When to get a flu shot in 2026: the best time, side effects, and myths vs facts

Short answer

When to get a flu shot for the 2026–27 season: for most adults, September or October. The CDC's interim guidance for this season, published September 1, 2026, says vaccination "should ideally be offered during September or October," advises most adults, particularly those 65 and older, to avoid July and August, and says vaccination should continue throughout the season (1). Protection takes about two weeks to build (2) and slowly fades over the following months (3), so a fall shot covers the winter peak best.

If you missed October, it is usually not too late: a shot in November, December or January still protects you for the rest of a season that most often peaks between December and February (4). The flu shot cannot give you the flu (2). The common side effects are a sore arm and, for some people, short-lived aches or a low fever.

If you felt rotten after a past flu shot, you're not imagining it, and being wary is not weakness. Below, we separate what the shot causes from what merely coincides with it.

Note: this article explains what research and guidelines say and is not medical advice; your doctor or pharmacist decides what fits you. Trouble breathing, swelling of the face or lips, hives or a racing heart in the minutes to hours after a shot means calling 911. With flu, difficulty breathing, chest pain or pressure, confusion, seizures, not urinating, or a fever or cough that improves and then returns worse means urgent care or 911.

When to get a flu shot in 2026

For most adults: in September or October, before flu spreads widely where you live. The CDC's 2026–27 interim considerations keep the July 2025 schedule's flu recommendations in effect, with one dose of the current trivalent (three-strain) vaccine each year (1). The CDC's public guidance puts it simply: "Ideally, everyone should be vaccinated by the end of October" (2).

When is the best time to get a flu shot in 2026?

September through October is the window most guidance points to. Two facts set it. Your body needs about two weeks after the shot to make antibodies (2), and US flu activity "most of the time" peaks between December and February (4). A shot in October is fully working by November and is still near its strongest when the peak arrives.

Children follow separate recommendations, including two doses for some younger children, so their timing can differ (1); ask your child's pediatrician. (See also: our guide to the flu shot for kids by age.)

Why not get the flu shot in July or August?

Because protection fades over the season, and an August shot may be weaker by February. That is the CDC's stated reason: possible waning of vaccine-induced immunity over the season. For most adults, particularly those 65 and older, it says July and August vaccination should be avoided unless a later shot might not be possible (1).

The evidence for waning:

• US Flu Vaccine Effectiveness Network, four seasons: effectiveness peaked shortly after vaccination, then fell about 7 percentage points a month for H3N2 and influenza B and 6–11 points for H1N1, staying above zero for at least five to six months. The authors note bias could explain part of it (3).

• Kaiser Permanente Northern California, seven seasons: the odds of testing positive rose about 16% for every extra 28 days since vaccination (5).

• Meta-analysis of 14 studies: effectiveness dropped significantly between days 15–90 and 91–180 for H3N2 and influenza B, not significantly for H1N1 (6).

The trade-off with waiting. A CDC model of adults 65 and older found that delaying everyone to October would increase hospitalizations if more than 14% of people who usually get vaccinated in August or September then missed their shot (7). Don't rush into August, and don't skip the shot waiting for the perfect week. The UK starts adults on October 1, citing "evidence of the waning of flu vaccine's effectiveness in adults" (8).

Who recommends what for adults in 2026–27

Why two US rows: the federal advisory process has been changing since 2025, and as of October 2026 the CDC keeps the July 2025 recommendations in effect, citing legal uncertainties (1). In September 2026, four physician organizations issued their own recommendations, informed by an evidence review from the University of Minnesota's Vaccine Integrity Project (10). For adults, both recommend a yearly shot and an enhanced vaccine at 65 and older.

How long does it take for the flu shot to work?

About two weeks. "It takes about two weeks after vaccination for antibodies to develop," the CDC says (2). Until then you have little or no vaccine protection, which is why the timing advice aims to finish vaccination before flu circulates widely.

How long does a flu shot last?

For one season, strongest in the first weeks and slowly declining, yet still measurable for at least five to six months in US data (3). The shot is yearly because protection wanes and the viruses change (2). For 2026–27, all three strains were updated, including a new H3N2 component after a variant called subclade K spread widely in 2025 (13).

