Attack Rate

00:01 Let's talk about the attack rate.

00:03 The attack rate is not truly a rate, it's a proportion but we use the term 'attack rate' because we've been doing it for a long time, we're not going to change.

00:14 It is a kind of incidence rate that we use specifically for infectious disease outbreaks.

00:21 So the primary attack rate or the incidence proportion is the percentage of the population that contracts the disease in an at-risk population during a specified time interval.

00:32 Okay, who's the at risk population? People who are not yet infected and who are likely to become infected.

00:39 And the proportion of people who become infected in a given time interval is the primary attack rate.

00:46 The secondary attack rate though is the percentage of the contacts of the people who became infected the first time who then go on to become infected.

00:57 That sounds complicated, right? Let me explain that again.

01:01 If a bunch of people become infected who are the contacts? their household members, their family members and so forth.

01:08 What percentage of those contacts then become infected? The difference between these two attack rates tells us something of the transmissibility of this disease.

01:18 How likely is it to move from one population to the next? How likely is it to transmit and find good penetration throughout society? So the primary attack rate is the number of new cases of the disease during a particular time interval divided by the population at the start of the time interval.

01:43 If you remember how we define incidence rate, traditionally, we use the average population size.

01:51 For the primary attack rate, we're using the population size at the start of the outbreak.

01:56 That's the key difference here in an attack rate versus a traditional incidence rate.

02:01 The secondary attack rate is the number of new cases among the contacts of the people became infected, divided by the total number of contacts available to us.

02:12 Here's an example of how we use attack rate.

02:15 So consider a dinner attended by 100 people.

02:19 12 of those people become ill.

02:22 So investigators interview all 100 of those people to determine something about their food consumption patterns at that dinner because they suspect something that they ate made them ill.

02:34 So the interviews reveal that 8 of the 12 people who got sick and 25 of the remaining people ate fish.

02:42 So we build our contingency table like so, fill in our data.

02:47 Now we can compute our attack rates.

02:50 So amongst those who ate fish, 8 out of the 33 became sick.

02:56 Out of those who did not eat the fish, 4 out of 67 became sick.

03:01 So that translates to attack rates of 24.2% and 6%.

03:08 Okay, what does that tell us? It tells us those who ate the fish were four times more likely to get sick than those who did not eat the fish.

03:17 That, of course, is the relative risk, and 4 is a pretty high relative risk.

03:21 And from this analysis, we can reliably conclude that the fish was the likely source of infection.