Can the influenza (flu) vaccine cause flu-like symptoms?

Can the Flu Vaccine Cause Flu-Like Symptoms?

The inactivated influenza vaccine contains only killed viruses and cannot cause actual influenza infection, but it can produce mild flu-like symptoms in some recipients. 1

Understanding What the Vaccine Can and Cannot Do

The vaccine cannot give you the flu. This is the most important point to emphasize when educating patients. The inactivated vaccine contains noninfectious killed viruses that are incapable of producing an active virus infection. 1

However, the killed vaccine can theoretically produce mild influenza-like symptoms by inducing some of the same cytokines associated with actual influenza disease. 1

Types of Reactions That Can Occur

Local Reactions (Most Common)

• Soreness at the injection site affects 10-64% of patients and typically lasts up to 2 days. 1, 2
• These local reactions are generally mild and rarely interfere with daily activities. 1

Systemic Flu-Like Symptoms (Less Common)

• Fever, malaise, myalgia, and other systemic symptoms can occur, typically beginning 6-12 hours after vaccination and persisting for 1-2 days. 1, 2
• These reactions most often affect persons who have had no prior exposure to the influenza virus antigens in the vaccine, such as young children. 1
• Recent placebo-controlled trials demonstrate that among elderly persons and healthy young adults, split-virus influenza vaccine is not associated with higher rates of systemic symptoms compared with placebo injections. 1

Gastrointestinal Symptoms

• Nausea and diarrhea have been reported after influenza vaccination, though they are less common than local reactions. 2

Important Clinical Context

A critical pitfall is attributing coincidental respiratory illness to the vaccine. Respiratory disease or other symptoms occurring after vaccination often represent coincidental illness unrelated to influenza vaccination. 1, 2, 3

The timing matters: if someone develops full influenza symptoms days after vaccination, this is almost certainly a coincidental infection, as:

• The vaccine takes 2-4 weeks to provide protection 4
• The person may have been exposed to influenza before vaccination or during the window before immunity develops 4

Biological Basis for Symptoms

Recent research demonstrates that subjective reports of local and systemic reactions correspond with biological indicators of inflammatory status. Women reporting significant soreness exhibited greater increases in serum TNF-α and MIF following vaccination, while those with systemic symptoms showed higher IL-6 at baseline and greater increases in MIF. 5

These inflammatory responses are normal and do not reflect actual infection or predict antibody responses. 5

Special Considerations for Live Attenuated Vaccine (LAIV)

For the intranasal live attenuated vaccine, which is different from the standard injectable vaccine:

• LAIV has the potential to produce mild signs or symptoms related to influenza virus infection because it contains live attenuated virus. 1
• The most common adverse events are nasal congestion, headache, myalgias, or fever. 4
• Even with LAIV, only mild and transient upper respiratory symptoms occur compared to placebo. 6

Clinical Bottom Line

When counseling patients, emphasize that:

• The vaccine cannot cause influenza infection 1
• Mild flu-like symptoms lasting 1-2 days can occur but are much less severe than actual influenza 1
• These symptoms represent normal immune responses, not infection 5
• The absolute risk of flu-like illness is approximately 5.5% higher during the first week following vaccination, but these symptoms do not result in decreased ability to perform usual daily activities 7