2. • Secondary attack rates are defined as the number of
exposed persons developing the disease within the range
of the incubation period following exposure to a primary
case.
• Developed to measure the spread of the infection within
the family, household or any closed aggregate of persons
who have had contact with a case or disease.
3. The primary case is excluded from both the numerator
and denominator.
4. • For example, there is a family of 6 consisting of 2 parents
(already immune) and 4 children who are susceptible to a
specific disease, say measles. There occurs a primary case
and within a short time 2 secondary cases among the
remaining children. Calculate the secondary attack rate.
no. of secondary cases = 2
no. of children susceptible = 3 (excluding primary case)
Therefore, SAR = 2/3 = 66.6%
5. USES
• To measure the spread of infection within a family,
household or any closed aggregate of persons who have had
contact with a case of disease.
• Vaccinees and non-vaccinees from several families can be
added to determine the overall attack rates in the
vaccinated and unvaccinated populations.
• To determine whether a disease of unknown aetiology (e.g.,
Hodgkin's disease) is communicable or not.
• To evaluate the effectiveness of control measures such as
isolation and immunization.
6. LIMITATIONS
• When the primary case is infective over a long period of
time {e.g., tuberculosis), duration of exposure should be
considered.
7. • Another limitation of secondary attack rate is to
identify "susceptibles". It is feasible only in diseases
such as measles and chickenpox where history can be
used as a basis for identification.
• Where there are numerous subclinical cases, secondary
attack rate has a limited meaning. Further spread cannot
be measured without laboratory investigations.
