Attack Rate What is it and how do

Attack Rate What is it and how do we calculate it?

Attack Rate Exposure: A potential risk factor can be an actual exposure, a behavior, or an attribute Outcome: A disease or health-related condition

Attack Rate l Attack rate is the number of people who get sick divided by the total number of people exposed or unexposed x 100% – – # of sick people/ total people exposed x 100% or # of sick people/ total people unexposed x 100%

Attack Rate l Epidemiologists use similar calculations to find out what things might cause people to get sick. l Epidemiologists would look at each food someone ate and determine the food specific attack rate.

Attack Rate l To calculate a food specific attack rate: – Number of people who ate a particular food and got sick Total number of people who ate that particular food x 100% – Example - # of people who ate fish and got sick Total # of people who ate fish x 100% 7 people/10 people x 100% = 70% attack rate

Attack Rate l l a= # of people who ate the food and got sick b= # of people who ate the food and did not get sick c= # of people who did not eat the food and got sick d= # of people who did not eat the food and did not get sick Sick Well Total Attack Rate Food a b a+b a/(a+b) x 100% Did not eat food c d c+d c/(c+d) x 100%

Attack Rate Fish l Sick Well Total Attack Rate 7 3 10 7/10=70% Did 7 rate table: 18 11/18=61% Example of 11 an attack not eat fish

Attack Rate Ratio l Epidemiologists can look at a ratio of attack rates – Example: Attack rate % for sick students who ate fish Attack rate % for sick students who did not eat fish 70%/61% = 1. 14

Attack Rate l Epidemiologists can compare attack rate ratios 1. 2. 3. 4. A ratio of 1. 0 means that there is no increased or decreased risk of getting sick by eating the fish. A ratio above 1. 0 means that there is an increased risk of getting sick by eating the fish. A ratio below 1. 0 means that there is a decreased risk of getting sick by eating the fish. However, since the ratio was 1. 15, very close to 1. 0, this means that the fish was not very likely to have been the reason that the students got sick.

Summary l You can calculate a food specific attack rate. l You can calculate an attack rate ratio. l This information will help you determine what food could be causing the outbreak. l Epidemiologists can use these calculations for lots of different diseases – not just with food.