Number Needed to Vaccinate Calculation
Based on the data provided, the number needed to vaccinate (NNV) to prevent one hospitalization is approximately 7.
Calculation Method
The NNV is calculated using the absolute risk reduction (ARR) between vaccinated and unvaccinated groups:
- Unvaccinated group: 200/1000 = 20% hospitalization rate
- Vaccinated group: 60/1000 = 6% hospitalization rate
- Absolute Risk Reduction (ARR): 20% - 6% = 14%
- NNV: 1/ARR = 1/0.14 = 7.14 ≈ 7 people
This means approximately 7 people need to be vaccinated to prevent one hospitalization.
Context and Interpretation
Comparison to Published Data
This calculated NNV of 7 is substantially more favorable than most published influenza vaccination studies:
- Pregnant women: The CDC reports an NNV of approximately 55 to prevent one case of influenza in mothers during the 2009 H1N1 pandemic 1
- General adult populations: Studies report NNV ranging from 40-50 for high-risk adults to several hundred for healthy younger adults 2
- Elderly populations: Among adults ≥65 years, the vaccine is 30-70% effective in preventing hospitalization, which translates to much higher NNV values than your scenario 1, 2
- COVID-19 booster data: During Omicron BA.1 predominance, the median NNV to prevent one hospitalization was 205 (range 44-615), with lower values of 110 for adults ≥65 years 3
Critical Considerations
Your scenario represents an unusually high-risk population or severe epidemic conditions where:
- The baseline hospitalization rate of 20% in unvaccinated individuals is extraordinarily high compared to typical influenza seasons 2
- For context, even high-risk adults typically have hospitalization rates of 80-400 per 100,000 (0.08-0.4%), not 20% 2
- Such high rates might be seen during pandemic conditions, where hospitalization rates can be tenfold higher than typical seasons 2
The vaccine effectiveness of 70% (reducing hospitalizations from 20% to 6%) is at the upper end of typical influenza vaccine performance 1, 2, 4
Methodological Caution
Traditional NNV calculations that ignore transmission effects can produce estimates biased by up to 1000-fold compared to calculations incorporating indirect vaccine effects through reduced community transmission 5. Your calculation represents a direct protection estimate only and likely underestimates the true population-level benefit of vaccination 5.
The NNV is highly dependent on local disease incidence, outcome severity, and patient risk factors 3. The value of 7 would only apply to populations with similar baseline risk profiles and during periods of similar disease activity 3, 6.