flu shot timing, duration and this year's updated vaccines before seasonal illness begins spreading in earnest.

According to the American Lung Association, the flu shot provides potentially lifesaving protection against severe illness and helps prevent spreading the virus to family members and other loved ones. The Centers for Disease Control and Prevention recommends getting vaccinated by late October, and even earlier in September if possible, to ensure protection is in place before flu activity picks up.

Once someone receives the vaccine, it takes approximately two weeks for the body to develop the antibodies needed to offer protection against the virus, according to the CDC. That vaccination is designed to protect a person throughout the current flu season, though immunity naturally declines over time, which is why both the CDC and Healthline recommend getting a new seasonal flu vaccine every year rather than relying on protection from a prior year's shot.

Flu season in the United States typically runs at its worst from October through May, with activity generally peaking during the winter months between December and February, according to both the CDC and the Cleveland Clinic. Flu season formally begins in the fall, typically in October, though the Cleveland Clinic notes that influenza viruses can circulate to some degree year-round, even outside the traditional peak window. Flu activity usually slows by May as the season winds down.

This year's flu vaccines have been specifically updated in response to the strain that dominated last year's flu season. The U.S. Food and Drug Administration has recommended updated formulations designed to protect against the influenza A H3N2 subclade K virus, the specific strain responsible for much of last season's flu activity, according to the CDC. That recommendation followed a March 12 meeting of the FDA's Vaccines and Related Biological Products Advisory Committee, at which the panel recommended three egg-based vaccines and three cell- or recombinant-based vaccines for the current season. The FDA subsequently shared that guidance directly with manufacturers of FDA-approved seasonal flu vaccines.

This season's flu shot remains a trivalent vaccine, meaning it targets three separate strains of the virus: two influenza A-type strains, covering both H1N1 and H3N2, along with an influenza B/Victoria lineage strain, according to the CDC.

A notable new option has also become available for certain patients this year. The FDA has authorized a new mRNA-based version of the seasonal flu vaccine, called mFLUSIVA, for use specifically in people ages 50 and older, marking an expansion of mRNA vaccine technology, previously more closely associated with COVID-19 vaccines, into the seasonal flu vaccine space.

The CDC recommends that everyone 6 months of age and older receive an annual flu vaccination. Certain groups face a heightened risk of severe complications from the flu and are considered especially important priorities for vaccination, including adults 65 and older, individuals with chronic health conditions, children 5 and younger, and pregnant women.

For those looking to get vaccinated, flu shots are widely available at local pharmacies, doctor's offices and health departments, many of which offer both walk-in availability and scheduled appointments. Cost is generally not a significant barrier for most people seeking vaccination, since the flu shot is typically free or available at low cost under most insurance plans.

Health officials have continued emphasizing the importance of annual vaccination given how significantly flu activity can vary from one season to the next, along with the reality that the specific strains circulating, and therefore the makeup of each year's vaccine, change from year to year based on ongoing global surveillance of circulating influenza viruses. That surveillance process is what allows regulators to identify emerging strains, such as last year's H3N2 subclade K, and update the following season's vaccine formulation accordingly, as reflected in this year's specifically updated shots.

With flu activity expected to begin climbing as the weather cools and typically peaking between December and February, health officials continue to emphasize that the two-week window needed for full vaccine protection to develop makes earlier vaccination, ideally by late October and as early as September when possible, the most effective strategy for ensuring protection is fully in place before flu season reaches its most active stretch.