What the Flu Vaccine Is For
The influenza vaccine is designed to prevent influenza infection and its complications, including hospitalization and death, by inducing protective antibodies against circulating influenza virus strains. 1, 2
Primary Purpose and Mechanism
The flu vaccine serves to protect against seasonal influenza by:
- Preventing influenza illness in approximately 70% of healthy persons under 65 years when vaccine strains match circulating viruses 1
- Reducing severe complications including pneumonia, hospitalization, and death, even when it doesn't completely prevent infection 1
- Protecting vulnerable populations who are at highest risk for serious complications, including young children (especially under 2 years), elderly persons, pregnant women, and those with chronic medical conditions 1
The vaccine contains three virus strains (typically two type A and one type B) representing influenza viruses predicted to circulate in the upcoming season, made from highly purified, inactivated (killed) viruses that cannot cause infection 1, 2
Who Should Receive It
Annual influenza vaccination is recommended for all persons ≥6 months of age without contraindications. 1
Priority groups include:
- Children 6 months through 18 years of age 1
- Adults ≥50 years of age 1
- Persons with chronic medical conditions (pulmonary, cardiovascular, metabolic diseases, immunosuppression) 1
- Pregnant women in second or third trimester during flu season 1
- Healthcare workers and caregivers of high-risk individuals to prevent transmission 1
- Residents of nursing homes and chronic-care facilities 1
Effectiveness in Different Populations
The vaccine's effectiveness varies by population:
- Healthy adults <65 years: 70% effective at preventing illness when well-matched 1
- Children and adolescents 6 months-17 years: 55% effective against medically attended illness 3
- Elderly in community settings: 30-70% effective at preventing hospitalization for pneumonia and influenza 1
- Nursing home residents: 50-60% effective at preventing hospitalization/pneumonia and 80% effective at preventing death, even when only 30-40% effective at preventing illness 1
Critical Benefits Beyond Illness Prevention
Even when the vaccine doesn't prevent infection entirely, it provides substantial benefit by:
- Reducing lower respiratory tract complications and secondary bacterial infections 1
- Decreasing hospitalization rates by 30-70% in elderly persons living independently 1
- Preventing death with 80% effectiveness in nursing home populations 1
- Reducing community transmission through herd immunity when vaccination rates are high in closed settings 1
Why Annual Vaccination Is Necessary
- Immunity declines within the year following vaccination 2
- Influenza viruses undergo frequent antigenic changes (antigenic drift), requiring updated vaccine strains each season 1
- Vaccine composition changes annually to match predicted circulating strains based on global surveillance 1, 2
Important Safety Information
The vaccine is safe and well-tolerated:
- Cannot cause influenza infection because it contains only inactivated viruses 2
- Most common side effect is soreness at injection site lasting up to 2 days in less than one-third of recipients 2
- Systemic reactions (fever, malaise, myalgia) occur infrequently, mainly in those without prior exposure 2
- Only absolute contraindication is previous severe allergic reaction (anaphylaxis) to any vaccine component 4
Special Considerations During COVID-19
Influenza vaccination is particularly important during ongoing SARS-CoV-2 circulation to reduce the burden of respiratory illnesses and preserve healthcare capacity. 1