Influvac (Influenza Vaccine)
Inactivated influenza virus surface antigen vaccine for seasonal flu prevention
Quick Facts About Influvac
Key Takeaways About Influvac
- Annual vaccination recommended: Influenza virus strains change each season, so the vaccine composition is updated yearly and re-vaccination is needed every autumn
- Cannot cause the flu: Influvac is an inactivated vaccine containing only surface proteins – it cannot cause influenza infection
- Protection takes 2–3 weeks: Antibodies develop within 2–3 weeks after vaccination, so get vaccinated before the flu season peaks
- Safe in pregnancy: WHO and EMA recommend flu vaccination for pregnant women at any stage – it also protects the newborn
- Egg allergy caution: Influvac is produced in eggs – individuals with confirmed severe egg anaphylaxis should discuss alternatives with their doctor
What Is Influvac and What Is It Used For?
Influvac is an inactivated influenza vaccine used for the prevention of seasonal influenza (the flu). It contains purified surface antigens – haemagglutinin and neuraminidase – from influenza virus strains recommended by the WHO for each flu season. The vaccine is approved for adults and children from 6 months of age.
Influenza, commonly known as the flu, is a contagious respiratory illness caused by influenza viruses that circulate worldwide. Seasonal influenza epidemics occur annually, typically during autumn and winter in temperate climates. The disease can range from mild to severe, and in vulnerable populations, it can lead to serious complications including pneumonia, hospitalization, and death. The World Health Organization estimates that seasonal influenza causes 3–5 million cases of severe illness and 290,000–650,000 respiratory deaths globally each year.
Influvac works by stimulating the immune system to produce protective antibodies against the influenza virus surface proteins. When a vaccinated person is subsequently exposed to the actual influenza virus, their immune system can recognize and neutralize the virus before it causes illness, or at least reduce the severity of the disease. The vaccine is manufactured by Abbott Biologicals B.V. (part of Mylan/Viatris) and has been used worldwide for decades with an established safety and efficacy profile.
The composition of Influvac is updated each year based on the WHO’s recommendations, which are made following global surveillance of circulating influenza strains. The quadrivalent formulation (Influvac Tetra) contains antigens from four influenza virus strains: two influenza A subtypes (H1N1 and H3N2) and two influenza B lineages (Victoria and Yamagata). This broader coverage replaced the older trivalent formulation to provide better protection against the diversity of circulating influenza B viruses.
The WHO recommends annual influenza vaccination especially for: adults aged 65 and older, pregnant women (at any trimester), individuals with chronic conditions (heart disease, lung disease, diabetes, kidney disease, liver disease, immunodeficiency), healthcare workers, residents of long-term care facilities, and children aged 6 months to 5 years. However, anyone aged 6 months and older can benefit from vaccination.
What Should You Know Before Taking Influvac?
Before receiving Influvac, inform your healthcare provider about any allergies (especially to eggs, chicken proteins, or previous influenza vaccines), current medications, pregnancy or breastfeeding status, and any current illness with fever. The vaccine should not be given to individuals with a history of severe allergic reaction to a previous dose or any of its components.
Contraindications
Influvac must not be administered to individuals who have had a severe allergic reaction (anaphylaxis) to a previous dose of any influenza vaccine or to any component of the vaccine, including egg proteins (ovalbumin, chicken proteins), formaldehyde, cetrimonium bromide (cetyl trimethylammonium bromide, CTAB), or polysorbate 80. If you have experienced anaphylaxis after a previous flu vaccination, you should not receive Influvac without specialist assessment.
Vaccination should be postponed in individuals who have a moderate or severe acute febrile illness. A minor illness such as a mild upper respiratory tract infection with or without low-grade fever is generally not a reason to postpone vaccination. However, the decision to postpone should be made by a healthcare professional based on the individual’s clinical status and the severity of symptoms.
Warnings and Precautions
As with all injectable vaccines, appropriate medical treatment and supervision should always be readily available in case of a rare anaphylactic reaction following administration. Influvac should be given with caution to individuals who are taking anticoagulant therapy or who have bleeding disorders, as intramuscular injection can cause bleeding at the injection site. In such cases, deep subcutaneous injection may be considered as an alternative administration route.
