Let's talk about the attack rate.
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.
It is a kind of incidence rate that we use
specifically for infectious disease outbreaks.
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.
Okay, who's the at risk population?
People who are not yet infected
and who are likely to become infected.
And the proportion of people who
become infected in a given time interval
is the primary attack rate.
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.
That sounds complicated, right?
Let me explain that again.
If a bunch of people become
infected who are the contacts?
their household members,
their family members and so forth.
What percentage of those
contacts then become infected?
The difference between these two
attack rates tells us something of the
transmissibility of this disease.
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.
If you remember how we define incidence rate,
traditionally, we use the average population size.
For the primary attack rate, we're using the
population size at the start of the outbreak.
That's the key difference here in an
attack rate versus a traditional incidence rate.
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.
Here's an example of how we use attack rate.
So consider a dinner attended by 100 people.
12 of those people become ill.
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.
So the interviews reveal that
8 of the 12 people who got sick
and 25 of the remaining people ate fish.
So we build our contingency
table like so, fill in our data.
Now we can compute our attack rates.
So amongst those who ate
fish, 8 out of the 33 became sick.
Out of those who did not eat
the fish, 4 out of 67 became sick.
So that translates to attack
rates of 24.2% and 6%.
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.
That, of course, is the relative risk,
and 4 is a pretty high relative risk.
And from this analysis, we can reliably conclude
that the fish was the likely source of infection.