A Simple Guide To Why You Need a Flu Shot Every Year

With Thanksgiving now behind us, we’re now truly in the thick of the winter holiday season. On a less exciting note, we’re also in the thick of flu season. 

That’s right: flu activity in the United States typically peaks somewhere between December and February. If you’re looking to avoid getting sick this flu season, this year’s flu shot is your best bet.

But have you ever wondered why we need flu shots each year? As this is  National Influenza Vaccination Week, read on as I break down the answer to this question. 

The Flu Is ‘Flu’id: A Constantly Changing Virus

Flu viruses are constantly changing. This is especially true of influenza A viruses, a major cause of seasonal flu and the only cause of flu pandemics.

There are generally two ways these viruses change – antigenic drift and antigenic shift. In order to understand these, we need to delve into some light virology. 

Antigenic Drift

Flu viruses have two proteins on their surface called hemagglutinin (HA) and neuraminidase (NA). These are the targets (antigens) that your antibodies recognize and bind to when your immune system is fighting a flu infection.

When flu viruses go about their lifecycle, they develop small changes (mutations) in their genes. Some of these mutations may affect HA and NA. 

As mutations accumulate over time, HA and NA proteins may become different enough that your existing antibodies cannot effectively recognize them. This makes it harder for your body to defend against the flu.

Antigenic drift is the main reason you need a flu shot annually. Every year, experts carefully select the viruses to include in the flu shot based on which ones they think will be most prevalent in the upcoming flu season.

This year’s flu vaccine is trivalent. That means it protects against three types of flu viruses, two influenza A viruses (H1N1 and H3N2) and one influenza B virus (Victoria).

Sometimes, the vaccine viruses are a “good match”, meaning they are similar enough to circulating flu viruses that they provide good protection. 

Other times, circulating flu viruses have drifted far enough from the vaccine viruses that the match and resulting protection is not as good. We may be seeing this in real time as reports of a new H3N2 variant called subclade K increase. 

Because subclade K emerged late in the southern hemisphere’s flu season (during summertime for us), it was too late to include it in this year’s flu vaccine. Researchers report that it’s a mismatch to the H3N2 virus included in this year’s flu shot.

Antigenic Shift

Antigenic shift is much less common. However, when it occurs, it can lead to a sudden, drastic change in the flu.

Flu viruses have eight separate gene segments. If two different flu viruses infect the same cell, these segments can mix and match in the host cell. This is called reassortment.

I like to explain it like this: Imagine that you’ve mixed M&M’s and Skittles into a bowl and choose to pick out eight pieces at random. Some of these pieces may be M&M’s, while others may be Skittles.

When antigenic shift happens, the resulting virus can end up with an HA, NA, or both that are very different from what our antibodies recognize. That means you have little or no immunity. This is typically how flu pandemics start.

It’s Not You, It’s Me: Our Immunity Changes Too

While the dynamic nature of flu viruses is a big reason that we need a new flu shot every year, we change too. Our immunity to the flu shot wanes with time.

Generally speaking, the protection gained from the flu shot lasts about six months.

One study in the journal Eurosurveillance measured vaccine protection over time using information from lab and health databases. It found that vaccine effectiveness dropped by 9% every 28 days starting 41 days post-flu shot.

That means that beginning a month and a half after your flu shot, your protection drops by about 10% with each passing month. 

By the time the next flu season rolls around, you’ll need a re-up on your immunity in the form of the updated flu shot.

It’s Not Too Late To Get Your Flu Shot

The CDC recommends that, with rare exceptions, everyone aged six months or older should get a seasonal flu shot each year.

The effectiveness of the flu shot at preventing flu always varies from year to year –last year’s shot was 56% effective overall.  While the flu shot won’t always prevent you from catching the flu, it has other very important benefits, including:

• Reducing the severity of illness due to flu

• Lowering the likelihood of hospitalization or death from flu

• Protecting others in the community that are at a higher risk of flu complications

Due to antigenic drift, the H3N2 portion of this year’s flu shot may not be as good at preventing overall illness due to H3N2 flu viruses. However, that’s just one piece of the puzzle. 

Early reports out of the U.K., where subclade K is the dominant circulating flu virus, suggest however that this year’s flu shot is still effective enough to protect against serious illnesses and hospitalizations from the flu. This is good news for those at serious risk of complications from the flu.

It’s also important to keep in mind that the flu shot can still help to protect you from illness due to H1N1 and influenza B viruses as well.

If you’re looking to get a flu shot, know that there are different flu vaccine options available. Your doctor can let you know which one is appropriate for you.

The Bottom Line

While getting stuck in the arm each fall or winter can be a (literal) pain, there are important reasons you need an annual flu shot. 

First, flu viruses are constantly changing, meaning that your immunity from last year may not protect you from flu viruses circulating this year. In addition to that, the immunity generated from the flu shot also wanes over time.

If you haven’t gotten your flu shot yet, it’s not too late. Talk to your doctor about which type of flu vaccine is recommended for you.

Additional References

• CDC: Flu season

• CDC: How flu viruses can change: “Drift” and “shift”

• CDC: Key facts about seasonal flu vaccine

• CDC: Seasonal flu vaccine effectiveness studies

• Cold Spring Harbor Perspectives in Medicine: The ecology and evolution of influenza viruses

• Eurosurveillance: Herd effect from influenza vaccination in non-healthcare settings: A systematic review of randomised controlled trials and observational studies

• Eurosurveillance: Measuring waning protection from seasonal influenza vaccination during nine influenza seasons, Ontario, Canada, 2010/11 to 2018/19

• Frontiers in Immunology: Influenza A virus cell entry, replication, virion assembly and movement

• Journal of the Association of Medical Microbiology and Infectious Disease Canada: Emergence of seasonal influenza A(H3N2) variants with immune escape potential warrants enhanced molecular and epidemiological surveillance for the 2025–2026 season

• Immunize.org: Ask the experts: Influenza: Vaccine recommendations

• Vaccine: Does influenza vaccination attenuate the severity of breakthrough infections? A narrative review and recommendations for further research

• Vaccine: Vaccine-associated reduction in symptom severity among patients with influenza A/H3N2 disease

• U.K. Health Security Agency: Pre-print: Early influenza virus characterisation and vaccine effectiveness in England in autumn 2025

• WHO: Influenza (seasonal)

Dr. Jill Seladi-Schulman, PhD

Featured in Healthline, Medical News Today, Journal of Virology

Jill Seladi-Schulman has many years of experience writing on a variety of medical and health-related topics. A scientist by training, she has a passion for communicating health and science information in a way that's clear and accessible to all.

Jill Seladi-Schulman has many years of experience writing on a variety of medical and health-related topics. A scientist by training, she has a passion for communicating health and science information in a way that's clear and accessible to all.