Is it too late to get a flu shot?

Usually not. The CDC says vaccination "should continue after October and throughout the influenza season" (1). In the UK, adults can get the 2026–27 shot until March 31, 2027 (12).

What if flu season is already under way?

You still gain protection for the weeks and months that remain, which often include the peak, since flu usually peaks between December and February (4). What you lose is the two weeks it takes the vaccine to start working (2): exposures in that gap are not covered. A December shot is less useful than an October one and much more useful than none.

I got the flu shot while already incubating the flu: what happens?

The shot won't stop an infection that has already started, since protection needs about two weeks, and it cannot add flu, because shots contain inactivated virus or a virus protein (2). We found no study showing that a shot given during the incubation period makes flu worse.

Coincidence is common. In a placebo-controlled trial of 849 working adults, a respiratory illness that happened to start around the injection was strongly linked to aches and fatigue afterward (odds ratio 4.6); the vaccine was not (14). The CDC defers shots during moderate or severe illness to avoid "diagnostic confusion" and stacking side effects on the illness (15), not because the vaccine worsens infection.

If flu symptoms start soon after your shot, treat it as possible flu: vaccination history "does not rule out" influenza, and the CDC recommends antiviral treatment as soon as possible for people who are hospitalized, severely ill or at higher risk (16); your clinician decides. Trouble breathing, chest pain, confusion, or symptoms that improve and then return worse mean urgent care or 911 (17).

Should I get a flu shot? What the numbers say

US and international guidance recommend a yearly flu shot for adults, with the strongest case for people at higher risk (1, 11)). Whether you get one is your decision with your doctor or pharmacist; here is what the evidence shows, including its limits.

"If I don't get vaccinated, I may not get sick anyway"

True for most people in most years. The seasonal attack rate is often quoted as 5–20%; a CDC analysis estimated that about 8% of Americans, vaccinated or not, got symptomatic flu each season in 2010–2016, ranging from 3% to 11% (18).

The Cochrane review of randomized trials in healthy adults aged 16 to 65 gives the absolute benefit: inactivated vaccines probably cut confirmed flu from 2.3% to 0.9%, so 71 people need vaccinating to prevent one case, and flu-like illness from 21.5% to 18.1%. Evidence on hospitalization in healthy adults was low-certainty (19).

The larger effect is on severe outcomes, measured in observational studies. In the high-severity 2024–25 season, among US adults hospitalized with respiratory illness, vaccination was 40% effective against flu hospitalization, 58% against ICU admission and 52% against death (20). The CDC estimates that season brought 710,000 flu hospitalizations and 45,000 deaths, and that vaccination prevented about 180,000 hospitalizations and 12,000 deaths (21). Among adults hospitalized with flu, vaccinated patients had 31% lower risk of death (22). Such designs can't fully exclude other differences between groups.

So without a shot your chance of avoiding flu is decent; the shot lowers it further and is linked to milder illness if you do get sick. The case is strongest at 65 and older, in pregnancy, and with chronic heart, lung, kidney or metabolic conditions.

Is the flu shot effective this year?

No 2026–27 estimate exists yet; first US interim estimates usually come out in late winter. Last season was a weaker year: interim 2025–26 effectiveness in adults was 22–34% against outpatient flu and 30% against flu hospitalization, lower than in recent seasons (23). For 2026–27, all three vaccine strains were updated, including a new H3N2 component chosen after the subclade K variant dominated last season (13). Whether that improves the match will only be known once the season is measured.

Flu shot side effects: what's common and how long they last

A sore arm is the most common one; aches, headache, tiredness, nausea or a low fever are possible, generally mild, and gone within a few days (2, 24). (See also: side effects of other vaccines, from shingles to RSV.)

How often they happen, from controlled data:

• In the 849-adult placebo-controlled trial, 63.8% of vaccine recipients reported arm soreness versus 24.1% with placebo. Systemic symptoms (fever, muscle aches, fatigue, malaise, headache) were reported by 34.1% after the vaccine and 35.2% after placebo, no real difference (14).

• Across randomized trials in the Cochrane review, inactivated vaccines raised fever from 1.5% to 2.3% (19).

