Inactivated Influenza Vaccine

The inactivated influenza vaccine contains purified surface antigens (haemagglutinin and neuraminidase) from influenza A and B virus strains. Flucelvax is prepared in Madin Darby Canine Kidney (MDCK) cell cultures rather than fertilised hen’s eggs, making it suitable for individuals with egg allergies and offering a more flexible manufacturing process. Egg-based and recombinant formulations are also available.

Vaccine Composition

Each 0.5 mL dose contains 15 µg of haemagglutinin per strain. Per the EMA Flucelvax EPAR, the vaccine includes:

• 2 influenza A strains: A(H1N1)pdm09 and A(H3N2).
• 1 influenza B strain (Victoria lineage) — the trivalent composition reflects the EU’s 2026/2027 transition.

Strain selection: changes annually based on WHO recommendations for the upcoming season. Each year’s formulation is dated separately and seasonal vaccines should not be used beyond their target season.

EU transition to trivalent: From the 2026/2027 season the EU recommends trivalent formulations. The B/Yamagata lineage has not been detected in circulation since March 2020, and several quadrivalent products (including Flucelvax Tetra and Fluad Tetra) had their EU marketing authorisations withdrawn on 23 February 2026.

Mechanism of Action

The vaccine contains inactivated viral surface antigens (it cannot cause infection). After administration, it stimulates production of neutralizing antibodies against each included strain and induces cellular immunity. Antibodies develop over 2–3 weeks, peak at approximately 4 weeks, and provide protection for approximately 6–12 months — hence the need for annual revaccination.

• Overall vaccine effectiveness: 40–60% when vaccine strains match circulating viruses; up to 80% in well-matched seasons; 10–30% in poorly matched seasons.
• Hospitalizations: 50–60% reduction.
• ICU admissions: 70–80% reduction.
• Mortality in high-risk groups: up to 80% reduction.
• Pregnancy: protects mother and infant (antibodies cross the placenta); reduces influenza in infants under 6 months and reduces preterm birth and low birth weight.
• Cardiovascular outcomes: reduces myocardial infarction and stroke in elderly populations.

Per the Flucelvax EPAR: prophylaxis of influenza in adults and children from 6 months of age. Flucelvax should be used in accordance with official recommendations.

Universal annual vaccination is recommended for all persons ≥6 months of age in most countries.

High-priority groups:

• Adults ≥65 years.
• Children 6 months to 5 years.
• Pregnant women (any trimester).
• Chronic medical conditions: cardiovascular disease, respiratory disease (asthma, COPD), diabetes, renal disease, liver disease, neurological disorders affecting respiratory function, morbid obesity (BMI ≥40), immunosuppression.
• Nursing home residents.
• Healthcare workers.
• Caregivers and household contacts of high-risk individuals.
• Household contacts of infants under 6 months.

• Hypersensitivity to the active substances, to any of the excipients, or to any component that may be present in trace amounts (e.g. for Flucelvax: beta-propiolactone, cetyltrimethylammonium bromide [CTAB], polysorbate 80).
• Severe allergic reaction (anaphylaxis) to a previous dose of any influenza vaccine.

Defer vaccination for moderate or severe acute illness (with or without fever).

Precaution: history of Guillain-Barré syndrome within 6 weeks of a previous influenza vaccination.

Not contraindications: minor illness, mild egg allergy (modern inactivated vaccines are safe to administer with observation), antibiotic allergy, breastfeeding, immunosuppression. Anaphylactic egg allergy: consider egg-free recombinant or cell-based formulations.

Most reactions are mild and resolve within 1–2 days. Frequencies below reflect the Flucelvax EPAR safety profile.

Adults (≥18 years), Very Common (>1 in 10)

Pain, erythema, and induration at the injection site; headache; fatigue; myalgia.

Children and adolescents (6 to <18 years), Very Common (>1 in 10)

Pain, erythema, induration, and bruising at the injection site; fatigue; myalgia; headache; loss of appetite.

Children (6 months to <6 years), Very Common (>1 in 10)

Tenderness, erythema, induration, and bruising at the injection site; sleepiness; irritability; fever; diarrhoea.

