Calculating Number Needed to Treat for Influenza Vaccination
The number needed to treat (NNT) to prevent one hospitalization for influenza in this scenario is 7.14, or approximately 7 patients.
Calculation Method
To calculate the NNT from the data provided, we need to follow these steps:
- Determine the absolute risk reduction (ARR)
- Calculate NNT as 1/ARR
Step 1: Calculate Absolute Risk Reduction (ARR)
- Hospitalization rate in unvaccinated group: 200 per 1000 (20%)
- Hospitalization rate in vaccinated group: 60 per 1000 (6%)
- Absolute risk reduction (ARR) = 20% - 6% = 14% (or 0.14)
Step 2: Calculate NNT
- NNT = 1/ARR = 1/0.14 = 7.14 ≈ 7
Clinical Significance
This NNT of 7 represents a substantial clinical benefit. It means that for every 7 people vaccinated against influenza, one hospitalization can be prevented. This is a very favorable NNT compared to many other preventive interventions in medicine.
Context Within Influenza Prevention Guidelines
The Advisory Committee on Immunization Practices (ACIP) consistently recommends annual influenza vaccination as the most effective means of reducing influenza-related hospitalizations and complications 1.
The calculated NNT of 7 in this scenario is considerably better than estimates from published studies:
- For children 6-23 months, the NNT to prevent one hospitalization ranges from 1031 to 3050 2
- For children 24-59 months, the NNT ranges from 4255 to 6897 2
- For adults, the NNT to prevent one influenza-like illness is approximately 29 3
High-Risk Populations
The effectiveness of influenza vaccination in preventing hospitalizations is particularly important for high-risk groups:
- Adults aged ≥65 years
- Children <5 years (especially those <2 years)
- Pregnant women
- Persons with underlying medical conditions
These groups experience higher rates of influenza-related hospitalizations and complications 1.
Impact on Healthcare System
With an estimated 114,000-226,000 influenza-related hospitalizations annually in the United States 1, an intervention with an NNT of 7 could substantially reduce this burden. The CDC estimated that vaccination prevented approximately 79,000 hospitalizations (17% of potential hospitalizations) during the 2012-13 influenza season 4.
Limitations to Consider
- The NNT calculated here is specific to the scenario presented and may not be generalizable to all populations or all influenza seasons
- Vaccine effectiveness varies by season, circulating strains, and population characteristics
- The quality of match between vaccine strains and circulating viruses affects effectiveness
Conclusion
With an NNT of 7, influenza vaccination in this scenario represents a highly effective intervention for preventing hospitalizations. This strong benefit supports current recommendations for widespread annual influenza vaccination, particularly among high-risk groups.