Secondary attack rates

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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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