Common (1–10%)

Malaise, shivering, arthralgia, low-grade fever (<38.5°C), nausea, sweating.

Uncommon (0.1–1%)

Moderate fever, lymphadenopathy, allergic reactions (rash, urticaria).

Rare (<0.1%)

Anaphylaxis (~1 per million doses), vasculitis, neurological complications, thrombocytopenia.

Very Rare

Guillain-Barré syndrome (~1–2 per million doses; far lower than the GBS risk from influenza infection itself).

Important: the inactivated vaccine cannot cause influenza. Respiratory illness in the days after vaccination is typically due to coincidental viral infection or pre-existing incubation, not the vaccine.

Route

Intramuscular injection (preferred). Deep subcutaneous injection is acceptable in patients with bleeding disorders.

Injection Site

• Adults and older children: deltoid muscle of the upper arm.
• Infants and young children: anterolateral thigh muscle.
• Avoid the gluteal region: reduced immunogenicity.

Dosing (per Flucelvax EPAR)

• Adults and children from 9 years: a single 0.5 mL dose.
• Children from 6 months to <9 years not previously vaccinated against seasonal influenza: two 0.5 mL doses given at least 4 weeks apart.
• Children from 6 months to <9 years previously vaccinated: a single 0.5 mL dose.
• Re-vaccinate annually with the current season’s formulation.

Timing

• Northern Hemisphere: optimal September–November; can vaccinate any time during flu season.
• Southern Hemisphere: optimal March–May.

Immunity develops 2–3 weeks after vaccination — vaccinate before the season for optimal protection.

Co-Administration

Can be given simultaneously with other vaccines at different injection sites. Safe with COVID-19, pneumococcal, Tdap, and shingles vaccines.

Procedure

• Inspect the vaccine visually; shake gently before use.
• Use an appropriate needle (typically 22–25 gauge, 1–1.5 inches in adults).
• Clean the injection site with an alcohol swab.
• Inject at 90° angle for IM administration.
• Do not administer intravenously, mix with other vaccines in the same syringe, or freeze (discard if frozen).

• Immunosuppressive therapy: may reduce immune response; vaccinate anyway (some protection is better than none); consider higher-dose formulations or additional doses.
• Anticoagulants: safe to administer; apply firm pressure to the injection site for 2 minutes; consider subcutaneous route if severe bleeding risk.
• No significant interaction with most medications, antibiotics, or antivirals.

Pregnancy and Breastfeeding

Safe and recommended in all trimesters of pregnancy. Protects both mother and infant. Antibodies cross the placenta and provide neonatal protection. Safe during breastfeeding.

Egg Allergy

Modern inactivated vaccines contain minimal egg protein and are safe for individuals with mild egg allergy when administered with observation. For anaphylactic egg allergy, use egg-free recombinant or cell-based formulations, or administer in a medically supervised setting.

Immunocompromised Patients

Inactivated influenza vaccines are safe in immunocompromised patients (HIV, cancer, transplant recipients, those on immunosuppressants). Immune response may be reduced. High-dose or adjuvanted formulations are available for selected populations (e.g., elderly).

Annual Vaccination

Required every year due to waning immunity (6–12 months) and antigenic drift in circulating viruses. Each season’s vaccine has a specific strain composition based on WHO recommendations.

Common Misconceptions

• The vaccine cannot cause influenza — it contains inactivated (killed) virus that cannot replicate.
• Mild post-vaccination symptoms reflect the immune response, not influenza.

Store at 2–8°C. Do not freeze (discard if frozen). Protect from light. Store in the original package. Maintain cold chain throughout transport and storage. Monitor refrigerator temperature daily.

Before administration, remove from the refrigerator just before use and inspect for particulates or discoloration.

12 months when stored properly at 2–8°C. Each year’s formulation has its own dating. Discard the previous season’s vaccine even if it has not expired.

Per the Flucelvax EPAR, the vaccine can only be obtained with a prescription. Widely available at pharmacies, clinics, and healthcare facilities. Must be administered by trained healthcare providers.