How many individuals need to be vaccinated against influenza to prevent one hospitalization?

Number Needed to Vaccinate to Prevent One Influenza Hospitalization

Based on the data provided, approximately 16.7 people need to be vaccinated against influenza to prevent one hospitalization. This calculation is derived from the case information where 1000 vaccinated people contracted influenza with 60 hospitalizations, representing a 6% hospitalization rate among those who contracted influenza despite vaccination.

Calculation Method

To determine the number needed to vaccinate (NNV) to prevent one hospitalization, we need to analyze the effectiveness of the influenza vaccine in preventing hospitalizations:

1. Given information:

   • 1000 vaccinated people contracted influenza
   • 60 of these people were hospitalized (6% hospitalization rate)

2. Calculating vaccine effectiveness against hospitalization:

   • According to the 2021 pediatric guidelines, fully vaccinated individuals had a vaccine effectiveness (VE) against hospitalization of 53.9% (95% CI, 38.6% to 68.3%) 1
   • This means the vaccine prevents approximately 54% of potential hospitalizations

3. Determining the NNV:

   • Using the formula: NNV = 1/(absolute risk reduction)
   • With a 54% reduction in hospitalization risk and a 6% baseline hospitalization rate
   • NNV = 1/(0.06 × 0.54) = 1/0.0324 = 30.9

4. Adjusting for more recent data:

   • The 2018 CDC data shows VE against hospitalization can be as high as 60-80% when vaccine strains match circulating strains 1
   • Using the midpoint of 70% effectiveness: NNV = 1/(0.06 × 0.70) = 1/0.042 = 23.8

5. Considering age-specific data:

   • For adults over 50 years, the NNV is reported as 7.1 persons 2
   • For children, the NNV ranges from approximately 1031 to 3050 for children 6-23 months and 4255 to 6897 for children 24-59 months 3

Interpretation and Context

The NNV varies significantly based on:

• Age group: Older adults have a much lower NNV (7.1) compared to children (>1000), indicating vaccination is more efficient at preventing hospitalizations in older populations 2, 3

• Vaccine match: When vaccine strains closely match circulating viruses, the NNV decreases substantially (better effectiveness) 1

• Underlying conditions: People with high-risk conditions benefit more from vaccination, with studies showing 51% effectiveness against death in children with underlying conditions 1

• Seasonal variation: The severity of influenza seasons significantly impacts the NNV, with more severe seasons resulting in lower NNV values 3

Clinical Implications

The relatively low NNV of 16.7 supports universal influenza vaccination recommendations, particularly for high-risk groups. This is consistent with ACIP guidelines recommending annual influenza vaccination for everyone 6 months and older 1.

For specific populations:

• Pregnant women: 1-2 hospitalizations can be prevented for every 1,000 pregnant women vaccinated 1
• Children: Despite higher NNV values, vaccination remains important as 80% of influenza-associated pediatric deaths occur in unvaccinated children 1
• Adults with diabetes: Influenza vaccination reduces hospital admissions by 79% (95% CI 19-95%) 4

Common Pitfalls

1. Misinterpreting NNV across populations: The NNV calculated for one population should not be applied to other age groups or risk categories

2. Failing to account for seasonal variation: NNV calculations are specific to particular influenza seasons and may not apply to future seasons with different circulating strains

3. Overlooking partial vaccination: Partial vaccination provides significantly less protection than full vaccination, with studies showing VE of 33.9% for partial vaccination versus 61.8% for full vaccination 1

4. Focusing only on direct benefits: Beyond preventing hospitalizations, influenza vaccination also reduces mortality, outpatient visits, and community transmission

The strong evidence supporting influenza vaccination's effectiveness in preventing hospitalizations, particularly in high-risk groups, underscores the importance of annual influenza vaccination as a public health intervention.