Secondary attack rates measure the spread of infection within a closed group exposed to a primary case, calculated as the number of secondary cases divided by the number of susceptible individuals excluding the primary case. They are used to evaluate disease spread within families and the effectiveness of control measures like isolation and immunization. Limitations include difficulty identifying susceptibles for diseases with subclinical cases and accounting for variable exposure durations.

1. SECONDARY ATTACK
RATES

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.

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