Flu Shot: When to Get It, Who Needs It and Side Effects

Flu Shot: When to Get It, Who Needs It and Side Effects

Almost everyone aged 6 months and older should get a flu shot every year, ideally in September or October so protection is in place before flu spreads. The vaccine takes about two weeks to work, side effects are usually mild, and the injected shot cannot give you the flu.

This guide is for adults deciding about their own flu shot, parents weighing it for children, and anyone caring for an older relative or someone with a long-term health condition.

Key takeaways

  • The CDC recommends a flu vaccine every season for everyone 6 months and older, with rare exceptions.
  • September and October are generally the best months to get vaccinated, and the CDC says everyone should ideally be vaccinated by the end of October.
  • Protective antibodies develop about two weeks after vaccination.
  • Flu shots contain inactivated (killed) virus, so they cannot cause flu. Common side effects are a sore arm, headache, mild fever and aches that pass within a few days.
  • CDC estimates that flu vaccination prevented 10 million flu illnesses and 180,000 hospitalizations in the 2024 to 2025 season.

When should you get a flu shot?

Aim for September or October. That timing gives your body the two weeks it needs to build protection before flu season usually picks up.

The CDC's key facts on seasonal flu vaccine say that September and October are generally good times to be vaccinated and that, ideally, everyone should be vaccinated by the end of October. The CDC advises most adults, especially people 65 and older, against getting vaccinated very early in July or August, because protection may fade before the season ends.

Missed the window? Get it anyway. Flu often circulates well into winter and spring. In England, for example, the NHS offers the vaccine through autumn and winter, with a final date of 31 March 2027 for this season, and says it usually takes up to 14 days to work.

You also don't need to space it away from other vaccines. The NHS notes that you can have the flu vaccine at the same time as COVID-19 and shingles vaccines.

Who should get the flu vaccine, and who should not?

Nearly everyone 6 months and older should get one. Only a small group should skip it or check with a doctor first.

Some people face a higher risk of serious flu complications, so vaccination matters even more for them. The NHS eligibility list for free flu vaccines gives a good picture of these higher-risk groups:

  • Adults 65 and older
  • People with long-term conditions such as asthma needing steroid treatment, COPD, heart or kidney disease, or type 1 or type 2 diabetes
  • People with a BMI of 40 or above
  • Pregnant women, at any stage of pregnancy
  • Care home residents and main carers of older or disabled people
  • People living with someone who has a weakened immune system
  • Frontline health and social care workers

According to the CDC's flu vaccine safety page, the people who should not get a flu shot are children younger than 6 months and anyone with a severe, life-threatening allergy to the flu vaccine or one of its ingredients. People with a history of Guillain-Barre syndrome, or who feel unwell on the day, should talk with a doctor first. The NHS adds that anyone with a high temperature should wait until they feel better.

How effective is the flu shot?

The flu shot does not block every infection, but it cuts the risk of getting sick and sharply lowers the chance of severe illness, hospital care and death.

Vaccine effectiveness means how much the vaccine reduces the risk of flu in vaccinated people compared with unvaccinated people in real-world conditions. It changes every season because flu viruses change and the vaccine is updated to match the strains experts expect.

Here's what CDC figures show:

MeasureWhat CDC reports
Doctor visits for fluRisk reduced by 40 to 60% when the vaccine matches circulating viruses (CDC benefits page)
2024 to 2025 season impactAn estimated 10 million illnesses, about 5 million medical visits, 180,000 hospitalizations and 12,000 deaths prevented
Children's deathsVaccinated children were 80% less likely overall to die from flu in CDC surveillance from 2016 to 2017 through 2024 to 2025
Adults hospitalized with fluA 2021 study linked vaccination to a 26% lower risk of ICU admission and a 31% lower risk of death from flu (CDC key facts)
Season to season variationOverall effectiveness estimates were 56% for 2024 to 2025 and 36% for 2025 to 2026, both preliminary (CDC effectiveness studies)

The numbers vary, which is normal. Even in a weaker season, vaccinated people who still catch flu often have a milder illness, a point both the CDC and the NHS make.

What are the side effects of the flu shot?

Most side effects are mild and fade within one to a few days. Serious reactions are rare.

The CDC lists these common reactions to the injected vaccine:

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

The nasal spray version can cause a runny nose, headache, sore throat and cough in adults, and wheezing or vomiting in some children.

Two rare risks get the most attention:

  • Severe allergic reaction. The CDC says these are rare but can be life threatening. Signs include hives, swelling of the face or throat and trouble breathing, and the CDC advises calling 911 if they appear.
  • Guillain-Barre syndrome (GBS). GBS is a rare disorder in which the immune system attacks the nerves, causing weakness. The CDC estimates any extra risk at about one to two additional cases per million doses, when an increase is seen at all. People are more likely to develop GBS after flu illness itself than after the vaccine. If you've had GBS before, our piece on medical decisions after Guillain-Barre syndrome covers why it's worth a careful conversation with your doctor.

