While it typically starts in October, vaccine effectiveness can affect its duration and severity

Lazy late summer days will soon give way to cool autumn nights. This means flu season is around the corner, too. The flu, or influenza, is a viral infection that attacks your nose, throat and lungs.

So, when exactly is flu season? And when does flu season start?

In the U.S., flu season can start as early as October. But sometimes, it doesn’t rear its head until January or February.

Infectious disease specialist Kristin Englund, MD, explains how we can predict the start of flu season.

How long does flu season last?

To get an idea of how heavy or light our flu season will be, we look to patterns in the Southern Hemisphere. And it’s why experts are foreseeing a flu season with more cases this year.

“We often use Australia as a way to predict what we’re going to have because they’re kind of a season ahead of us,” Dr. Englund explains. “This year, they’ve seen a rise in the number of cases of influenza. And we are certainly going to see a significant rise in the number of cases of influenza this year as well.”

When it comes to last year’s flu season in the U.S., the U.S. Centers for Disease Control and Prevention (CDC) estimates that from October 1, 2022 through April 30, 2023, there were 27 million to 54 million flu illnesses, with 300,000 to 650,000 flu hospitalizations — and 19,000 to 58,000 deaths related to the flu.

As for what to expect this year? Dr. Englund says flu season won’t be as light as it was a few years ago.

When is flu season over?

There are reported cases of the flu in the U.S. year-round. But when it comes to flu season, the CDC says flu cases typically start to rise in October and peak between December and February. In some years, flu season months may extend into May.

“Influenza A season usually ends around March, but that is when we can see influenza B rising, so the flu season is usually around until April or May,” says Dr. Englund. “You can check out actual numbers of flu cases on your local health department’s website.”

Is flu season the same each year?

No, says Dr. Englund. Think of the last few years and how other viruses like COVID-19 and respiratory syncytial virus (RSV) affected flu season.

“During the height of COVID, when people were limiting contact and gatherings, flu cases dropped as well — since both viruses spread in similar ways," she says.

With the introduction of the COVID-19 vaccines and the lifting of mask requirements, people started going out more. This shifted the following year’s flu season, when we saw more cases of RSV.

Other factors like the kind of circulating strain of influenza, timing of flu season, how well the flu vaccine works and how many people get the flu shot can all play a role in how the flu season will unfold.

Bottom line?

Even in lighter flu seasons, the virus can cause serious illness — especially in young children, older adults, pregnant women and people with chronic conditions. Getting a flu shot, practicing good handwashing and staying home when sick are simple steps that can lower your risk and help protect those around you.