How Long Does the Flu Shot Last? Timing and Side Effects

October 9, 2026 avinashmittal

Flu shot season is here, and two questions come up every year: how long will this shot actually protect me, and why do I feel so tired afterward? Here’s what the Centers for Disease Control and Prevention (CDC) and research say about flu shot timing, protection and side effects.

Quick answer: How long does the flu shot last? It protects you for one flu season, roughly six months. Protection starts about two weeks after the shot, according to the CDC, and slowly fades over the following months. That’s why you need a new flu shot every year, ideally in September or October.

How Long Does the Flu Shot Last?

A flu shot is designed to cover a single flu season. In the U.S., flu activity often peaks between December and February but can continue into spring. The CDC gives two reasons you need a new shot each year:

Your protection declines over time. Antibody levels drop in the months after vaccination.
Flu viruses keep changing. Each year’s vaccine is updated to match the strains expected to spread.

How Fast Does Protection Fade?

A large Kaiser Permanente Northern California study of nearly 45,000 people who tested positive for flu, reported by CIDRAP, found that the odds of getting the flu rose about 16% for every 28 days after vaccination. People vaccinated more than 154 days earlier had about twice the odds of testing positive compared with people vaccinated recently.

That doesn’t mean the shot stops working after a few months. It still lowers your risk of serious illness, but its protection is strongest in the first few months.

When Should You Get a Flu Shot?

Timing	What the CDC says
September and October	Generally good times for most people to get vaccinated
July or August	Most adults, especially those 65 and older, should generally wait, because protection may fade before the season ends
After October	Still worth getting, since flu can spread well into spring
Two weeks after the shot	Antibodies have developed and protection begins

If you’re also due for a checkup, see our guide on how often you should get a physical.

Does the Flu Shot Make You Tired?

It can. Feeling tired for a day or so is a common, mild reaction. It happens because your immune system is responding to the vaccine and building protection. It doesn’t mean you have the flu.

According to the CDC, common side effects include:

Soreness, redness or swelling where the shot was given
Headache
Low-grade fever
Muscle aches
Tiredness

The CDC says these effects are usually mild and short-lasting, especially compared with the flu itself.

How to Feel Better After Your Flu Shot

Move your arm: gentle movement can ease soreness.
Use a cool compress on the injection site.
Rest and drink fluids if you feel tired or achy.
Ask about pain relievers: check with your doctor or pharmacist about over-the-counter options.

When to Call a Doctor

Serious reactions are rare. Get emergency help if you have signs of a severe allergic reaction, such as trouble breathing, swelling of the face or throat, hives, a fast heartbeat or dizziness, usually within minutes to hours after the shot. Also call your doctor if a fever is high or side effects last more than a couple of days.

Frequently Asked Questions

Can you get the flu from the flu shot?

No. Flu shots are made with inactivated virus or a single viral protein, so they can’t give you the flu. Mild aches or tiredness after the shot are signs of your immune system responding.

How long after the flu shot are you protected?

About two weeks. The CDC says that’s how long it takes for antibodies to develop and provide protection.

Does the flu shot make you tired?

It can. Tiredness, headache and muscle aches are common, mild reactions that usually go away within a day or two.

Should I get the flu shot in August?

Most adults, especially people 65 and older, should generally wait until September or October, because protection may fade before the end of flu season. Getting it early is still better than not getting it at all.

Do I need a flu shot every year?

Yes. Protection fades over time and flu viruses change, so the vaccine is updated every year.

The Bottom Line

How long does the flu shot last? About one flu season. Protection starts roughly two weeks after the shot and slowly fades, so September or October is the best time to get it. Feeling tired afterward is common and usually passes within a day or two. For other seasonal bugs, see our guide to COVID symptoms in 2026.