Each fall, medical experts recommend that nearly everyone 6 months or older be vaccinated against the flu — before flu season is in full swing. (Flu cases typically start to ramp up in October.)

The flu shot has been around since the 1940s and been proven every year since to be the most effective way to prevent infection with the influenza virus. But there are lots of misunderstandings and misconceptions about it.

Maybe you’re a fan of the flu shot and never miss a season, or maybe you’re wary of it, simply forget about it, or figure you won’t get sick — and that it won’t be so bad if you do. No matter which category you’re in, learning all you can about the vaccine can help you make informed decisions going forward.

The flu vaccine is updated every year to match the specific strains of the influenza virus experts predict will be most prominent during flu season. To determine this, the World Health Organization (WHO) Global Influenza Surveillance and Response System (GISRS), made up of 129 WHO member states, conducts year-round surveillance to determine which flu strains are actively circulating around the globe.

“These flu experts try to anticipate what the dominant strains are going to be nine months down the road,” explains William Schaffner, MD, a professor of infectious diseases at Vanderbilt University School of Medicine in Nashville, Tennessee. “They’re often on target, but on some occasions, the flu virus that circulates varies from the vaccine. How effective the influenza vaccine is changes from year to year, too.”

In other words, flu virus experts have to base their recommendations on a moving target, which means the vaccine can’t be perfect. Even so, according to the Centers for Disease Control and Prevention (CDC), when the flu shot and circulating viruses are a good match, it helps lower the risk of needing to see a doctor for the flu by 40 to 60 percent.

All available flu vaccines for the 2024–2025 season target three strains of influenza:

• H1N1, a subtype of influenza A
• H3N2, a subtype of influenza A
• Victoria lineage virus, a subtype of influenza B

“In regards to influenza, the recommendation could not be simpler: Most healthy people over 6 months old should be vaccinated,” says Schaffner.

This guidance comes with rare exceptions.

According to the CDC, it’s safe for people with an egg allergy to get a flu shot — even if it’s egg based. But people who have allergies to some other substances may not be good candidates for a flu shot. So, if you have allergies, talk to your doctor before you get vaccinated.