What are normal reactions after receiving an influenza (flu) vaccination?

Normal Reactions After Receiving a Flu Shot

The most common normal reactions after receiving a flu shot include soreness at the injection site lasting up to 2 days, affecting 10-64% of recipients, as well as mild systemic symptoms like fever, malaise, and myalgia that typically begin 6-12 hours after vaccination and persist for 1-2 days. 1

Local Reactions

• Soreness at the vaccination site is the most frequent side effect, typically lasting less than 2 days and reported by 10-64% of vaccine recipients 1
• Local reactions are generally mild and rarely interfere with a person's ability to conduct everyday activities 1
• Redness and swelling at the injection site may also occur 2
• In a controlled trial, body aches (25.1%) were reported more frequently after inactivated influenza vaccine than placebo injection (20.8%) 1

Systemic Reactions

• Fever, malaise, myalgia, and other systemic symptoms can occur after vaccination with inactivated vaccine 1
• These systemic reactions typically begin 6-12 hours after vaccination and can persist for 1-2 days 1
• Systemic symptoms most often affect persons who have had no previous exposure to the influenza virus antigens in the vaccine (e.g., young children) 1
• Among older persons and healthy young adults, administration of split-virus influenza vaccine is not associated with higher rates of systemic symptoms compared with placebo injections 1
• Children may experience higher rates of fever after vaccination compared to adults, with 11.5% of children aged 1-5 years, 4.6% of children aged 6-10 years, and 5.1% of children aged 11-15 years experiencing fever 1, 3

Important Facts About Flu Vaccine Reactions

• The influenza vaccine contains only noninfectious killed viruses and cannot cause influenza 1
• Respiratory disease or other symptoms after vaccination often represent coincidental illness unrelated to influenza vaccination 1
• Gastrointestinal symptoms like nausea and diarrhea have been reported after influenza vaccination, though they are less common than local reactions 3
• In a study of healthy working adults, no differences were seen between vaccine and placebo groups for systemic symptoms of fever, myalgias, fatigue, malaise, or headaches 4
• Women reporting being very sore at the injection site exhibited greater increases in certain inflammatory markers (serum TNF-α and MIF) in the days following vaccination compared to those with no or mild soreness 5

When to Seek Medical Attention

• Immediate allergic reactions (such as hives, angioedema, allergic asthma, or systemic anaphylaxis) occur rarely after influenza vaccination 1
• These reactions probably result from hypersensitivity to certain vaccine components; the majority are most likely related to residual egg protein 1
• Persons who have had hives or swelling of the lips or tongue, or who have experienced acute respiratory distress or collapse after eating eggs should consult a physician for appropriate evaluation 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 2

Management of Normal Side Effects

• For local reactions, applying cold compresses can help reduce pain and swelling 2
• Rest the affected arm but maintain gentle movement to prevent stiffness 2
• Acetaminophen can be used for pain relief and fever if needed 2
• Ensure adequate hydration, especially if experiencing systemic symptoms 2

Common Pitfalls to Avoid

• Mistaking normal local reactions for infection or allergic reactions 2
• Using antibiotics for inflammatory local reactions 2
• Assuming that post-vaccination symptoms are caused by the vaccine when they may be due to coincidental illness 1