The objective of most field trials is to measure the effect of an intervention in reducing disease rates. The results of such studies may be used to estimate the impact that an intervention might have on disease rates if it was introduced into a public health programme. In such circumstances, the overall effect is much influenced by the coverageachieved by the programme.
The prevented fraction among individuals exposed to an intervention measure is defined as the percentage of the disease incidence in such individuals that has been prevented due to having received the intervention. For example, if the efficacy of BCG vaccination against TB is 60%, among persons who receive BCG vaccination, 60% of the TB cases that would have developed otherwise have been prevented by the vaccination. For vaccine studies, the prevented fraction is directly equivalent to the vaccine efficacy, but the former term may be used for interventions other than vaccines.
The prevented fraction is computed by subtracting the disease risk in individuals with the intervention measure (for example, an anti-leprosy vaccine) from the disease risk in individuals without the intervention, and expressing the difference as a proportion of the latter. For example, if the annual incidence of leprosy is 2.8 per thousand in the vaccinated and 4.2 in the unvaccinated, the prevented fraction is equal to[(4.2−2.8)/ 4.2]= 0.33 (or 33%).
If the relative risk (R) (of disease in those who receive the intervention, compared to those who do not) is known, the prevented fraction may be obtained by calculating(1− R). For example, if the relative risk of developing malaria in homes where mos-quito-nets are used is a quarter of that in homes where they are not used, the prevented fraction is equal to 1−0.25 , i.e. 75%.
The population prevented fraction is defined as the proportion of cases of the disease
in the total population that have been prevented by the intervention. If the relative risk (R) and the proportion of individuals in the population who receive the intervention measure (P) are known, the population prevented fraction is obtained by calculating P(1−R). Thus, the extent of reduction possible in disease incidence in the total population, if all individuals were to receive the intervention measure(P =1), is (1−R).
Consider a situation in which the annual incidence of TB is 2.0 per thousand in those who do not receive BCG vaccination and 0.8 per thousand in those who do, i.e. the relative risk in those vaccinated is 0.8/2.0 = 0.4. Table 21.20 shows the fraction of all cases prevented by the intervention, according to the disease incidence in the total population and the vaccination coverage.