Practical Manual - Malareo

1999 (which was year 1 of the 10 year study) = 57 people
=57 x 10 = 570 person years
2000 = 15 =15 x 9 = 135 person years etc
This is done for each year. At the end of the study 250 people were not infected (340 – 90) = 250 x 10 =
2500 person years Add all these person years together to get the influence of all affected individuals per
year
=570+135+40+28+6+5+12+4+0+2= 802 person years
Add the cumulative infected total person years to the total uninfected years =2500
=802+2500=3302 person years.
The incidence rate is the total number of infected divided by the total cumulative person years in the
sample or study = 90/3302 = 0.027256 new cases of malaria per year per person
In other words Incidence rate is 27 anticipated new cases per thousand people each year.
It therefore shows anticipated risk of infection.
Prevalence
Prevalence is a lot simpler and is referred to as the total number of cases of malaria within the entire
population of an area at a given point in time or over the entire period. It is the number of affected
divided by the total population.
So in 1999 the prevalence was 57/340 = 16.7% which is extremely high! Over 3 people in 20 had malaria.
However we want to know what the average prevalence is over all years as prevalence varies with time
due to availability of breeding sites etc.
To work it out you need to add all the cases together and divide it by the total population
The total number of cases is therefore 57+15+5+4+1+1+3+2+0+2= 90
The average total population over this period is 340 the field Pop05 (Population from 2005)
So if we look at the Monzi example:
Total Prevalence of malaria within the population over the 10 year period is (90/340) x 100 =26.5%
QGIS for Malaria Page 59