Influenza Vaccine

Influenza vaccine is a preventive biological product designed to protect against influenza, a contagious respiratory illness caused by influenza viruses. It helps the immune system recognize specific viral strains and develop an immune response before exposure to the infection. Depending on the formulation, influenza vaccines may contain inactivated viral antigens, recombinant viral proteins, or live attenuated viruses that have been weakened so they can stimulate immunity without causing typical influenza illness in eligible recipients. Most injectable influenza vaccines are inactivated or recombinant, while live attenuated formulations are administered as a nasal spray in countries where they are approved and available. Influenza viruses change over time, so vaccine composition is reviewed and updated regularly to target strains expected to circulate during an upcoming influenza season. Vaccination can reduce the likelihood of influenza infection and may lower the risk of severe disease, hospitalization, and complications, although protection varies with age, immune status, circulating strains, and vaccine match. Annual vaccination is generally recommended because immunity decreases over time and circulating viruses evolve. Influenza vaccines are particularly important for older adults, young children, pregnant women, healthcare workers, and people with chronic medical conditions. They are preventive vaccines, not treatments for an existing influenza infection.

Influenza vaccine development began in the twentieth century after researchers established that influenza was caused by a virus. Early inactivated influenza vaccines were developed in the 1930s and used during the Second World War, followed by broader civilian vaccination programs. Since then, vaccine technology has expanded to include split-virus and subunit vaccines, cell-culture-based formulations, recombinant vaccines, adjuvanted vaccines, and live attenuated nasal vaccines. The World Health Organization coordinates global surveillance and issues recommendations for the viral strains to be included in seasonal vaccines, generally reviewing the composition twice each year for the Northern and Southern Hemisphere seasons. National regulatory authorities, including the United States Food and Drug Administration (FDA), the European Medicines Agency (EMA), and relevant national licensing authorities such as India's Central Drugs Standard Control Organisation (CDSCO), oversee the approval and quality requirements applicable to individual products in their jurisdictions. Influenza vaccines have received regulatory approvals over several decades, rather than through a single approval date for the entire vaccine category. Product-specific approval dates, formulations, age indications, and manufacturing methods differ. Clinical evidence supporting their use comes from immunogenicity studies, safety evaluations, effectiveness studies, and ongoing surveillance of influenza-related illness and complications.

Influenza vaccine is used to prevent seasonal influenza caused by influenza A and B viruses represented in the vaccine formulation. It is recommended for eligible individuals according to national immunization guidelines, age, medical history, and local influenza activity. Vaccination is especially important for older adults, children, pregnant women, healthcare workers, and individuals with chronic respiratory, cardiovascular, metabolic, renal, hepatic, neurological, or immunocompromising conditions that increase the risk of influenza-related complications. It may also help reduce influenza transmission and protect vulnerable people in households and healthcare settings. Inactivated and recombinant vaccines are used according to their licensed indications, while live attenuated nasal vaccines are suitable only for eligible age groups and clinical circumstances. Influenza vaccine may be administered alongside other recommended vaccines when permitted by applicable guidance. It does not treat an existing influenza infection and does not protect against every respiratory virus or every influenza strain. The appropriate formulation should be selected according to local approvals and current vaccination recommendations.

Influenza vaccine is generally administered once per influenza season, with the formulation, dose, route, and timing determined by the individual's age, medical history, and the product's approved prescribing information. Most injectable inactivated and recombinant vaccines are given intramuscularly, although certain products may have different approved administration instructions. Live attenuated influenza vaccines are administered intranasally where available and approved. Many adults receive a single seasonal dose, while children receiving influenza vaccination for the first time may require two doses separated by the recommended interval under applicable immunization guidelines. The vaccine composition is updated periodically to reflect expected circulating strains. Vaccination should ideally take place before influenza activity increases locally, although the appropriate timing varies by region and season. Treatment duration is not applicable because influenza vaccine is preventive rather than therapeutic. Healthcare professionals should verify the current product information, age eligibility, recommended dose, and local immunization schedule before administration.

Common side effects of influenza vaccine are usually mild and temporary. Injectable vaccines may cause pain, redness, swelling, or tenderness at the injection site, along with headache, tiredness, muscle aches, or a low-grade fever. These effects generally resolve within a few days. Live attenuated nasal vaccines may cause a runny or blocked nose, sore throat, headache, or other mild respiratory symptoms. The type and frequency of side effects vary according to the vaccine formulation, age, and individual response. These reactions are associated with the body's immune response and do not usually indicate influenza infection. Most people can continue normal daily activities after vaccination. Persistent, worsening, or unusual symptoms should be assessed by a healthcare professional, particularly when accompanied by significant breathing difficulty, facial swelling, or other signs of a serious allergic reaction.

Serious adverse events following influenza vaccination are uncommon but can occur. Severe allergic reactions, including anaphylaxis, are rare and require immediate medical attention. A history of a severe allergic reaction to a previous influenza vaccine or one of its components must be reviewed before selecting a vaccine product. Guillain–Barré syndrome occurring within six weeks of a previous influenza vaccination is generally considered a precaution requiring individual assessment. Live attenuated nasal vaccines have additional restrictions and may be unsuitable for pregnant individuals, immunocompromised people, and certain patients with asthma or other medical conditions, depending on age and applicable guidance. Moderate or severe acute illness may warrant postponing vaccination until the person has recovered. Influenza vaccines cannot guarantee complete protection, and vaccinated individuals may still develop influenza or other respiratory infections. Suspected serious adverse reactions should be evaluated promptly and reported according to local pharmacovigilance requirements. Further doses or alternative formulations should be considered only after clinical assessment and review of the relevant product information.

Before vaccination, healthcare professionals should review the patient's age, previous influenza vaccinations, history of severe allergic reactions, current illness, underlying medical conditions, pregnancy status where relevant, and medications that may affect immune function. People receiving chemotherapy, immunosuppressive medicines, or other treatments that weaken immunity may require careful vaccine selection and timing. Inactivated and recombinant influenza vaccines are generally suitable for many immunocompromised individuals, although their immune response may be reduced. Live attenuated vaccines have more restrictive eligibility requirements. Egg allergy alone is not necessarily a reason to avoid influenza vaccination; the appropriate product and setting should follow current national recommendations and product information. Classic drug interactions are generally limited, but immunosuppressive therapies may affect vaccine effectiveness, and concurrent vaccines may require specific timing or administration considerations. Patients taking anticoagulants or experiencing bleeding disorders may need precautions for intramuscular administration. Baseline laboratory testing is not routinely required for healthy individuals, although additional assessment may be appropriate in selected clinical circumstances.

Prescribers and pharmacists should select influenza vaccine according to the patient's age, clinical risk factors, local regulatory approvals, and the current seasonal formulation. Confirm the patient's vaccination history, assess previous serious allergic reactions, and check contraindications and precautions before administration. Follow the product-specific instructions for dose, route, storage, handling, and expiry, maintaining the required cold chain throughout distribution and storage. Do not freeze vaccines unless the specific product information explicitly permits it, and protect products from conditions that could compromise their quality. Explain that protection develops after vaccination rather than immediately and that annual vaccination is generally needed because circulating influenza strains and immune protection change over time. Patients should be counselled about expected local and systemic reactions and advised to seek urgent medical care for symptoms suggestive of anaphylaxis. Document the product name, batch number, administration date, and relevant details according to local practice. For patients receiving immunosuppressive therapy or those with a complex allergy history, coordinate with the treating clinician to determine the most appropriate vaccine and timing.