The immune response to Influvac may be reduced in individuals who are immunocompromised, whether due to disease (such as HIV infection) or immunosuppressive therapy (including corticosteroids, chemotherapy, or biological agents). These individuals may not develop a full protective antibody response after vaccination. Despite this, vaccination is still recommended for immunocompromised persons because the disease itself poses a greater risk than any potential vaccine side effects.
Antibody responses may also be lower in elderly individuals (aged 65 and older) compared with younger adults. Nevertheless, the benefits of vaccination in this age group are well-established, as influenza carries a particularly high risk of complications and death in the elderly.
Influvac is produced using embryonated chicken eggs and may contain trace amounts of egg proteins (ovalbumin). Individuals with a history of severe anaphylactic reaction to egg proteins should be vaccinated only under medical supervision with appropriate resuscitation equipment available, or may be offered an egg-free alternative vaccine where available. Individuals with mild egg allergy (e.g., hives only) can generally receive Influvac safely under standard observation.
Pregnancy and Breastfeeding
Inactivated influenza vaccines, including Influvac, have been used extensively in pregnant women. A large body of evidence, including data from large observational studies and systematic reviews, supports the safety of inactivated influenza vaccination during pregnancy. The WHO, the European Medicines Agency (EMA), and national health authorities worldwide recommend influenza vaccination for all pregnant women, at any stage of pregnancy.
Vaccination during pregnancy not only protects the mother – who is at increased risk of severe influenza complications – but also confers passive protection to the newborn infant through transplacental transfer of maternal antibodies. This is particularly important because infants under 6 months of age cannot be vaccinated themselves and are at high risk of severe influenza. Studies have shown that maternal vaccination can reduce the risk of influenza infection in infants by approximately 50% during the first months of life.
Influvac can also be used during breastfeeding. Inactivated vaccines do not pose any risk to the breastfed infant, and breastfeeding is not a contraindication to vaccination. In fact, some maternal antibodies may be passed through breast milk, potentially offering additional modest protection to the nursing infant.
How Does Influvac Interact with Other Drugs?
Influvac can be administered concurrently with other vaccines (at different injection sites) and is compatible with most medications. However, immunosuppressive drugs may reduce the immune response to the vaccine, and there are theoretical interactions with anticoagulants due to the injection route.
Influvac can be given at the same time as other vaccines, including the COVID-19 vaccine, pneumococcal vaccine, and shingles vaccine, provided they are administered at different injection sites and preferably in different limbs. Co-administration does not generally reduce the immune response to either vaccine or increase the frequency of adverse reactions. This is supported by clinical data and is recommended by major health authorities to improve vaccination uptake.
| Drug / Drug Class | Type | Effect | Recommendation |
|---|---|---|---|
| Immunosuppressants (e.g., corticosteroids, methotrexate, biologics) | Major | Reduced immune response to vaccine | Still vaccinate; may need additional dose. Consult specialist. |
| Anticoagulants (e.g., warfarin, heparin, DOACs) | Moderate | Increased bleeding risk at injection site | Use fine needle; apply pressure for 2+ minutes. Consider subcutaneous route. |
| Other inactivated vaccines (e.g., pneumococcal, COVID-19) | Minor | No clinically significant interaction | Can be co-administered at different injection sites. |
| Live vaccines (e.g., MMR, varicella) | Minor | No interference (Influvac is inactivated) | Can be given simultaneously or at any interval. |
| Theophylline | Minor | Rare reports of transiently altered theophylline levels | Monitor if clinically indicated; generally no action needed. |
Major Interactions
The most significant interaction involves immunosuppressive medications. Patients receiving high-dose corticosteroids (equivalent to ≥20 mg prednisone daily for ≥14 days), chemotherapy, or biological immunosuppressants (such as rituximab, which depletes B cells) may have a substantially reduced antibody response to Influvac. For patients on rituximab, vaccination timing should ideally be coordinated with treatment cycles – vaccinating at least 2 weeks before the next rituximab dose or 6 months after the last dose provides the best chance of an adequate immune response. Despite reduced efficacy, vaccination is still recommended for these patients, as even a partial immune response may provide some clinical benefit.