• In an Israeli cohort that reported symptoms daily, 66.9% had no systemic reaction at all after a flu shot (25).

Can you get the flu from the flu shot?

No. "A flu vaccine cannot cause flu," the CDC states: shots are made with inactivated viruses or a single virus protein (2). A day of aches is the immune response, not influenza.

Does the flu shot make you sick?

It can make you feel off for a day or two, but not with flu. The placebo-controlled trial above found the same rate of systemic symptoms with vaccine and placebo, and the strongest link to feeling ill afterward was a coincidental cold (14). Shots are given in fall, when colds circulate, so some illness after vaccination is timing.

Does the flu shot make you tired?

Some people feel tired for a day or so; the CDC lists fatigue among possible side effects (2). In the controlled trial, fatigue was no more common after the vaccine than after placebo (14).

Rare but real: allergic reactions, fainting and Guillain-Barré syndrome

• Severe allergic reaction: difficulty breathing, hoarseness or wheezing, swelling around the eyes or lips, hives, paleness, a fast heartbeat or dizziness, most likely within minutes to a few hours. Call 911 (24).

• Fainting can occasionally follow any injection (24); if it has happened before, say so and sit for the shot.

• Guillain-Barré syndrome (GBS), a rare nerve disorder: if flu vaccines raise its risk, the CDC says the increase is small, about one to two extra cases per million doses (26). In Ontario data, the attributable risk was 1.03 GBS hospitalizations per million vaccinations versus 17.2 per million flu-related health-care visits (2770104-X)).

• Bell's palsy: one Israeli health-records study linked flu vaccination to about 1.3 additional cases of this one-sided facial weakness per 10,000 people, with no increase in the other adverse events examined (25). That is a single study's association, not proof of cause. Sudden facial drooping can also be a stroke sign: call 911 (38).

What to avoid after a flu shot

Very little. No CDC or NHS guidance we reviewed lists foods, drinks or activities to avoid after a flu shot. What research supports:

• Exercise is fine if you feel well. The American College of Cardiology, writing about COVID-19 vaccines, said vaccination itself "does not require a set time away from exercise training" (28). If you're achy or feverish, use the logic in working out while sick.

• Alcohol: we found no guideline and no human trial showing that a drink after a flu shot reduces protection.

• Short sleep around the shot: in a meta-analysis, objectively measured sleep under six hours a night around vaccination was associated with a markedly weaker antibody response in healthy adults, mainly in men (29). If you can, pick a week when you can sleep normally; see sleep debt: can you catch up.

• Time of day: in a UK trial of 276 adults 65 and older, morning shots produced higher antibody responses to two of three strains than afternoon shots (30). That measured antibodies, not illness; any convenient time beats a missed shot.

• Pain relievers: for a sore arm or aches afterward, ask your pharmacist what suits you and your other medicines.

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

Usually yes. CDC immunization guidance lists "mild acute illness with or without fever" among conditions commonly mistaken for reasons not to vaccinate, and says the safety and efficacy of vaccinating people with mild illness "have been documented" (15). A runny nose or slight sore throat is not a reason to skip it.

Flu shot while sick with a fever

Postpone it if you're moderately or severely ill. The CDC says "vaccination should be deferred for persons with a moderate or severe acute illness," with or without fever, so that illness symptoms aren't confused with side effects (15, 1). Get vaccinated once you've recovered. If your illness could be flu and you're at higher risk, call your clinician, since the CDC recommends antiviral treatment as soon as possible for higher-risk patients (16).

Flu and COVID vaccine: can you get both at once?

Yes, if you're eligible for both. "People can get a flu vaccine and COVID-19 vaccine at the same visit," the CDC says; people who got both were slightly more likely to have fatigue, headache and muscle aches, "mostly mild" (31). In the UK ComFluCOV randomized trial of 679 adults, giving a flu vaccine alongside a COVID-19 vaccine raised no safety concerns and preserved antibody responses to both (3202329-1)). Whether you need a COVID-19 vaccine this fall is a separate question for your doctor or pharmacist.

