Number needed to vaccinate

Number needed to vaccinate (NNV) is a metric used in the evaluation of vaccines,^[1][2][3] and in the determination of vaccination policy. It is defined as the average number of patients that must be vaccinated to prevent one case of disease. It is a specific application of the number needed to treat metric (NNT).

NNV is the inverse of the absolute risk reduction of the vaccine. If the incidence in the vaccinated population is ${\displaystyle I_{e}}$, and the incidence in the unvaccinated population is ${\displaystyle I_{u}}$, then the NNV is ${\displaystyle 1/(I_{u}-I_{e})}$.

For example, one study reported a number needed to vaccinate of 5206 for invasive pneumococcal disease.^[4]

In order to determine a NNV, it is necessary to identify a specific population and a defined endpoint, because these can vary:^{[citation needed]}

• Tuberculosis vaccination rates are much higher in Europe than in the United States.
• When evaluating a vaccine against chickenpox, it is necessary to define whether or not the endpoint would include shingles due to reactivation.
• If evaluating a HIV vaccine, the NNV may vary depending upon the expected standard of care in the absence of a vaccine, which may vary from continent to continent.
• If an infectious disease is acute and highly lethal, there may be large differences in the impact of the vaccine upon incidence and prevalence.

Despite the limitations, the NNV can serve as a useful resource. For example, it can be used to report the results of computer simulations of varying vaccination strategies.^[5]

• Number needed to treat