Influenza Vaccine
Overview
Influenza vaccines are biologic immunizations designed to protect individuals from seasonal influenza, a contagious respiratory virus infection caused by influenza A and B viruses that can result in fever, cough, sore throat, malaise, and serious complications. These vaccines include inactivated influenza vaccines, recombinant influenza vaccines, adjuvanted vaccines, high-dose vaccines, and live attenuated intranasal vaccines. The aim of influenza vaccination is to stimulate the immune system to produce antibodies against the circulating strains of influenza viruses predicted for each season, thereby reducing the risk of infection and severe illness. Because influenza viruses evolve rapidly, vaccine strains are reviewed annually and updated based on global surveillance to match the most likely circulating viruses. Influenza vaccines are typically administered once a year before or during influenza season and are recommended for individuals aged six months and older, with special considerations for older adults and high-risk groups. The clinical value of annual influenza vaccination is widely recognized in reducing hospitalizations, complications, and deaths related to influenza, particularly among vulnerable populations.
Background and Date of Approval
Influenza vaccines have a long regulatory history with multiple products approved over many decades in the United States and other countries to provide active immunization against influenza A and B viruses. Traditional trivalent and quadrivalent inactivated influenza virus vaccines and live attenuated intranasal vaccines have been licensed since the mid-20th century, with ongoing updates in composition each year to match circulating viral strains. For example, the vaccine product Fluarix, an inactivated influenza vaccine, was originally licensed in the United States on August 31, 2005 and received a full traditional approval on October 1, 2009 for the prevention of seasonal influenza illness in adults. Other vaccines such as Flucelvax and Fluad have received more recent approvals for use in persons aged six months and older for prevention of disease caused by influenza virus subtypes contained in the vaccine. Regulatory agencies including the United States Food and Drug Administration and equivalent authorities internationally support annual review of vaccine composition to ensure relevance to circulating strains.
Uses
Influenza vaccines are indicated for active immunization against seasonal influenza disease caused by influenza A and B virus strains contained in the vaccine formulation. They are recommended annually for persons six months of age and older to reduce the risk of influenza infection and associated complications. Specific formulations such as high-dose or adjuvanted vaccines are recommended for older adults aged 65 years and above to enhance immune response. Live attenuated intranasal vaccines are indicated for certain age groups, such as individuals aged two through 49 years who are not contraindicated for this route. Vaccination is particularly important for individuals at increased risk of severe disease, including the elderly, young children, pregnant people, and those with chronic medical conditions, as well as individuals in close contact with high-risk persons.
Administration
Influenza vaccines are administered by intramuscular injection for inactivated, recombinant, high-dose, and adjuvanted formulations, usually in the upper arm for older children and adults or the thigh for infants and young children. The live attenuated influenza vaccine is administered as a nasal spray, with one spray per nostril. The typical schedule involves a single dose each influenza season, ideally before influenza circulation increases, although people may be vaccinated at any point during the season. Children aged six months through eight years who have not previously been vaccinated or whose vaccination history is unknown may require two doses separated by at least four weeks during their first season of vaccination. Specific dosing volumes and schedules vary by age, vaccine type, and prior vaccination history, and clinicians follow current immunization guidelines to determine the appropriate regimen.
Side Effects
Common side effects following influenza vaccination are generally mild and self-limited and may include pain, redness, or swelling at the injection site, low-grade fever, fatigue, headache, and muscle aches. For live attenuated intranasal vaccines, mild upper respiratory symptoms such as runny nose, nasal congestion, sore throat, and cough may occur. In children, irritability and decreased activity may be observed. These expected reactions reflect immune activation and typically resolve within a few days without specific treatment.
Warnings
Serious adverse events following influenza vaccination are uncommon. Rare allergic reactions, including anaphylaxis, may occur and require immediate medical attention. Individuals with a known severe allergic reaction to a component of the vaccine, including egg protein or other constituents, should not receive that specific vaccine formulation. Careful assessment of risk factors such as a history of Guillain-Barré syndrome within six weeks of a prior influenza vaccination may inform clinical decision-making. Acute severe illness may warrant deferring vaccination until recovery. Clinicians balance potential benefits and risks in individuals with complex medical conditions.
Precautions
Prior to vaccination, reviewing the individual’s medical history, including previous allergic reactions and immunization history, is important. Influenza vaccines can generally be administered concomitantly with other routine vaccines at different anatomical sites. Drug interactions are uncommon because influenza vaccines stimulate an immune response rather than undergo systemic pharmacologic metabolism, but specific interactions such as reduced efficacy of live attenuated nasal vaccines when administered around the time of influenza antiviral medications may occur. Individuals with immunocompromising conditions may exhibit attenuated immune responses and require tailored guidance and follow-up.
Expert Tips
Effective influenza vaccination practice involves appropriate patient selection based on age, risk profile, and prior vaccination history. Educating patients and caregivers about expected mild reactions and the importance of annual vaccination supports adherence to seasonal recommendations. Observing vaccinated individuals briefly for signs of immediate allergic reactions is standard practice. Accurate documentation of vaccine type, dose, and date ensures clear records for future clinical care. Coordinating influenza vaccination with other preventive health services optimizes patient outcomes during respiratory virus seasons.