(b) Secondary attack rate (SAR): is defined as the number of exposed persons developing the disease within the range of incubation period following exposure to a primary case.
Formula:
Number of exposed persons developing the disease
within the range of incubation period
SAR = ------------------------------------------------------------------X 100
Total number of exposed/ susceptible contacts
The denominator consists of all persons who are exposed to case. More specifically, the denominator may be restricted only to susceptible contacts. The primary case is excluded from both numerator and denominator.
Example: Suppose 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 is a primary case and within a short time 2 secondary cases among the remaining children. The secondary attack rate is 2/3 or 66.6%.
Exercise 1 – During an eight hour work shift at a factory , 30 employee visited the company physician with the complaints of nausea , vomiting headaches & dizziness.
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