Please Note: Asecondary attack rateis sometimes calculated to document the difference between community transmission of illness versus transmission of illness in a household, barracks, or other closed population. It is calculated as:

1.Shigellosis is the third most common bacterial enteric disease.It is highly contagious, and does not have a marked seasonal trend. Transmission of Shigella occurs when people come into contact with the stool of a person infected with Shigella or eat food contaminated with Shigella. Food can become contaminated due to contact with infected individuals, contact with flies that have bred in infected feces, contact with human sewage in agricultural fields, swallowing recreational or drinking water contaminated by infected fecal matter, or exposure to feces through sexual contact.

During an outbreak of shigellosis 18 persons in 18 different households became ill in a town of 1,000. The overall attack rate is 1.8% (18/1,000 * 100%).

One incubation period later, 17 people in the same households as the primary cases developed shigellosis. These 17 people are the "Number of Cases Among Contacts of Primary Cases".

There was a total of 86 people in the 18 households.

What is the secondary attack rate? Hint, 86 goes in the denominator. However, in this denominator we must subtract out the people who have already had shigellosis.

Incidence Rate (Incidence Density)

We calculate an incidence rate (incidence density) when everyone in the study population is not followed for the same amount time. Both cumulative incidence and incidence rate use the number of new cases as the numerator. However, an incidence rate uses some measure of "person-time" as the denominator. If a quiz or test questions gives you person-time (e.g. person-years, person-months, etc.) consider whether you should calculate the incidence rate.

2.In a certain health district, a group of 80 school children that lacked antibody to measles were followed for a total of 125-person months for infection with measles. At the end of two months of follow-up, 10 children had acquired antibody to measles. What is the incidence rate for measles infection in this population of children?