Number needed to vaccinate
Number needed to vaccinate (NNV) is a metric used in the evaluation of vaccines, and in the determination of vaccination policy. It is a specific application of the number needed to treat metric (NNT) that incorporates the implications of herd immunity.
NNV is the statistical inverse of incidence i.e. 1/incidence. In other words, in case of the vaccination for a disease with incidence of 1 per 1000, the NNV is 1000.
In order to determine a NNV, it is necessary to identify a specific population and a defined endpoint, because these can vary:
- 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.
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