Minor Interactions
Influvac is an inactivated vaccine and does not interfere with live vaccines. It can be administered at the same time as, or at any interval before or after, any other vaccine. When co-administered with other injectable vaccines, different injection sites should be used. There are rare, older reports of influenza vaccination transiently affecting serum levels of certain drugs metabolized by the cytochrome P450 system (such as theophylline and warfarin), but these findings have not been consistently replicated and are not considered clinically significant by current guidelines.
What Is the Correct Dosage of Influvac?
The standard dose of Influvac is 0.5 mL administered as a single intramuscular or deep subcutaneous injection once per year. Children aged 6 months to 8 years who are receiving influenza vaccine for the first time should receive two doses at least 4 weeks apart.
Adults
Adults (18 years and older)
Dose: 0.5 mL (one pre-filled syringe), administered as a single dose once per year.
Route: Intramuscular injection, preferably in the deltoid muscle of the upper arm. Deep subcutaneous injection is an alternative, particularly for patients on anticoagulant therapy.
Timing: Ideally administered in autumn (September–November in the Northern Hemisphere, March–May in the Southern Hemisphere) before the onset of the influenza season. Vaccination can still be beneficial if given later during the season.
Children
Children (6 months to 17 years)
Children aged 6–35 months: 0.25 mL or 0.5 mL (depending on the product and national recommendations) as a single dose. Some national guidelines recommend a full 0.5 mL dose for all children; follow local recommendations.
Children aged 3–17 years: 0.5 mL as a single dose.
First-time vaccination (children 6 months to 8 years): Two doses of 0.5 mL given at least 4 weeks apart. In subsequent seasons, only a single annual dose is required.
Route: Intramuscular injection in the anterolateral thigh (children under 1 year) or deltoid muscle (older children).
Elderly
Elderly (65 years and older)
Dose: 0.5 mL as a single annual dose, the same as for younger adults.
While the immune response may be somewhat reduced in older adults, the standard dose formulation of Influvac has been shown to provide meaningful protection. In some countries, high-dose or adjuvanted influenza vaccines are preferentially recommended for this age group to elicit a stronger immune response. Discuss with your healthcare provider which vaccine is most appropriate.
Missed Dose
If you miss your annual influenza vaccination, you can still receive the vaccine at any point during the flu season. Vaccination is beneficial as long as influenza viruses are circulating, which can extend from autumn through spring. There is no concept of a “missed dose” requiring catch-up in the same way as multi-dose medication regimens – simply get vaccinated as soon as possible. For children who are receiving two doses in their first year of vaccination and miss the second dose, it should be given as soon as possible, maintaining the minimum 4-week interval from the first dose.
Overdose
Overdosage with Influvac is unlikely given the single-dose pre-filled syringe presentation. In the rare event that more than the recommended dose is administered, the individual should be monitored for any adverse reactions. No specific symptoms of overdose have been reported with inactivated influenza vaccines. If an accidental overdose occurs, management is supportive and symptomatic. Contact your healthcare provider or poison control centre for guidance.
What Are the Side Effects of Influvac?
Influvac is generally well-tolerated. The most common side effects are injection site reactions (pain, redness, swelling) and mild systemic symptoms (headache, fatigue, muscle aches, low-grade fever). These usually resolve within 1–2 days. Serious side effects such as anaphylaxis are very rare.
Like all vaccines, Influvac can cause side effects, although not everybody gets them. Most side effects are mild to moderate and resolve spontaneously within a few days. The safety profile of Influvac has been extensively studied in clinical trials and through post-marketing surveillance over many decades. Below is a summary of side effects organized by frequency, based on data from clinical trials and post-marketing reports.
Very Common
- Pain at injection site
- Redness at injection site
- Swelling at injection site
- Headache
- Fatigue / malaise
- Muscle aches (myalgia)
Common
- Low-grade fever (≥37.5°C)
- Sweating
- Joint pain (arthralgia)
- Chills / shivering
- Hardening (induration) at injection site
- Bruising (ecchymosis) at injection site
Uncommon
- Nausea
- Diarrhoea
- Itching (pruritus) at injection site
- Generalised skin reactions (rash, urticaria)
- Lymphadenopathy (swollen lymph nodes)
Rare
- Anaphylaxis / severe allergic reaction
- Neuralgia (nerve pain)
- Paraesthesia (tingling, numbness)
- Febrile convulsions
- Transient thrombocytopenia (low platelet count)
- Guillain-Barré syndrome (extremely rare, estimated <1–2 per million vaccinations)
Injection site reactions are the most frequently reported side effects and are a normal indication that the immune system is responding to the vaccine. These reactions are typically mild and resolve within 24–48 hours without treatment. Applying a cool compress and taking paracetamol (acetaminophen) can help relieve discomfort if needed.
