Attack Rate

1
Attack Rate

• What is it and how do we calculate it?

2
Attack Rate

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

3
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

4
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.

5
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

6
Attack Rate

• 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 ab a/(ab) x 100
Did not eat food c d cd c/(cd) x 100
7
Attack Rate

• Example of an attack rate table

Sick Well Total Attack Rate
Fish 7 3 10 7/1070
Did not eat fish 11 7 18 11/1861
8
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

9
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.

10
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.