Flu shot for seniors: the high-dose flu vaccine and other options

At 65 and older, the CDC recommends preferentially a high-dose, recombinant or adjuvanted vaccine (1); in the US these are sold as Fluzone High-Dose, Flublok and Fluad. The AAFP adds mRNA flu vaccines when available, and says people should not delay vaccination to get a specific product (9).

What the trials show, in absolute terms:

• High-dose vs standard-dose, 31,989 adults 65+ in the US and Canada: confirmed flu in 1.4% vs 1.9%, a relative efficacy of 24.2% (33).

• Pooled Danish and Spanish trials, 466,320 adults: hospitalization for flu or pneumonia 0.56% vs 0.62% (relative effectiveness 8.8%), and laboratory-confirmed flu hospitalization 0.11% vs 0.16% (31.9%). Deaths were similar in both groups (3401742-8)).

Both trials were funded by the high-dose vaccine's manufacturer. The gain over a standard shot is modest but measurable.

Who should not get the senior flu shot?

People who have had a severe allergic reaction, such as anaphylaxis, to a previous dose of flu vaccine or to a component of that vaccine; each product has its own contraindications, listed by the CDC (1). A prior case of GBS after a flu shot is a precaution, not an automatic no. Your doctor or pharmacist checks this before the shot.

Flu shot while pregnant

The CDC says a flu shot can be given "at any time during pregnancy," in any trimester; pregnant women should get an inactivated or recombinant shot, not the nasal spray (1). In the third trimester, the CDC allows an earlier July or August shot because it can protect the baby during the first months of life (1). ACOG also recommends flu vaccination during pregnancy and postpartum (10).

In a randomized trial in Bangladesh, infants of mothers vaccinated during pregnancy had 63% fewer laboratory-confirmed flu cases in their first six months, and mothers had 36% fewer feverish respiratory illnesses (35). Cochrane rates protection in pregnancy as modest and found no link with miscarriage or newborn death in observational data (19). Your obstetric clinician can advise on timing.

Flu shot myths vs facts

Does the flu shot weaken your immune system?

No good evidence says so. The idea traces partly to a 2012 trial of 115 children in Hong Kong, where vaccinated children had more non-flu respiratory infections over nine months (36). A larger US analysis of children under five and adults 50 and older with respiratory illness, across six seasons, found no link between flu vaccination and detection of other respiratory viruses (37). The vaccine trains a specific response to flu; it does not use up your defenses.

Is the flu shot a live vaccine?

No. Flu shots contain inactivated virus or a single virus protein. The nasal spray (FluMist) is the live, weakened option; its viruses can't reproduce in the lungs (2). The spray is not used in pregnancy or for people with weakened immune systems.

Can I get a flu shot with an egg allergy?

Yes. The CDC says people with egg allergy "may receive any influenza vaccine" otherwise appropriate for their age and health (1).

Living with flu season: the questions people ask next

Can I work out after a flu shot?

Yes, if you feel fine. Vaccination itself does not require time away from exercise, according to the American College of Cardiology's guidance on COVID-19 vaccines (28). If the flu shot leaves you with fever or body aches, skip hard training until they pass, which usually takes no more than a few days (24). Chest pain, fainting or severe shortness of breath at any point means 911. See working out while sick.

Why do I keep getting sick even though I had a flu shot?

Because the flu shot protects against influenza only. Colds, RSV, COVID-19 and other viruses circulate at the same time, and in fall many people catch one around their vaccination date (14). The shot also isn't perfect against flu itself: in 2025–26 its effectiveness in adults was about 22–34% against outpatient flu (23). Frequent infections have many possible causes; why do I keep getting sick walks through them and when to check.

My resting heart rate went up after my flu shot. Is that normal?

A brief rise is possible, especially with fever, but on average flu shots seem to change it little. In a study of Israeli adults wearing Garmin smartwatches, no statistically significant change in heart rate or an HRV-based stress score was found after flu vaccination, while COVID-19 boosters produced a larger rise in the first two days (25). If your resting heart rate stays up for days, or comes with palpitations, dizziness or breathlessness, call your doctor; high resting heart rate causes explains other reasons it climbs.

Do I need a flu shot every year?

Yes, according to the CDC, for two reasons: protection declines over time, and flu viruses change, so the vaccine is reformulated each year (2). For 2026–27 all three strains were updated (13).

