What are the indications, dosing schedule, administration route, contraindications, and safety profile of Influvac (inactivated split‑virion influenza vaccine) for individuals aged 6 months and older, including pregnant women?

Influvac Vaccine: Clinical Overview

Indications

Influvac is indicated for annual influenza prophylaxis in all individuals aged ≥6 months, including pregnant women, with no upper age limit. 1

• Universal vaccination is recommended for all persons ≥6 months who do not have contraindications, regardless of risk status 1
• Priority groups requiring special outreach include:
  • Children aged 6 months through 8 years (especially those <2 years who have higher hospitalization rates) 1, 2
  • Adults ≥50 years 1
  • Pregnant women at any stage of pregnancy 1
  • Persons with chronic medical conditions (asthma, diabetes, cardiovascular disease, immunosuppression, neurologic disorders) 1
  • Healthcare workers and household contacts of high-risk individuals 1

Dosing Schedule

Most individuals require only ONE dose per influenza season. 3 The critical exception is children aged 6 months through 8 years receiving influenza vaccine for the first time or who received only one dose in previous seasons—these children require TWO doses administered ≥4 weeks apart. 1, 3, 2

Specific Dosing by Age:

• Children 6-35 months: 0.25 mL or 0.5 mL (depending on product formulation) 1
• Children ≥36 months and adults: 0.5 mL 1
• Two-dose requirement: Children <9 years who are vaccine-naïve or received only one prior dose need two doses ≥4 weeks apart; the second dose should ideally be administered before December 1, 3
• Annual revaccination is mandatory because immunity declines within one year and viral strains change annually 1, 3

Critical Point on Repeat Dosing:

Administering multiple doses of the same seasonal vaccine formulation within a single season provides NO additional benefit and is NOT recommended, even in high-risk patients. 3 Studies show limited or no improvement in antibody response with repeat dosing during the same season. 1

Administration Route

Influvac is administered via intramuscular (IM) injection. 1

• Adults and older children: Deltoid muscle 1
• Infants and young children: Anterolateral aspect of the thigh 1
• Needle length: Use needles >1 inch for adults and older children to ensure adequate muscle penetration 1

Contraindications

Absolute contraindication: Anaphylactic hypersensitivity to eggs or other vaccine components 1

However, recent guidance has evolved: Persons with egg allergy of any severity may receive any licensed, age-appropriate influenza vaccine (including Influvac) without special precautions beyond those for any vaccine. 1 For those with severe egg allergy and high risk for influenza complications, appropriate allergy evaluation and desensitization may be considered. 1

Precautions (NOT absolute contraindications):

• Acute febrile illness: Defer vaccination until symptoms abate, though minor illnesses with or without fever do NOT contraindicate vaccination 1
• History of Guillain-Barré Syndrome (GBS): The risk of vaccine-associated GBS is estimated at 1-2 cases per 1 million vaccinees, which is substantially lower than the risk from influenza infection itself 4

Safety Profile

Influvac demonstrates an excellent safety profile established over 25 years of clinical use with 6,415 subjects studied. 5

Common Adverse Events (Mild-to-Moderate, Transient):

• Local reactions (most common): Pain, redness, or swelling at injection site affecting 10-64% of recipients, lasting <2 days 1, 6, 5
• Systemic reactions: Headache (16-44%), fatigue (25.7%), muscle aches, malaise, arthralgia 1, 6
• Fever: Occurs in 10-35% of children <2 years but rarely in older children and adults 1
• Treatment-emergent adverse events: Occurred in only 13.7% of subjects in comprehensive safety analysis 5

Important Safety Considerations:

• Inactivated vaccines CANNOT cause influenza because they contain killed viruses 1
• Serious adverse events: Occur at rates <1% in healthy children and adults 1
• Pregnancy safety: Vaccination during pregnancy is safe and provides protection to both mother and infant, with demonstrated efficacy in reducing laboratory-confirmed influenza in infants and febrile respiratory illness in mothers 6
• Febrile seizures: A slight increase was noted when administered concomitantly with 13-valent pneumococcal conjugate vaccine in young children during 2010-2012, but this should not preclude vaccination 1

Optimal Timing

Vaccination should ideally occur between September and November, with campaigns after mid-October preferred to ensure vaccine availability. 1, 3, 2 However, vaccination should continue throughout the influenza season as long as viruses are circulating—late vaccination is better than no vaccination. 1, 3

Vaccine Effectiveness

Vaccine effectiveness varies by age, immune status, and match between vaccine and circulating strains:

• Well-matched seasons: 70-90% effectiveness in preventing laboratory-confirmed illness in healthy children and young adults 4, 7
• Suboptimal match: Effectiveness may be lower (e.g., 16% against A(H3N2) in 2021-22 season), but vaccine still prevents serious outcomes including hospitalization and death 8
• Mortality reduction: 65% effectiveness against influenza-associated death in children overall, 51% in children with underlying conditions 2
• Severe illness prevention: 75% reduction in life-threatening influenza illness in children 2
• Historical impact: Up to 80% of pediatric influenza deaths occur in unvaccinated or incompletely vaccinated children 2

Common Pitfalls to Avoid

• Do NOT delay vaccination waiting for a specific brand or "optimal timing"—any licensed, age-appropriate vaccine should be administered when available 3, 2
• Do NOT administer multiple doses of the same seasonal formulation within one season—this provides no benefit 3
• Do NOT use live attenuated vaccine (LAIV/FluMist) in patients with chronic medical conditions, immunosuppression, cardiovascular disease, or COPD—only inactivated vaccines like Influvac are appropriate 3, 2
• Do NOT assume prior vaccination provides multi-year protection—annual vaccination is mandatory due to antigenic drift and waning immunity 3
• Do NOT withhold vaccination from egg-allergic patients—current guidelines permit vaccination with appropriate monitoring 1