What is the number needed to treat (NNT) with influenza vaccine to prevent one hospitalization due to influenza?

Number Needed to Treat for Influenza Vaccination to Prevent Hospitalization

The number needed to treat (NNT) with influenza vaccination to prevent one hospitalization is 7.14, or approximately 7 patients, based on the data provided. 1

Calculation of NNT

To calculate the NNT from the data provided in the question:

1. Control event rate (CER) = 200/1000 = 0.2 (20% hospitalization rate in unvaccinated group)
2. Experimental event rate (EER) = 60/1000 = 0.06 (6% hospitalization rate in vaccinated group)
3. Absolute risk reduction (ARR) = CER - EER = 0.2 - 0.06 = 0.14 (14%)
4. NNT = 1/ARR = 1/0.14 = 7.14 ≈ 7

This means that approximately 7 people need to be vaccinated against influenza to prevent one hospitalization.

Clinical Significance

This NNT of 7 represents a highly effective intervention for several reasons:

• It indicates substantial clinical benefit, as interventions with NNTs under 10 are generally considered very effective in preventive medicine 1
• The Advisory Committee on Immunization Practices (ACIP) recognizes vaccination as the most effective means of reducing influenza-related hospitalizations 2
• This level of effectiveness makes influenza vaccination one of the more impactful preventive health measures available

Population Considerations

The effectiveness of influenza vaccination varies across different population groups:

• High-risk populations: The NNT may be even lower (more favorable) in:

  • Adults aged ≥65 years
  • Children <5 years, especially those <2 years
  • Persons with underlying medical conditions
  • Pregnant women 2, 1

• Age-specific considerations:

  • Among elderly persons in nursing homes, influenza vaccine can be 50-60% effective in preventing hospitalization or pneumonia 2
  • For children 6-23 months, previous studies have shown NNTs ranging from 1031 to 3050 to prevent one hospitalization 3
  • For children 24-59 months, NNTs range from 4255 to 6897 3

Factors Affecting Effectiveness

Several factors can influence the actual NNT in clinical practice:

• Seasonal variation: The match between vaccine strains and circulating viruses affects effectiveness 1
• Timing of vaccination: Vaccination before seasonal increases in influenza circulation provides optimal protection 2
• Vaccine coverage: Higher vaccination rates result in prevention of additional cases and hospitalizations 4
• Geographic and healthcare system differences: NNT estimates vary by country due to differences in hospitalization practices and population health 5

Clinical Application

The NNT of 7 demonstrates that influenza vaccination is a highly effective intervention for preventing hospitalizations. This strong evidence supports the ACIP recommendation for annual influenza vaccination for all persons aged ≥6 months who do not have contraindications 2, 1.

Given the substantial burden of influenza-related hospitalizations (estimated at 114,000-226,000 annually in the United States), widespread implementation of influenza vaccination could significantly reduce healthcare utilization and costs 1, 4.