Number Needed to Vaccinate (NNV) Calculation
Based on the data provided, the number needed to vaccinate (NNV) to prevent one hospitalization is 7.
Calculation Method
The NNV is calculated using the absolute risk reduction (ARR) between the unvaccinated and vaccinated groups:
- Unvaccinated group: 200/1000 = 20% hospitalization rate
- Vaccinated group: 60/1000 = 6% hospitalization rate
- Absolute Risk Reduction (ARR): 20% - 6% = 14% = 0.14
- NNV = 1/ARR = 1/0.14 = 7.14 ≈ 7
This means approximately 7 persons over 50 years of age need to be vaccinated to prevent one hospitalization from influenza.
Clinical Context and Interpretation
Comparison to Real-World Data
This calculated NNV of 7 represents an exceptionally optimistic scenario compared to actual clinical evidence:
- The CDC reports that influenza vaccine is 30%-70% effective in preventing hospitalization among elderly persons living outside nursing homes 1, 2
- Real-world NNV values for preventing hospitalization in adults ≥65 years typically range from several hundred to over 2,000, depending on vaccine effectiveness and seasonal influenza burden 2, 3
- A recent 2025 study found NNV values of 2,262 to 7,662 for high-dose versus standard-dose vaccines in preventing hospitalizations among adults ≥65 years 3
Why This Scenario Differs
The 70% reduction in hospitalizations (from 20% to 6%) in your scenario represents:
- Unusually high baseline hospitalization rate: The 20% hospitalization rate in unvaccinated individuals is approximately 100-200 times higher than typical seasonal influenza hospitalization rates for this age group 2
- Exceptional vaccine effectiveness: The 70% reduction is at the upper limit of typical influenza vaccine performance 2
- This scenario might represent a severe pandemic season or a highly selected high-risk population 2
Mortality and Quality of Life Considerations
Vaccination remains critically important despite variable NNV values because:
- More than 90% of influenza deaths occur in adults ≥65 years, with 20,000-40,000 deaths during severe epidemic years 2
- Among nursing home residents, vaccination is 80% effective in preventing death 1, 2
- The vaccine prevents 54,000 to 430,000 hospitalizations annually in the United States, with 63% occurring among persons ≥65 years 1
Clinical Recommendation
For individuals over 50 years of age, annual influenza vaccination is strongly recommended regardless of the specific NNV calculation, as the intervention:
- Substantially reduces hospitalization risk even when absolute numbers vary by season 1, 2
- Provides critical mortality benefit, particularly in high-risk populations 1, 2
- Demonstrates cost-effectiveness with net societal savings among older adults 1
- Prevents secondary complications including pneumonia and exacerbations of chronic cardiopulmonary conditions 1