Number needed to vaccinate
Encyclopedia
Number needed to vaccinate (NNV) is a metric used in the evaluation of vaccine
s, 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
.
In order to determine a NNV, it is necessary to identify a specific population and a defined endpoint, because these can vary:
For example, one study reported a number needed to vaccinate of 5206 for invasive pneumococcal disease.
Still, 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.
Vaccine
A vaccine is a biological preparation that improves immunity to a particular disease. A vaccine typically contains an agent that resembles a disease-causing microorganism, and is often made from weakened or killed forms of the microbe or its toxins...
s, and in the determination of vaccination policy. It is a specific application of the number needed to treat
Number needed to treat
The number needed to treat is an epidemiological measure used in assessing the effectiveness of a health-care intervention, typically a treatment with medication. The NNT is the average number of patients who need to be treated to prevent one additional bad outcome...
metric (NNT) that incorporates the implications of herd immunity
Herd immunity
Herd immunity describes a form of immunity that occurs when the vaccination of a significant portion of a population provides a measure of protection for individuals who have not developed immunity...
.
In order to determine a NNV, it is necessary to identify a specific population and a defined endpoint, because these can vary:
- TuberculosisTuberculosisTuberculosis, MTB, or TB is a common, and in many cases lethal, infectious disease caused by various strains of mycobacteria, usually Mycobacterium tuberculosis. Tuberculosis usually attacks the lungs but can also affect other parts of the body...
vaccination rates are much higher in Europe than in the United States. - When evaluating a vaccine against chickenpoxChickenpoxChickenpox or chicken pox is a highly contagious illness caused by primary infection with varicella zoster virus . It usually starts with vesicular skin rash mainly on the body and head rather than at the periphery and becomes itchy, raw pockmarks, which mostly heal without scarring...
, it is necessary to define whether or not the endpoint would include shingles due to reactivation. - If evaluating a HIV vaccineHIV vaccineAn HIV vaccine that protects vaccinated individuals from HIV infection is the goal of many HIV research programmes. Currently, there is no effective vaccine against HIV, the virus that causes AIDS...
, 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 incidenceIncidenceIncidence may refer to:* Incidence , a measure of the risk of developing some new condition within a specified period of time* Incidence , the binary relations describing how subsets meet...
and prevalencePrevalenceIn epidemiology, the prevalence of a health-related state in a statistical population is defined as the total number of cases of the risk factor in the population at a given time, or the total number of cases in the population, divided by the number of individuals in the population...
.
For example, one study reported a number needed to vaccinate of 5206 for invasive pneumococcal disease.
Still, 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.