What is it and how do we calculate it? 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 Before understanding the definition of an attack rate, you should know the definitions for exposure and outcome. An exposure is a potential risk factor in a public health study. This risk factor can be an actual exposure (tobacco smoke), a behavior (sedentary lifestyle), or attribute (age). An outcome is a disease or health-related condition in a public health study.

Attack Rate 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% An attack rate can be the number of people who are sick divided by the total number of people exposed to the risk factor. It can also be the number of people who are sick divided by the total number of people unexposed to the risk factor. In this lesson plan, the risk factor is the food that each student ate.

Attack Rate Epidemiologists use similar calculations to find out what things might cause people to get sick. Epidemiologists would look at each food someone ate and determine the food specific attack rate. Your role today is very similar to the role of an epidemiologist.

Attack Rate 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 We know from completing our charts that there were at a total of 10 people who ate the fish and 7 of them were sick.

Attack Rate Sick Well Total Attack Rate Food a b a+b a/(a+b) x 100% 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% This is an attack rate table that an epidemiologist would use to calculate attack rates. You will complete a table very similar to this one.

Attack Rate Example of an attack rate table: Sick Well Total Sometimes, an epidemiologist will use a table to help them organize their information and calculate attack rates for each food. This is an example of part of the table. 70% of the students who ate fish were sick. 61% of the students who did not eat fish were sick. Sick Well Total Attack Rate Fish 7 3 10 7/10=70% Did not eat fish 11 18 11/18=61%

Attack Rate Ratio 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 This attack rate ratio reads: Students who ate the fish were 1.14 times more likely to get sick than students who did not eat the fish.

Attack Rate Epidemiologists can compare attack rate ratios 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. A good rule of thumb for determining a relationship for attack rate ratios (or risk ratios) in foodborne disease: >5 Very strong association (increased risk) 3.0 -<5.0 Strong association (increased risk) 1.7 -<3.0 Moderate association (increased risk) 1.3-1.6 Weak association (increased risk) 0.9-1.2 Probably no association 0.5-0.8 Weak association (decreased risk)

Summary You can calculate a food specific attack rate. You can calculate an attack rate ratio. This information will help you determine what food could be causing the outbreak. Epidemiologists can use these calculations for lots of different diseases – not just with food. [Please read slide] [Created by Lauren Almaguer, CDC Science Ambassor, 2004.]