Number needed to vaccinate explained

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).

Derivation

NNV is the inverse of the absolute risk reduction of the vaccine. If the incidence in the vaccinated population is

I _{e} , and the incidence in the unvaccinated population is I _{u} , then the NNV is 1 /(I _{u} - I _{e)} .

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

Significance

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.^[5]

See also

• Number needed to treat

Notes and References

1. Kelly H, Attia J, Andrews R, Heller RF . The number needed to vaccinate (NNV) and population extensions of the NNV: comparison of influenza and pneumococcal vaccine programmes for people aged 65 years and over . Vaccine . 22 . 17–18 . 2192–2198 . June 2004 . 15149776 . 10.1016/j.vaccine.2003.11.052 .
2. Brisson M . Estimating the number needed to vaccinate to prevent herpes zoster-related disease, health care resource use and mortality . Can J Public Health . 99 . 5 . 383–386 . 2008 . 19009921 . 10.1007/BF03405246. 6976184 .
3. Lewis EN, Griffin MR, Szilagyi PG, Zhu Y, Edwards KM, Poehling KA . Childhood influenza: number needed to vaccinate to prevent 1 hospitalization or outpatient visit . Pediatrics . 120 . 3 . 467–472 . September 2007 . 17766517 . 10.1542/peds.2007-0167 . 25894405 .
4. Mooney JD, Weir A, McMenamin J, etal . The impact and effectiveness of pneumococcal vaccination in Scotland for those aged 65 and over during winter 2003/2004 . BMC Infect. Dis. . 8. 53 . 2008 . 18433473 . 2386805 . 10.1186/1471-2334-8-53 . free .
5. Van Rie A, Hethcote HW . Adolescent and adult pertussis vaccination: computer simulations of five new strategies . Vaccine . 22 . 23–24 . 3154–65 . August 2004 . 15297068 . 10.1016/j.vaccine.2004.01.067 .