From the Guidelines
The Number Needed to Treat (NNT) for flu shots is approximately 10-50, with the most recent and highest quality study suggesting that vaccinating 10-50 people can prevent one case of influenza. This variability in NNT is due to factors such as season, population, and flu strain match. For healthy adults, the NNT is typically around 40, while for older adults it's approximately 30-40 1. Children often have better response rates with an NNT of about 10-15.
Key Points to Consider
- The effectiveness of flu vaccines typically ranges from 40-60% in seasons with good vaccine-to-circulating strain matches 1.
- The NNT is higher (less effective) in years with poor matches between the vaccine and circulating strains.
- Annual flu vaccination remains recommended for everyone 6 months and older because influenza causes significant morbidity and mortality each year.
- Even when the vaccine doesn't prevent infection entirely, it often reduces symptom severity and complications, including hospitalization and death.
Recommendations
- The American Academy of Pediatrics (AAP) recommends influenza vaccination of everyone 6 months and older, including children and adolescents, during the 2024–2025 influenza season 1.
- The AAP recommends any licensed influenza vaccine product appropriate for age and health status and does not prefer one product over another.
- The number of influenza vaccine doses recommended for children remains unchanged in the 2024–2025 influenza season and depends on the child’s age at first dose administration and influenza vaccination history.
Important Considerations
- Influenza vaccine should be offered to children as soon as it becomes available, especially to those recommended to receive 2 doses 1.
- The recommended dose(s) ideally should be received by the end of October for optimal protection before the influenza.
- When a child is recommended to receive 2 doses of vaccine in a given season, the doses do not need to be the same brand.
- A child may receive a combination of IIV and LAIV if appropriate for age and health status.
From the Research
Influenza Vaccination Effectiveness
The effectiveness of influenza vaccination can be measured by the Number Needed to Treat (NNT) to prevent one case of influenza.
- The NNT can be calculated using the formula: NNT = 1 / (vaccine efficacy or effectiveness)
- However, the provided studies do not directly report the NNT for influenza vaccination.
Vaccine Effectiveness
- A study from 2019 2 reported an overall adjusted vaccine effectiveness of 47% (95% CI = 34%-57%) against all influenza virus infection associated with medically attended acute respiratory illness (ARI).
- Another study from 2022 3 reported an overall vaccine effectiveness of 16% (95% CI = -16% to 39%) against medically attended outpatient ARI associated with influenza A(H3N2) virus, which is considered not statistically significant.
- A study from 2019 4 estimated the vaccine effectiveness against outpatient, medically attended, laboratory-confirmed influenza to be 38% (95% CI, 31%-43%).
Prevention of Severe Influenza
- A study from 2022 5 reported an adjusted effectiveness of 60.7% (20.5-80.5) for the prevention of severe cases of influenza in a general acute hospital.
- The study from 2019 4 estimated that influenza vaccination prevented 7.1 million illnesses, 3.7 million medical visits, 109,000 hospitalizations, and 8,000 deaths in the United States during the 2017-2018 season.
Calculation of NNT
- Using the vaccine effectiveness reported in the studies, the NNT can be calculated as follows:
- For an overall vaccine effectiveness of 47% 2, NNT = 1 / 0.47 ≈ 2.13
- For an overall vaccine effectiveness of 16% 3, NNT = 1 / 0.16 ≈ 6.25
- For a vaccine effectiveness of 38% 4, NNT = 1 / 0.38 ≈ 2.63
- For an adjusted effectiveness of 60.7% 5, NNT = 1 / 0.607 ≈ 1.65
Note: These calculations are based on the reported vaccine effectiveness and should be interpreted with caution, as the NNT can vary depending on the specific population, setting, and outcome being measured.