I had a bad reaction last year. Should I skip it this year?

Not necessarily; tell the person vaccinating you what happened. Sore arms, aches and a day of fever are expected reactions, not reasons to stop. A severe allergic reaction to a previous flu vaccine is a contraindication for many products, and GBS within six weeks of a previous dose is a precaution (1). In one study, people with stronger reactions to a flu shot also tended to react more to a COVID-19 booster (25); planning a quieter day after the shot is reasonable.

How to bring this up with your doctor or pharmacist

What to say. "I'd like this season's flu shot. Does anything about my health change which one I should get, or when?"

What to mention. Your age; pregnancy; heart, lung, kidney, liver or metabolic conditions; a weakened immune system or medicines that suppress it; a severe allergic reaction to a previous vaccine; Guillain-Barré syndrome after a past flu shot; fainting with injections; and whether you're sick today.

What to ask.

• "Which vaccine fits me this year? Is a high-dose, adjuvanted or recombinant one available?" (65 and older)

• "Can I get my COVID-19 or other vaccines at the same visit?"

• "If I get flu symptoms this winter, should I contact you early about antiviral treatment?"

When to seek care. After a shot, trouble breathing, swelling of the face or lips, hives or a racing heart means 911 (24). With flu, difficulty breathing, persistent chest or abdominal pain, persistent dizziness or confusion, seizures, not urinating, severe weakness, or fever or cough that improves and then returns worse needs urgent care (17).

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How Welltory helps — and what it cannot do

The limits first. Welltory is a general wellness product, not a medical device. It does not diagnose, detect, predict, monitor, prevent, treat or mitigate any condition, including flu, vaccine reactions or Guillain-Barré syndrome. It can't tell you whether you have flu, whether your vaccine worked, or when to get one.

You note the shot; the app records the rest. Add a tag from the plus sign at the top of the feed ("flu shot", "sore arm", "achy"). When the Today screen marks a stress or rest stretch, it asks "What happened?" or "What helped you rest?"; tap a suggested tag, type, or just talk, and Welltory picks out the tags, which you can correct before saving.

Look at the next night against your own history. Sleep analysis shows how your heart rate changed overnight and whether it ran higher than usual; resting heart rate, HRV, stress minutes and Battery are recorded the same way every day (Battery and stress minutes need iOS with an Apple Watch or Oura; detailed sleep analysis needs an Apple Watch). In the smartwatch study above, flu shots on average didn't measurably shift heart rate or an HRV-based stress score, unlike COVID-19 boosters (25), so your numbers may look unchanged. A night that looks different is your own observation, not a verdict.

Over the season, My Patterns shows what your tags have in common. My Patterns collects tags from stress and rest stretches (iOS with an Apple Watch or Oura), and patterns become visible at around seven tagged events in the current month. One shot a year won't make a pattern; tagged colds, short nights and busy weeks across fall might.

A record you made, for an appointment if you need one. The Journal (Premium) keeps your measurements, tags and notes; from the web app you can export a CSV (Dashboard → choose a chart → Export). A dated "flu shot, achy two days" helps when you're asked about last year's reaction. Any pattern is an association to discuss, not a cause.

The same red flags apply here, in the same type size: trouble breathing, facial or lip swelling, hives or a racing heart after a shot means 911; with flu, difficulty breathing, chest pain, confusion, seizures, or symptoms that improve and then return worse mean urgent care or 911 (17, 24).

Where to find other people in the same situation. If you are also living with an energy-limiting condition, Welltory runs a paid, moderated community called Energy Lab for women aged 18 to 65 living with ME/CFS, long COVID, fibromyalgia, POTS, MCAS and similar. It runs alongside the app: you keep collecting your own data, and the Lab is where people learn to read it together, with a medical board answering the science. It is education and peer support, not medical care, it does not replace your clinician, and there is a 14-day money-back guarantee on a first purchase.

How we made it

Made with AI tools, then edited and fact-checked by the Welltory team. See our Editorial & AI policy.

Data analysis by Jane Smorodnikova, co-founder of Welltory and the person who built the methodology behind how we read physiological data.

Written by Tatsiana Yashyna.

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