Flu shot myths vs facts

Most reasons people skip the flu shot come from a few persistent myths. Here is what the evidence says.

  • Myth: The flu shot gives you the flu. Fact: The CDC says the viruses in flu shots are killed, so you cannot get flu from the shot. Feeling achy for a day is the immune system responding, not infection.
  • Myth: I'm healthy, so I don't need it. Fact: The CDC recommendation covers everyone 6 months and older. Studies cited by the CDC found vaccination reduced the risk of flu death by more than 85% in healthy children.
  • Myth: I got one last year, so I'm covered. Fact: Protection fades and the strains change, so the NHS and CDC both recommend a new vaccine every year.
  • Myth: People with egg allergy can't have it. Fact: The CDC says people with egg allergy may get any flu vaccine, egg-based or not, and no extra precautions beyond those for any vaccine are recommended.
  • Myth: It's not safe in pregnancy. Fact: The CDC says pregnant women should get a flu shot to protect themselves and their babies, and a CDC study found infant flu risk was cut by about one third after maternal vaccination.

If a claim about vaccines spreads quickly on social media, check it against a public health source first. Our article on how fake health news spreads explains why myths travel so fast, and the story of a measles death linked to vaccine refusal shows the stakes.

Your flu shot checklist

  1. Book your shot for September or October, or as soon as you can if it's later in the season.
  2. Tell the person giving the shot about any severe vaccine allergy, past Guillain-Barre syndrome, pregnancy or current fever.
  3. Ask which vaccine type suits your age. Some products are designed specifically for adults 65 and older.
  4. Combine appointments if it helps: COVID-19 and shingles vaccines can be given at the same visit.
  5. Expect a sore arm for a day or two. Moving the arm gently can help.
  6. Keep up everyday protection: hand washing, staying home when sick, and good sleep and nutrition. Our guide to nutrient-rich foods for immune health covers the food side.

Common mistakes

  • Waiting until you hear flu is "going around." By then you still need two weeks for protection to build.
  • Skipping it after a mild reaction last year. A sore arm or one day of aches is not a reason to stop. Discuss true allergic reactions with a doctor.
  • Confusing flu with other viruses. The seasonal flu shot protects against seasonal influenza, not colds, COVID-19 or bird flu strains in animals.
  • Assuming you can't have it while mildly unwell. A minor cold usually isn't a reason to delay. A high temperature is.

When to see a doctor

Call emergency services right away if, after a flu shot, you have:

  • Trouble breathing, wheezing or throat tightness
  • Swelling of the face, lips or tongue, or widespread hives
  • A fast heartbeat, dizziness or weakness

See a doctor promptly if you notice tingling or weakness that starts in your legs and spreads upward in the weeks after vaccination or after a flu illness, or a high fever that lasts more than a couple of days. With flu itself, get urgent help for difficulty breathing, chest pain, confusion, severe weakness, or symptoms that improve and then return worse. Check with a doctor before vaccination if you have had a severe reaction to a previous dose or a history of Guillain-Barre syndrome.

Frequently asked questions

Is it too late to get a flu shot in December or January?

No. The CDC prefers September or October, but flu commonly circulates through winter and into spring, so a later shot still offers protection. The NHS offers vaccination until the end of March for this season. Remember the two-week lag, so getting it sooner rather than later in the winter is better.

How long does a flu shot last?

Protection builds over about two weeks and lasts through a typical flu season, then declines. That's why the CDC advises most older adults against vaccinating in July or August, and why everyone needs a new dose each year with updated strains.

Can I get the flu shot if I have a cold?

Usually yes. A mild illness like a cold is not normally a reason to delay. The NHS advises waiting if you have a high temperature, and the CDC suggests talking with a doctor if you're not feeling well on the day. When in doubt, ask the clinic before your appointment.

Why do I feel tired after a flu shot?

Tiredness, mild aches and a low fever are signs your immune system is responding. The CDC lists them as common side effects that usually go away on their own within a few days. Rest, fluids and gentle movement help. Get checked if symptoms are severe or last longer.

What to do next

  1. Check whether you're eligible for a free or covered flu vaccine through your doctor, pharmacy or workplace, and book a date this week.
  2. If you care for an older relative or a baby under 6 months, make sure everyone around them is vaccinated.
  3. Write down any past vaccine reactions so you can mention them at the appointment.
  4. Set a yearly reminder for September so next season's shot happens on time.

This article is for general information and is not medical advice. Talk with your doctor or pharmacist about which flu vaccine is right for you.

Sources

Last reviewed: October 2026.

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