What is the absolute risk reduction of influenza vaccine over the last 5 years?

Absolute Risk Reduction of Influenza Vaccine Over the Last 5 Years

Based on the most recent high-quality evidence, influenza vaccination in community-dwelling adults ≥65 years provides an absolute risk reduction of 3.6% for influenza illness (from 6% to 2.4%) and 2.5% for influenza-like illness (from 6% to 3.5%), translating to a Number Needed to Vaccinate of 30 to prevent one case of influenza and 42 to prevent one case of ILI. 1

Population-Specific Absolute Risk Reductions

Elderly Adults (≥65 years)

• Mortality reduction: Influenza vaccination reduces mortality risk by 48% in community-dwelling elderly adults, though the absolute baseline mortality rate varies by season and population 2, 3
• Hospitalization reduction: 27% relative risk reduction for all-cause hospitalization, with absolute rates varying from 30-70% effectiveness in preventing pneumonia and influenza hospitalizations depending on the season 2, 3
• Influenza illness: Absolute risk reduction of approximately 3.6% (from 6% to 2.4%) in a single season 1
• Influenza-like illness: Absolute risk reduction of 2.5% (from 6% to 3.5%) 1

Cardiovascular Disease Patients

• Cardiovascular mortality: In the FLUVACS trial, vaccination reduced cardiovascular death from 8% to 2% at one year—an absolute risk reduction of 6% 2
• Composite cardiovascular endpoints: Absolute risk reduction of 12% (from 23% to 11%) for cardiovascular death, nonfatal MI, or severe ischemia 2
• Myocardial infarction: Observational studies show 19-45% relative risk reduction, with absolute risk reductions varying by baseline cardiovascular risk 2

Healthy Adults (<65 years)

• Influenza illness: When vaccine and circulating strains are well-matched, vaccination prevents influenza in 70-90% of healthy adults <65 years, though absolute attack rates vary by season (typically 5-15% in unvaccinated populations) 2
• Work absenteeism: 32-45% reduction in lost workdays, with absolute reductions depending on baseline influenza attack rates 2

Children (6 months to 18 years)

• Influenza illness: Vaccine efficacy ranges from 66% in children 6-24 months (absolute risk reduction from 15.9% to 5.5% = 10.4% ARR in one study year) to 77-91% in children 1-15 years 2
• Influenza-associated otitis media: Approximately 30% reduction in incidence 2
• Pediatric mortality: 65% effectiveness against influenza-associated death (95% CI 54-74%), with only 22% of eligible children who died having received vaccination 2

Critical Context: Variability Across Seasons

The absolute risk reduction varies substantially by season based on:

• Vaccine-strain matching: Well-matched seasons show ARR of 10-15% in high-risk populations; poorly matched seasons may show minimal benefit 2
• Circulating strain virulence: H3N2-predominant seasons typically cause higher baseline attack rates (15-20%) compared to H1N1 seasons (5-10%) 2
• Population baseline risk: Nursing home residents have baseline influenza attack rates of 30-40% unvaccinated versus 10-20% vaccinated (ARR 10-20%) 2

Enhanced Vaccine Formulations: Greater Absolute Benefit

• High-dose vaccines in elderly: Provide additional 3.4% absolute reduction in respiratory-related hospitalizations (from 3.9% to 3.4%) and 1.2% absolute mortality reduction (from 18.3% to 17.1%) compared to standard-dose vaccines 2, 3
• These enhanced formulations should be preferentially used in all adults ≥65 years 3

Common Pitfalls in Interpreting ARR

Avoid these errors when counseling patients:

• Season-to-season variability: A single season's ARR does not predict future seasons; baseline attack rates fluctuate from 3-20% depending on circulating strains 2, 1
• Population heterogeneity: Community-dwelling elderly have different baseline risks than nursing home residents (30-40% attack rates unvaccinated) 2
• Outcome selection matters: ARR for preventing death (0.5-2% absolute reduction) differs dramatically from ARR for preventing any ILI (2-5% reduction) 2, 1

Number Needed to Vaccinate (NNV)

Based on recent systematic review data:

• NNV = 30 to prevent one case of laboratory-confirmed influenza in elderly adults 1
• NNV = 42 to prevent one case of influenza-like illness in elderly adults 1
• NNV = 17 to prevent one cardiovascular death in acute coronary syndrome patients (based on 6% ARR) 2

Limitations of Current Evidence

The Cochrane systematic review notes that certainty of evidence is low to moderate due to diagnostic uncertainty and study design limitations, though the direction of benefit is consistent across studies 1. The absolute risk reductions presented represent single-season estimates and should not be extrapolated to multi-year cumulative benefits without considering annual re-vaccination 1.