Can the influenza (flu) vaccine cause nausea and diarrhea?

Flu Vaccine and Gastrointestinal Side Effects

Yes, the influenza (flu) vaccine can cause nausea and diarrhea in some individuals, though these side effects are generally mild and self-limiting. According to multiple guidelines from the Advisory Committee on Immunization Practices (ACIP), gastrointestinal symptoms are recognized potential side effects of influenza vaccination.

Common Side Effects of Influenza Vaccine

Local Reactions

• The most frequent side effect is soreness at the vaccination site, affecting 10-64% of patients and typically lasting less than 2 days 1
• These local reactions are usually mild and rarely interfere with a person's ability to conduct daily activities 1

Systemic Reactions

• Systemic symptoms can include:
  • Fever
  • Malaise
  • Myalgia (muscle aches)
  • Nausea and vomiting
  • Diarrhea 1
• These symptoms typically begin 6-12 hours after vaccination and can persist for 1-2 days 1
• Systemic reactions most commonly affect persons who have had no previous exposure to influenza virus antigens in the vaccine, such as young children 1

Gastrointestinal Side Effects Specifically

• Studies have shown that nausea and vomiting were reported more often among adults being treated with oseltamivir (an influenza antiviral) than among persons receiving placebo, and these symptoms might be lessened if oseltamivir is taken with food 1
• In clinical studies of zanamivir (another influenza antiviral), diarrhea and nausea were among the most common adverse effects reported 1
• For the influenza vaccine itself, gastrointestinal symptoms like nausea and diarrhea have been reported, though they are less common than local reactions 1

Important Considerations

• The influenza vaccine contains only noninfectious killed viruses and cannot cause influenza 2
• Respiratory disease or other symptoms after vaccination often represent coincidental illness unrelated to influenza vaccination 1, 2
• In placebo-controlled studies among adults, administration of split-virus influenza vaccine was not associated with higher rates of systemic symptoms when compared with placebo injections 1, 3
• A randomized, double-blind, placebo-controlled trial found no differences between vaccine and placebo groups for systemic symptoms including fever, myalgias, fatigue, malaise, or headaches 3

Risk Factors for Side Effects

• Female sex, age younger than 40 years, and coincidental upper respiratory tract illness are independently associated with higher rates of systemic symptoms 3
• Children may experience higher rates of fever after vaccination compared to adults:
  • 11.5% of children aged 1-5 years
  • 4.6% of children aged 6-10 years
  • 5.1% of children aged 11-15 years 1

Managing Side Effects

• Symptoms are generally mild and self-limiting, resolving within 1-2 days 1
• One study found that acetaminophen significantly reduced the incidence of side effects without affecting the antibody response to the vaccine 4
• Taking acetaminophen at the time of vaccination and at 4,8, and 12 hours afterward may help reduce the incidence of side effects 4

Despite these potential side effects, influenza vaccination remains the most effective strategy to prevent influenza and its complications, especially in high-risk populations 2. The benefits of vaccination in preventing serious illness, hospitalization, and death substantially outweigh the potential for mild side effects in most individuals.