What are the common side effects of the influenza (flu) vaccination?

Common Flu Shot Side Effects

The most common side effect of the influenza vaccine is soreness at the injection site, affecting 10-64% of recipients and typically lasting less than 2 days, followed by systemic symptoms like fever, malaise, and myalgia that occur infrequently and resolve within 1-2 days. 1

Local Reactions at Injection Site

• Soreness, redness, tenderness, or swelling at the injection site is the most frequently reported side effect, occurring in 10-64% of vaccinees 1
• These local reactions typically last less than 2 days and are generally mild, rarely interfering with daily activities 1
• In healthcare personnel studies, injection site soreness was reported in 33.4% of recipients, with skin redness in 18.1% 2
• Local reactions can be managed with cold compresses and acetaminophen, with gentle arm movement to prevent stiffness 3

Systemic Reactions

Fever, malaise, and myalgia can occur but are infrequent, most commonly affecting persons with no previous exposure to the vaccine antigens (such as young children) 1

• These systemic symptoms typically begin 6-12 hours after vaccination and persist for 1-2 days 1
• In controlled trials among healthy adults and elderly persons, split-virus influenza vaccine showed no higher rates of systemic symptoms compared to placebo injections 1
• Healthcare personnel reported myalgia in 17.7%, fatigue in 17%, and febrile sensation in 15.2% after vaccination 2
• Body aches were reported more frequently after vaccination (25.1%) compared to placebo (20.8%) in one controlled trial 1
• Management includes acetaminophen for fever and discomfort, adequate hydration, and rest as needed 3

Gastrointestinal Symptoms

• Nausea and diarrhea have been reported but are less common than local reactions 4
• These gastrointestinal symptoms are recognized by the Advisory Committee on Immunization Practices (ACIP) as potential side effects, though infrequent 4

Rare Serious Adverse Events

Immediate allergic reactions (hives, angioedema, allergic asthma, systemic anaphylaxis) occur rarely and result from hypersensitivity to vaccine components, most commonly residual egg protein 1

• Persons with severe egg allergy who have experienced hives, lip/tongue swelling, or acute respiratory distress after eating eggs should consult a physician before vaccination 1
• Seek immediate medical attention for signs of allergic reactions, persistent high fever, severe swelling extending beyond the injection site, or symptoms persisting beyond 2-3 days 3

Guillain-Barré syndrome (GBS) has not been clearly associated with influenza vaccines since the 1976 swine flu vaccine 1

• If current vaccines pose any GBS risk, it is estimated at approximately one additional case per million persons vaccinated 1
• This minimal risk is substantially less than the risk of severe influenza complications that vaccination prevents 1

Critical Points to Understand

The influenza vaccine contains only noninfectious killed viruses and cannot cause influenza 1, 4

• Respiratory illness occurring after vaccination represents coincidental illness unrelated to the vaccine 1
• In one study, the absolute risk of flu-like illness was only 5.5% higher during the first week post-vaccination compared to the third week, with no corresponding increase in disability 5
• Adverse reactions typically begin within 24 hours in 70.6% of cases, with 89% resolving within 1-3 days 2

Special Populations

Children may experience higher rates of fever compared to adults, with 11.5% of children aged 1-5 years experiencing fever after vaccination 4

Elderly patients (especially those over 90) may warrant more careful monitoring due to decreased physiological reserve, comorbidities, and higher risk of dehydration if systemic symptoms occur 3

Common Pitfalls to Avoid

• Do not mistake normal local inflammatory reactions for infection or prescribe antibiotics for injection site reactions 3
• Do not attribute coincidental respiratory illness to the vaccine when it represents unrelated infection 1
• Do not withhold vaccination due to minor illnesses with or without fever, particularly mild upper respiratory infections or allergic rhinitis 1