Number Needed to Vaccinate to Prevent One Hospitalization
The number needed to vaccinate (NNV) to prevent one hospitalization in individuals over 50 years old is 7.1 persons.
Calculation Method
To calculate the NNV, we need to determine the absolute risk reduction (ARR) between the unvaccinated and vaccinated groups, then calculate the reciprocal of this value:
- Unvaccinated group: 200 hospitalizations per 1000 people (20%)
- Vaccinated group: 60 hospitalizations per 1000 people (6%)
- Absolute risk reduction (ARR) = 20% - 6% = 14%
- NNV = 1/ARR = 1/0.14 = 7.1
Interpretation and Context
This NNV of 7.1 means that approximately 7 individuals over 50 years old would need to be vaccinated to prevent one hospitalization. This represents a highly effective intervention, as relatively few people need to be vaccinated to achieve a meaningful clinical outcome.
Importance of Vaccination in Older Adults
The Advisory Committee on Immunization Practices (ACIP) guidelines emphasize that vaccination of older adults is particularly important for preventing serious complications and reducing the risk of hospitalization and death 1. Among elderly persons, influenza vaccine is 30%-70% effective in preventing hospitalization for pneumonia and influenza 1.
Factors Affecting NNV Calculations
Several important considerations when interpreting NNV calculations:
NNV calculations that exclude transmission effects (as in this direct calculation) may overestimate the number needed to vaccinate, as they don't account for indirect protection through herd immunity 2
The effectiveness of vaccination varies by:
- Age of recipient (may be lower in those >70 years) 1
- Match between vaccine and circulating virus strains
- Individual immune response
The CDC reports that influenza vaccination can reduce both healthcare costs and productivity losses, with substantial reductions in hospitalization and death among older adults 1
Clinical Application
When counseling patients over 50 years old about vaccination:
- Emphasize that only about 7 people need to be vaccinated to prevent one hospitalization
- Note that vaccination is particularly effective in preventing severe outcomes in this age group
- Explain that the benefits extend beyond the individual to the community through reduced transmission
Common Pitfalls in NNV Calculations
- Failing to account for cumulative effects over time can grossly underestimate vaccine impact 3
- Not considering indirect effects through reduced transmission 2
- Using only short-term data rather than considering the full duration of protection
The NNV of 7.1 demonstrates that vaccination in this population is a highly efficient intervention for preventing hospitalizations, supporting strong recommendations for vaccination of adults over 50 years old.