A ceain community has 100 children out of whom 28 are immunised against measles; 2 of them acquire measles simultaneously; Subsequently 14 get measles. Assuming the efficacy of the vaccine to be 100%; What is the Secondary Attack rate?

Secondary attack rate (SAR) is defined as "the number of exposed person developing the disease within the range of incubation period, following exposure to the primary case". The denominator consists of all persons who are exposed to the case. No. of exposed persons developing the disease within the range of incubation period SAR= ------------------------------------------------------------------------------------*100 Total number of exposed/"susceptible" contacts In the given question No of exposed developing the disease is =14 No of susceptible children: (total no of children - no of immunized children)=100-28=72 -2=70 Two children develops measles (primary case) so 72-2=70 SAR = 14/70*100=20% thus 20% is the secondary attack rate Ref: Park's Textbook of Preventive and Social Medicine By K. Park, 19th Edition, Pages 56, 94.