Systemic symptoms such as headache, fatigue, muscle aches, and low-grade fever are also common and reflect the body’s immune response. These are not influenza illness – the vaccine cannot cause the flu. Symptoms usually appear within 6–12 hours after vaccination and resolve within 1–2 days.
The risk of Guillain-Barré syndrome (GBS) after influenza vaccination is extremely small, estimated at approximately 1–2 additional cases per million vaccinations. For most individuals, the benefits of influenza vaccination greatly outweigh this very small potential risk. Individuals with a history of GBS within 6 weeks of a previous influenza vaccination should discuss the risks and benefits with their healthcare provider before being revaccinated.
Seek emergency medical help if you experience signs of a severe allergic reaction (anaphylaxis) after vaccination, including: difficulty breathing, swelling of the face, lips, tongue or throat, rapid heartbeat, dizziness or collapse, or widespread hives (urticaria). These reactions are very rare but require immediate treatment.
How Should You Store Influvac?
Influvac must be stored in a refrigerator at 2°C to 8°C (36°F to 46°F). Do not freeze. Protect from light. The vaccine should be used within the expiry date printed on the package.
Proper storage of Influvac is essential to maintain its effectiveness. The vaccine must be kept in a refrigerator at a temperature between 2°C and 8°C at all times. It must never be frozen, as freezing destroys the vaccine’s structure and renders it ineffective. If Influvac has been accidentally frozen, it must be discarded and not administered.
The pre-filled syringe should be stored in the original packaging to protect it from light. Before administration, the vaccine should be allowed to reach room temperature and then gently shaken to produce a uniform suspension. After removing from the refrigerator, the vaccine should be used promptly. Any unused vaccine or waste material should be disposed of in accordance with local requirements for pharmaceutical waste.
Keep Influvac out of the sight and reach of children. Do not use the vaccine after the expiry date stated on the carton and syringe label. The expiry date refers to the last day of that month. As Influvac is formulated specifically for each flu season, leftover vaccine from a previous season should not be used in subsequent seasons, even if it has not expired.
What Does Influvac Contain?
Influvac contains inactivated influenza virus surface antigens (haemagglutinin and neuraminidase) from the WHO-recommended strains for the current season, along with excipients including potassium chloride, potassium dihydrogen phosphate, disodium phosphate dihydrate, sodium chloride, calcium chloride dihydrate, magnesium chloride hexahydrate, and water for injections.
Active Ingredients
The active substances in Influvac are the surface antigens (haemagglutinin and neuraminidase) of inactivated influenza virus strains, propagated in fertilized chicken eggs. The quadrivalent formulation (Influvac Tetra) contains antigens from four virus strains: two influenza A strains (typically an A/H1N1 subtype and an A/H3N2 subtype) and two influenza B strains (one from each of the Victoria and Yamagata lineages). Each 0.5 mL dose contains 15 micrograms of haemagglutinin from each of the four recommended strains, for a total of 60 micrograms of haemagglutinin.
The specific virus strains included in the vaccine change each year, as determined by the WHO based on global influenza surveillance data. The WHO issues separate recommendations for the Northern Hemisphere (typically in February) and the Southern Hemisphere (typically in September) influenza seasons.
Inactive Ingredients (Excipients)
The excipients in Influvac include:
- Potassium chloride – buffer component
- Potassium dihydrogen phosphate – buffer component
- Disodium phosphate dihydrate – buffer component
- Sodium chloride – isotonicity agent
- Calcium chloride dihydrate – stabilizer
- Magnesium chloride hexahydrate – stabilizer
- Water for injections – solvent
Influvac may contain trace amounts of egg proteins (ovalbumin), formaldehyde, cetrimonium bromide (CTAB), and polysorbate 80, which are used during the manufacturing process. Gentamicin may also be present in trace amounts as a residual from the production process. The vaccine does not contain any preservatives (such as thiomersal/thimerosal) and does not contain any adjuvants.
