Can the Flu Shot Make You Sick?
No, the inactivated influenza vaccine cannot cause influenza because it contains only noninfectious, killed viruses that are incapable of replication. 1, 2, 3
Why the Vaccine Cannot Cause Influenza
The Advisory Committee on Immunization Practices (ACIP) explicitly states that because influenza vaccine contains only noninfectious viruses, it cannot cause influenza. 1 Any respiratory illness that occurs after vaccination represents coincidental illness unrelated to the vaccine itself. 1, 2, 4
What Side Effects Actually Occur
Local Reactions (Most Common)
- Soreness at the injection site is the most frequent side effect, reported by fewer than one-third of vaccinees and lasting up to 2 days. 1, 2, 3
- In a placebo-controlled trial, 63.8% of vaccine recipients reported arm soreness compared to 24.1% of placebo recipients, but these reactions were mild and infrequently resulted in decreased arm use. 5
Systemic Symptoms (Less Common)
- Fever, malaise, myalgia, and other systemic symptoms occur infrequently, most often affecting persons who have had no prior exposure to influenza virus antigens (such as young children). 1, 2
- These reactions begin 6-12 hours after vaccination and persist for 1-2 days. 1, 2
- Importantly, a randomized placebo-controlled trial found no difference in systemic symptoms between vaccine and placebo recipients (35.2% placebo vs 34.1% vaccine, p=0.78). 5
Gastrointestinal Symptoms
- Nausea and diarrhea have been reported but are less common than local reactions. 2
- These symptoms typically occur within 6-12 hours of vaccination and resolve within 1-2 days. 2
Common Pitfalls to Avoid
The most important clinical pitfall is attributing coincidental respiratory illness to the vaccine. When patients develop influenza-like symptoms after vaccination, this represents:
- Coincidental infection with influenza or other respiratory viruses that occurred around the time of vaccination 1, 4
- The vaccine takes 2-4 weeks to provide full protection, so infection can occur during this window 6
- Respiratory viruses circulating in the community are unrelated to vaccine administration 4
Risk Factors for Side Effects
Certain populations experience higher rates of side effects:
- Female sex (odds ratio 1.5) 5
- Age younger than 40 years (odds ratio 1.6) 5
- Children aged 1-5 years experience fever in 11.5% of cases, compared to 4.6% in ages 6-10 and 5.1% in ages 11-15 2
Special Considerations
Pregnancy
- Influenza vaccination during pregnancy is safe and does not cause adverse fetal effects. 1, 7
- The vaccine can be administered during any trimester. 7
Allergic Reactions
- Immediate allergic reactions (hives, angioedema, anaphylaxis) occur rarely. 1
- Persons with severe egg allergies should consult a physician before vaccination due to residual egg protein in the vaccine. 1, 3