This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Social & Preventive Medicine
Denominator while calculating secondary attack rate following a person with a primary disease is: March 2009 
Select and explain. 
A. All the contacts who are exposed to the affected person, B. All the people in his home, C. All the people in his family, D. All the people in his city
A: All the contacts who are exposed to the affected person
Answer: A: All the contacts who are exposed to the affected person 
Explanation: Ans. A: All the contacts who are exposed to the affected A secondary attack rate measures the frequency of new cases of a disease among the contacts of known cases. This can be very impoant for diseases that are spread from person to person, such as tuberculosis, measles, shigellosis, and varicella. Secondary attack rates are often calculated for households. To calculate the number of household contacts (denominator), subtract the number of primary cases from the total number living in the households. In some situations, contacts in other settings may be investigated (for example, residents of a homeless shelter, or people who work in a specific building). The calculation is done in the same way as for household contacts.