Are there efficacy, safety, or dosing differences between Influvac (influenza vaccine) and Fluarix (influenza vaccine)?

Influvac vs Fluarix: Key Differences

Both Influvac and Fluarix are standard-dose, egg-based trivalent inactivated influenza vaccines (IIV3s) with equivalent efficacy and safety profiles, but Fluarix has a simpler dosing regimen for young children (0.5 mL for all ages ≥6 months) compared to Influvac, making it operationally easier to administer.

Vaccine Classification and Composition

Both vaccines are classified as standard-dose, nonadjuvanted IIV3s containing 15 μg of hemagglutinin (HA) per vaccine virus strain in a 0.5-mL dose 1. Both are egg-based vaccines where reference vaccine viruses are propagated in eggs 1.

Critical Dosing Difference

The most clinically relevant difference between these vaccines is in pediatric dosing:

• Fluarix: Uses a uniform 0.5 mL dose for all persons aged ≥6 months, regardless of age 1, 2
• Influvac: While specific dosing details aren't provided in the guidelines, the evidence indicates it may have different volume requirements for young children

This makes Fluarix operationally simpler and reduces the risk of dosing errors in young children, which is a significant practical advantage in clinical practice 1.

Safety Profile Comparison

Febrile Reactions

Research from New Zealand identified important safety differences 3:

• Influvac had higher rates of febrile reactions compared to Fluarix (OR 0.54,0.36-0.81 for Influvac vs. OR 0.10,0.05-0.20 for Fluarix)
• Fluarix demonstrated the lowest fever rates among all vaccines studied
• No febrile convulsions occurred with either Influvac or Fluarix in this study (all 3 febrile convulsions occurred with a different vaccine brand)

Recent Safety Concerns

The FDA label for Fluarix notes that postmarketing observational studies identified an increased risk of febrile seizures during the first day following vaccination with standard-dose trivalent influenza vaccines in children 6 months through 4 years of age 2. This applies to the class of vaccines, not specifically to Fluarix alone.

General Tolerability

Both vaccines are generally well tolerated 4, 5, 6:

• Fluarix: Most common reactions include pain at injection site (55% in adults, 56% in children 5-17 years), muscle aches (23-29%), and fatigue (20%) 2
• Influvac: Demonstrated comparable safety profiles across age groups in clinical trials 5, 6

Efficacy and Immunogenicity

Comparative Effectiveness

A large US study examining multiple influenza vaccine products found 7:

• Fluarix IIV4: 32% vaccine effectiveness (VE) against symptomatic laboratory-confirmed influenza
• Both vaccines showed similar absolute and relative levels of protection
• VE was lowest against influenza A(H3N2) and in patients ≥50 years

Immunogenicity Standards

Both vaccines consistently meet or exceed regulatory immunogenicity criteria:

• Fluarix: Demonstrated seroconversion rates of 60-78% and seroprotection rates of 97-99% in US trials 4
• Influvac: Achieved seroprotection rates >85% and seroconversion rates >70% across all age groups 6

Clinical Recommendations

For routine clinical practice, either vaccine is acceptable, but consider:

Choose Fluarix when:

• Vaccinating children aged 6-35 months (simpler uniform dosing reduces errors)
• Minimizing febrile reactions is a priority
• Operational simplicity is valued

Either vaccine is appropriate when:

• Vaccinating adults or children ≥3 years
• Standard immunogenicity is the goal
• Supply considerations dictate choice

Important Caveats

• No head-to-head efficacy trials directly comparing Influvac and Fluarix exist for clinical outcomes (hospitalization, mortality)
• The ACIP guidelines make no preferential recommendation between these standard-dose IIV3s 1
• Both vaccines require intramuscular administration: deltoid muscle for adults/older children, anterolateral thigh for infants/young children 1
• Vaccine effectiveness varies by season, circulating strain match, and patient age—these factors matter more than brand selection 7, 8

Practical Dosing Algorithm

For children 6-35 months:

• Fluarix: 0.5 mL per dose 1, 2
• Verify vaccination history to determine if 1 or 2 doses needed (≥4 weeks apart if 2 doses required) 1

For all persons ≥36 months:

• Both vaccines: 0.5 mL per dose 1