From the Guidelines
The influenza vaccine is associated with several side effects, with soreness at the vaccination site being the most frequent, reported in less than one-third of vaccinees, followed by systemic reactions such as fever, malaise, myalgia, and other symptoms, which occur infrequently, particularly in individuals with no prior exposure to the influenza virus antigens. The side effects of the influenza vaccine, listed in order of frequency, are:
- Soreness at the vaccination site, lasting up to 2 days, reported in less than one-third of vaccinees 1
- Systemic reactions, including:
- Fever
- Malaise
- Myalgia
- Other systemic symptoms, which occur infrequently, particularly in individuals with no prior exposure to the influenza virus antigens, such as young children 1
- Immediate, presumably allergic reactions, such as:
- Hives
- Angioedema
- Allergic asthma
- Systemic anaphylaxis, which occur rarely, likely due to hypersensitivity to residual egg protein in the vaccine 1 It is essential to note that the benefits of flu vaccination in preventing influenza and its complications far outweigh the risks for the vast majority of people, and individuals with severe egg allergies or other concerns should consult their healthcare provider for guidance on vaccination 1.
From the Research
Side Effects of Influenza Vaccine
The side effects of the influenza vaccine can vary in frequency and severity. According to the available evidence, the following side effects have been reported:
- Local side effects, such as pain, redness, and swelling at the injection site, have been reported in approximately 11.3% of participants who received the influenza vaccine, compared to 5.1% of those who received a placebo 2
- Systemic side effects, such as fever, headache, and fatigue, have been reported in approximately 35.1% of individuals who received the influenza vaccine, although this difference was not statistically significant compared to the placebo group 2
- Severe side effects, such as Guillain-Barré syndrome, are rare and have been reported in approximately 1-2 cases per million doses of vaccine administered 3
- Febrile seizures have been reported in children, particularly those under the age of 2 years, although the exact frequency is not well established 4
Comparison of Side Effects by Vaccine Type
Different types of influenza vaccines may have varying rates of side effects. For example:
- High-dose, inactivated, trivalent influenza vaccine has been associated with higher rates of side effects, such as local and systemic reactions, compared to standard-dose vaccine 5
- Live attenuated influenza vaccine may be associated with a slight risk of asthma exacerbations, whereas inactivated trivalent influenza vaccine is not 3
Factors Influencing Side Effects
Several factors can influence the frequency and severity of side effects, including:
- Age: Older adults may be more likely to experience side effects, such as local reactions, although the evidence is not consistent 5, 2
- Underlying medical conditions: Individuals with certain medical conditions, such as egg allergy or immunocompromise, may be at increased risk of side effects or may require special precautions when receiving the vaccine 3
- Vaccine manufacturer: Emerging data suggest that rates of reactogenicity may vary by manufacturer, although more research is needed to confirm this finding 6