Number Needed to Vaccinate: Eight
The number needed to vaccinate (NNV) to prevent one hospitalization is 8 based on the data you've presented to the Chamber of Commerce.
Calculation Method
The NNV is calculated using the absolute risk reduction (ARR) between unvaccinated and vaccinated groups:
- Unvaccinated hospitalization rate: 200/1,000 = 20% (0.20)
- Vaccinated hospitalization rate: 60/1,000 = 6% (0.06)
- Absolute Risk Reduction (ARR): 20% - 6% = 14% (0.14)
- NNV = 1/ARR = 1/0.14 = 7.14, which rounds to 8
This means you need to vaccinate 8 people over age 50 to prevent one hospitalization from influenza.
Clinical Context and Supporting Evidence
This calculation aligns with the substantial body of evidence supporting influenza vaccination in adults over 50. The ACIP guidelines 1 specifically recommend vaccination for persons aged 50-64 years because this group has increased prevalence of high-risk conditions, with 24%-32% having one or more chronic medical conditions that place them at elevated risk for influenza-related hospitalization.
Among elderly persons living outside nursing homes, influenza vaccine is 30%-70% effective in preventing hospitalization for pneumonia and influenza 2. Recent evidence confirms vaccination's protective effect, with one study showing a 47% reduction in hospitalization risk among vaccinated individuals presenting with influenza-like illness 3.
Key Points for Your Presentation
- The 14% absolute risk reduction (from 20% to 6% hospitalization rate) represents a clinically meaningful benefit
- This NNV of 8 is quite favorable compared to many preventive interventions
- The calculation assumes similar influenza attack rates in both groups and focuses specifically on the outcome of hospitalization prevention
- This metric is particularly compelling for public health advocacy, as it translates vaccine effectiveness into concrete, understandable terms for community stakeholders