Number Needed to Vaccinate (NNV) Calculation
The number needed to vaccinate to prevent one hospitalization is 7 (or approximately 7.14). This is calculated using the absolute risk reduction between unvaccinated (20% hospitalization rate) and vaccinated (6% hospitalization rate) individuals aged ≥50 years who contracted influenza.
Calculation Method
The NNV is determined by the following formula:
NNV = 1 / Absolute Risk Reduction (ARR)
Where ARR = Risk in unvaccinated - Risk in vaccinated
- Unvaccinated hospitalization rate: 200/1000 = 20% (0.20) 1
- Vaccinated hospitalization rate: 60/1000 = 6% (0.06) 1
- Absolute Risk Reduction: 0.20 - 0.06 = 0.14 (14%)
- NNV: 1/0.14 = 7.14 ≈ 7
Clinical Context and Interpretation
This NNV of 7 represents an exceptionally favorable outcome that warrants careful interpretation:
The calculated vaccine effectiveness of 70% (reducing hospitalizations from 20% to 6%) is at the upper end of typical influenza vaccine performance, as the CDC reports vaccine effectiveness is typically 30-70% for preventing hospitalization in elderly persons living outside nursing homes 2, 1
This scenario assumes all individuals already contracted influenza, which differs from real-world NNV calculations that account for both infection prevention and hospitalization prevention among those infected 1
Real-world NNV values are substantially higher because they must account for the probability of infection itself. The CDC reports that among adults ≥65 years, typical NNV values range from approximately 40-50 for high-risk adults to several hundred for healthy younger adults 1
Important Caveats
This calculation has significant limitations:
The question presents a conditional probability scenario (hospitalization given infection), not the typical population-level NNV used in public health decision-making 1, 3
Traditional NNV calculations that exclude transmission effects can produce estimates biased by up to 1000-fold compared to calculations incorporating indirect vaccine effects 3
The 14% absolute risk reduction demonstrated here is considerably higher than typical influenza vaccine performance in preventing hospitalization, suggesting either optimal vaccine-strain matching or a selected high-risk population 2
Mortality and Quality of Life Considerations
The clinical significance extends beyond hospitalization prevention:
Among adults ≥65 years, influenza vaccine is 80% effective in preventing death in nursing home residents, even when effectiveness against illness itself is only 30-40% 2
More than 90% of influenza deaths occur in adults ≥65 years, with 20,000-40,000 deaths during severe epidemic years 2, 1
Influenza vaccination reduces not only hospitalization rates but also shortens hospital stays by an average of 2.4 days for all causes and 4.9 days for lung disease 4