How Long Do Flu Shot Side Effects Last?

Quick Answer

1–3 days is the typical duration for flu shot side effects. The most common side effect -- arm soreness at the injection site -- begins within hours, peaks at 24–48 hours, and resolves by day 2–3. Systemic side effects like low-grade fever, fatigue, muscle aches, and headache affect about 10–35% of recipients and typically last 1–2 days. These side effects are signs your immune system is building protection, not signs of illness.

Common Side Effects Timeline

Side Effect	Onset	Peak	Duration
Arm soreness/pain	6–12 hours	24–48 hours	2–3 days
Redness at injection site	6–24 hours	24–48 hours	2–3 days
Swelling at injection site	6–24 hours	24–48 hours	2–3 days
Fatigue	6–12 hours	12–24 hours	1–2 days
Headache	6–24 hours	12–24 hours	1–2 days
Muscle aches	6–12 hours	12–36 hours	1–2 days
Low-grade fever	6–24 hours	12–24 hours	1–2 days
Nausea	1–12 hours	12–24 hours	1 day

Injection Site Reactions

Arm soreness is by far the most common side effect, affecting 60–80% of flu shot recipients:

• The pain comes from the immune response at the injection site, not from the needle itself
• Moving your arm and using the muscle actually helps reduce soreness faster
• Applying a cool compress for 10–15 minutes several times a day reduces swelling
• Over-the-counter pain relievers (ibuprofen, acetaminophen) are effective but should not be taken before the shot, as they may reduce the immune response
• Getting the shot in your non-dominant arm minimizes disruption to daily activities
• A small, hard lump at the injection site may persist for 1–2 weeks and is harmless

Systemic Side Effects

About 10–35% of people experience whole-body side effects. These are mild and self-limiting:

Fatigue and malaise: A general feeling of tiredness or being "run down" for 1–2 days. This is your immune system redirecting energy toward building antibodies. Rest and hydration help.

Muscle aches and body pains: Mild, flu-like achiness lasting 1–2 days. This is caused by the inflammatory cytokines your immune system produces in response to the vaccine.

Low-grade fever: A temperature of 99–100.4 degrees F lasting up to 24–48 hours. Fever over 101 degrees F is uncommon and usually resolves within a day. This is a normal immune response.

Headache: Mild to moderate, lasting 1–2 days. Usually responds well to over-the-counter pain relievers.

Side Effects in Children vs. Adults

Children (6 months – 8 years):

• Arm soreness: 1–2 days
• Low-grade fever: more common than in adults, lasting 1–2 days
• Irritability and fussiness: 1–2 days
• Children getting a flu shot for the first time need two doses, 4 weeks apart
• Febrile seizures are rare (about 1 in 1,000 in young children with fever)

Adults:

• Arm soreness: 2–3 days
• Systemic effects: 1–2 days
• Older adults (65+) who receive the high-dose or adjuvanted vaccine may experience slightly stronger side effects, which resolve in the same 1–3 day timeframe

Nasal Spray Vaccine (FluMist) Side Effects

The nasal spray live attenuated vaccine has different side effects:

• Runny nose or congestion: 1–3 days
• Sore throat: 1–2 days
• Cough: 1–3 days
• Mild headache or tiredness: 1–2 days
• No arm soreness (no injection)
• Available for healthy, non-pregnant individuals ages 2–49

Rare but Serious Side Effects

Allergic reaction (anaphylaxis): Extremely rare, occurring in approximately 1.3 per million doses. Symptoms include hives, swelling, difficulty breathing, rapid heartbeat, and dizziness. This occurs within minutes to hours of vaccination, which is why providers ask you to wait 15 minutes after your shot.

Guillain-Barre Syndrome (GBS): An extremely rare neurological condition estimated at 1–2 additional cases per million flu shots. Symptoms include weakness and tingling in the legs that progresses upward. The risk of GBS from the flu itself is higher than the risk from the vaccine.

Shoulder injury (SIRVA): Caused by the needle being placed too high on the shoulder or too deep, damaging the bursa or rotator cuff. Symptoms include persistent shoulder pain and limited range of motion lasting weeks to months. This is an injection technique issue, not a vaccine reaction.

When to Call Your Doctor

• Side effects lasting more than 3 days or getting worse after 48 hours
• Fever above 101.3 degrees F that persists beyond 48 hours
• Signs of allergic reaction (hives, facial swelling, difficulty breathing) -- call 911
• Severe arm pain or inability to move your arm after 3–4 days
• Signs of infection at the injection site (spreading redness, warmth, pus)
• Weakness, numbness, or tingling in the legs (seek immediate medical attention)

Tips for Minimizing Side Effects

• Stay hydrated before and after your shot
• Move your arm frequently after the injection to reduce soreness
• Get vaccinated in your non-dominant arm for comfort
• Apply a cool compress to the injection site for swelling
• Take over-the-counter pain relievers after (not before) the shot if needed
• Plan to rest the evening of your vaccination if you tend to get fatigued
• Avoid strenuous exercise with the vaccinated arm for 24–48 hours