Influvac is produced using embryonated chicken eggs and may contain residual egg proteins. The ovalbumin content per dose is typically very low (≤1 microgram). Current guidelines from the EMA and most national health authorities state that individuals with egg allergy – including those with a history of anaphylaxis to egg – can generally receive egg-based inactivated influenza vaccines, provided appropriate medical supervision and monitoring is available. However, egg-free vaccine alternatives exist and may be preferred for individuals with a history of severe egg anaphylaxis.
Frequently Asked Questions About Influvac
No. Influvac is an inactivated (killed) vaccine that contains only purified surface proteins from the influenza virus – not whole live virus. It is physically impossible for this vaccine to cause influenza infection. Some people may experience mild symptoms such as low-grade fever, muscle aches, or fatigue for 1–2 days after vaccination. These are normal immune responses to the vaccine, not the flu. They indicate that your body is building protection against influenza.
The effectiveness of Influvac varies from season to season, depending on how well the vaccine strains match the circulating influenza viruses. In seasons with good strain matching, inactivated influenza vaccines typically reduce the risk of laboratory-confirmed influenza by 40–60% in the general adult population. Even when effectiveness is lower due to strain mismatch, vaccination still reduces the risk of severe illness, hospitalization, and death. A Cochrane systematic review found that inactivated influenza vaccines prevent approximately 1 case of influenza for every 40 adults vaccinated in a typical season.
Yes. The WHO, EMA, and all major national health authorities recommend influenza vaccination for pregnant women at any stage of pregnancy. Inactivated influenza vaccines like Influvac have been given to millions of pregnant women with a strong safety record. Vaccination during pregnancy protects the mother from severe influenza complications and also passes protective antibodies to the baby, providing protection during the first months of life before the infant can be vaccinated. Influvac is also safe during breastfeeding.
Yes. Influvac can be safely administered at the same time as other vaccines, including COVID-19 vaccines, pneumococcal vaccines, and shingles vaccines. When given together, the vaccines should be injected at different sites, ideally in different arms. Clinical studies have shown that co-administration does not reduce the immune response to either vaccine or increase the risk of side effects. This approach is recommended by health authorities to improve vaccine coverage and reduce the number of healthcare visits needed.
Annual vaccination is necessary for two reasons. First, influenza viruses constantly mutate through a process called antigenic drift, meaning the strains circulating each year are different from previous years. The WHO reviews global surveillance data and updates the recommended vaccine composition each year to match the expected circulating strains. Second, the immune protection generated by influenza vaccination wanes over time, typically declining significantly after about 6 months. For these reasons, a new vaccination each autumn provides the best protection for the upcoming flu season.
Current evidence supports that individuals with egg allergy, including those with a history of anaphylaxis to egg, can receive egg-based inactivated influenza vaccines like Influvac. The ovalbumin content in modern influenza vaccines is very low (typically ≤1 microgram per dose). Major guidelines from the EMA, CDC, and other authorities no longer recommend a prolonged observation period solely because of egg allergy. However, individuals with a history of severe egg anaphylaxis should be vaccinated in a healthcare setting where anaphylaxis can be treated. Egg-free influenza vaccines (e.g., cell-based or recombinant vaccines) are available in some countries and may be preferred for these individuals.
References
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- Turner PJ, Southern J, Andrews NJ, et al. Safety of live attenuated influenza vaccine in atopic children with egg allergy. Journal of Allergy and Clinical Immunology. 2015;136(2):376–381.
- Kwong JC, Vasa PP, Campitelli MA, et al. Risk of Guillain-Barré syndrome after seasonal influenza vaccination and influenza health-care encounters: a self-controlled study. Lancet Infectious Diseases. 2013;13(9):769–776.
- British National Formulary (BNF). Influenza vaccine (inactivated). London: National Institute for Health and Care Excellence (NICE); 2025.
- World Health Organization (WHO). Recommended composition of influenza virus vaccines for use in the 2025–2026 northern hemisphere influenza season. Geneva: WHO; 2025.
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