This is defined as
the number of exposed and susceptible people who became cases/ total number of exposed and susceptible people
and is expressed as a percentage.
Note: Attack rate is a proportion, not a rate, since there is no time unit in the calculation.
Attack rates are calculated for acute conditions which occur within hours to days after exposure.
Secondary Attack Rate:
The number of exposed at risk people who develop the condition within range of incubation period/ Total number of at risk people who got exposed
expressed as a percentage.
The infectious case who exposed the at risk people to infection, is not included in the calculation. Similarly, those who are immune are excluded from the denominator.
It is typically calculated in a closed population like a household, barracks, etc. This is due to practical considerations- it is easier to accurately determine the denominator. That said, secondary attack rate can be calculated in other situations as well, as long as the required data is available. Knowledge of the incubation period is critical to calculate secondary attack rate.
100 students live in a hostel, of whom 50 have been vaccinated against Measles. A case of Measles attended a hostel function during the prodromal period. Six days later 5 students developed Measles. Ten days after the function, 20 students developed Measles and 5 more days later, 10 students developed Measles. What is the secondary attack rate?
As 50 students were previously vaccinated, only 50 students are susceptible to Measles infection. Of these, 5 developed Measles outside the range of incubation period (7-21 days) since Measles occurred 6 days after the exposure. These are excluded from the calculation, leaving us with 45 susceptible students. This is the denominator.
The number of susceptible students who developed Measles within range of incubation period is 20+10= 30. This is the numerator.
Substituting the values, we calculate Secondary Attack Rate as: 30/45 *100 (%) = 66.67%