Computation and Measurement of Risk - Mathematics and Statistics for the Environment - page 188

Often, the total number of contacts in the denominator is calculated as the total population in the
households of the primary cases minus the number of primary cases. For a secondary attack rate,
10 n usually is 100%. An example of calculating secondary attack rates is illustrated below.
■ EXAMPLE 8.7
Problem: Consider an outbreak of shigellosis in which 18 persons in 18 different households all
became ill. If the population of the community was 1000, then the overall attack rate was 18/1000
× 100% = 1.8%. One incubation period later, 17 persons in the same households as these “pri-
mary” cases developed shigellosis. If the 18 households included 86 persons, calculate the second-
ary attack rate.
Solution:
Secondary attack rate = [17/(86 - 18)] × 100% = (17/68) × 100% = 25.0%
8.3.2 i nCidenCe r ate or p erson -t ime r ate
Incidence rate or person-time rate is a measure of incidence that incorporates time directly into the
denominator. A person-time rate is generally calculated from a long-term cohort follow-up study,
wherein enrollees are followed over time and the occurrence of new cases of disease is documented.
Typically, each person is observed from an established starting time until one of four “end points”
is reached: onset of disease, death, migration out of the study (“lost to follow-up”), or the end of the
study. Similar to the incidence proportion, the numerator of the incidence rate is the number of new
cases identified during the period of observation. However, the denominator differs. The denomi-
nator is the sum of the time each person was observed, totaled for all persons. This denominator
represents the total time the population was at risk of and being watched for disease. Thus, the
incidence rate is the ratio of the number of cases to the total time the population is at risk of disease:
Number of new cases of diseaseorinjury during specified period
Time each person was observed, totaledfor allpersons
In a long-term follow-up study of morbidity, each study participant may be followed or observed
for several years. One person followed for 5 years without developing disease is said to contribute
5 person-years of follow-up.
What about a person followed for one year before being lost to follow-up at year 2? Many
researchers assume that persons lost to follow-up were, on average, disease-free for half the year,
and thus contribute 1/2 year to the denominator. Therefore, the person followed for one year before
being lost to follow-up contributes 1.5 person-years. The same assumption is made for participants
diagnosed with the disease at the year 2 examination—some may have developed illness in month
1, and others in months 2 through 12. So, on average, they developed illness halfway through the
year. As a result, persons diagnosed with the disease contribute 1/2 year of follow-up during the
year of diagnosis.
DID YOU KNOW?
A cohort study is a systematic review of the management of patients with disease and their
contacts. A “cohort” is a group of disease cases counted over a specific period of time, usually
3 months. Disease cases are reviewed for the patient's clinical status, the adequacy of the med-
ication regimen, treatment adherence of completion, and the results of contact investigation.