What are the risks of the live Flumist (influenza vaccine, live, attenuated) versus the injectable (influenza vaccine, inactivated) vaccine?

Risk Comparison: Live Attenuated (FluMist) vs Injectable Influenza Vaccine

The injectable inactivated influenza vaccine (IIV) is safer than live attenuated influenza vaccine (LAIV/FluMist) for high-risk populations, while LAIV carries specific contraindications related to viral shedding, immunocompromise, and respiratory disease that make it unsuitable for many patients who would benefit most from influenza prevention.

Key Safety Distinctions

Live Attenuated Vaccine (LAIV/FluMist) Specific Risks

LAIV is contraindicated in multiple high-risk populations where IIV remains safe and indicated:

• Immunocompromised patients (including transplant recipients, HIV-positive individuals, and those on immunosuppressive therapy) should never receive LAIV due to theoretical risk of disseminated vaccine virus infection 1
• Patients with cardiovascular disease should not receive LAIV because it can cause influenza illness in this high-risk population, whereas IIV is specifically recommended for CVD patients 1
• Persons with asthma, reactive airways disease, or chronic pulmonary/cardiovascular disorders are contraindicated from receiving LAIV 1
• Children under 2 years and adults over 49 years cannot receive LAIV, while IIV is approved for persons ≥6 months 1, 2

Viral Shedding Concerns with LAIV

A critical safety distinction is that LAIV recipients shed live vaccine virus, creating transmission risks:

• Vaccine virus can be shed for 2-7 days after LAIV administration 1, 2
• Healthcare workers and household contacts of severely immunosuppressed patients should receive IIV preferentially, not LAIV 1, 2
• If LAIV is administered to contacts of immunosuppressed persons, they should refrain from contact with severely immunosuppressed patients for 7 days post-vaccination 1
• IIV contains only inactivated virus and causes no viral shedding, making it safer for contacts of vulnerable populations 1

Respiratory Safety Profile

LAIV carries specific respiratory risks not seen with IIV:

• Increased wheezing and asthma exacerbations have been documented in young children (18-35 months) receiving LAIV 1, 3
• Children and adolescents receiving aspirin or salicylates are contraindicated from LAIV due to Reye syndrome risk with live influenza virus 1
• IIV cannot cause asthma exacerbations, while LAIV may pose slight risk 4
• Common LAIV side effects include runny nose/nasal congestion (20-78%), headache (2-46%), and in children, fever (0-26%) and vomiting (3-13%) 1

Injectable Vaccine (IIV) Safety Profile

IIV has a well-established safety record with minimal contraindications:

• Most common adverse events are local reactions: sore arm and injection site redness 5
• Systemic symptoms (fever, malaise) are less common than with LAIV 5
• Guillain-Barré Syndrome (GBS) risk is extremely low: estimated at most 1-2 cases per 1 million vaccinees, far less than the risk of severe influenza complications 1, 5
• IIV is safe in immunocompromised patients, including transplant recipients, where it is specifically recommended 1, 6

Contraindications Summary

LAIV Absolute Contraindications 1:

• Age <2 years or >49 years
• Immunodeficiency or immunosuppressive therapy
• Asthma, reactive airways disease, or chronic cardiopulmonary disorders
• Pregnancy
• Children/adolescents on aspirin therapy
• History of Guillain-Barré syndrome
• Severe egg allergy (should receive IIV instead) 1

IIV Contraindications 1:

• Anaphylactic hypersensitivity to eggs (though persons with mild egg allergy experiencing only hives can receive IIV with precautions) 1
• Severe allergic reaction to vaccine components
• GBS within 6 weeks of previous influenza vaccination is a precaution (not absolute contraindication) 1

Clinical Context for Decision-Making

For transplant recipients and severely immunocompromised patients:

• LAIV is absolutely contraindicated posttransplant 1
• Standard-dose IIV (15 μg antigen/virus) is the recommended formulation 1
• Household contacts should preferentially receive IIV rather than LAIV 1, 2

For patients with chronic medical conditions:

• IIV is the appropriate choice for all patients with diabetes, renal dysfunction, hemoglobinopathies, or cardiovascular disease 1
• These populations have higher mortality from influenza and require the proven safety profile of IIV 1

For healthy persons aged 5-49 years:

• Both vaccines are options, with no preferential recommendation when both are appropriate 1
• LAIV offers intranasal administration advantage but requires careful screening for contraindications 1

Critical Pitfalls to Avoid

• Never assume LAIV is "safer" because it avoids injection - the live virus nature creates multiple serious contraindications 1, 2
• Do not give LAIV to anyone with even mild immunocompromise - this includes patients on low-dose corticosteroids for asthma 1
• Screen carefully for household contacts of immunosuppressed persons before administering LAIV 1, 2
• Remember that cardiovascular disease is a contraindication to LAIV - this is frequently overlooked 1