The attack rate, in epidemiology, is the proportion of people within a population who are infected with a certain disease, having previously been healthy. This term is also known as incident ratio. This information is used mainly to determine the cause of the emergence of an epidemic in some region.
By determining the attack rate, you can investigate where the epidemic arose and then combat the cause. This rate is calculated by dividing the number of people who got sick by the number of people who are at risk of getting sick (that is, the number of healthy people in a given area).
The attack rate can be considered a biostatistic, as it measures the influence of a certain disease on a set of living beings that inhabit a region.
What is it for?
The main objective of the attack rate is to prevent a certain disease from spreading further in a region. By determining the attack rate, an in-depth study of the causes of a disease can be carried out, in order to then combat them and prevent major epidemics.
In addition, the attack rate is used to determine the fatality of a disease and to know how many people it has killed in a region.
It fulfills the function of determining only the new cases of a disease within a population. The cases of a disease that are registered in people who have already convalesced are not considered for the calculation of the attack rate, but in the prevalence rate.
Usually a certain measure of time is used to carry out this study. This allows real-time analysis of the emergence of an epidemic. That is, by studying a specific time it is possible to know when the disease arose and as a consequence of what it did.
Basically, the attack rate is the incidence of new cases encompassed within the same unit of time.
How is it calculated?
The attack rate is calculated relatively easily. Simply divide the number of people who have been affected by the epidemic (or disease) by the number of people who are considered to be at risk of being affected by it.
Determine the risk
Determining risk is the first and most intuitive step when it comes to calculating the attack rate. When you study a group of healthy people exposed to the environment where a disease is present, it is possible to know how easily these people are infected.
The proportion of people who have had the disease compared to those who have not is evaluated to obtain an estimate of the number of people who are likely to develop the disease.
No exact figure is achieved, but the larger the sample of exposed people studied, the greater the probability of determining overall risk. This will allow the attack rate to be calculated more effectively in any population group.
problems
When determining the risk to calculate the attack rate, certain problems may arise in the investigation.
The first of these risks is called "competent risk." Competent risk is the probability that a person has to die while the study of the disease is being carried out, but not from the disease, but from external causes.
For example, if you are conducting a study of an epidemic within a group of soldiers in Ukraine, it is likely that some of the soldiers being studied will die in war before determining the outcome of the study.
The second cause is the difficulty of studying the same people for a long period. In many cases, people may simply not show up at the study site and this makes it difficult to know if the person died or just did not show up for other reasons.
When a person does not appear at the study site, without having previously determined a reason, the person is considered lost and the state of their health is uncertain.
Incidence time
One of the terms that must be taken into account when conducting an attack rate study is the inability to distinguish the emergence of a risk within a study time.
That is, when a study is being carried out for a large amount of time, it is indifferent to the risk whether the disease arises in the first month or the second year. As long as the disease arises within the time period studied, the result is the same for the attack rate.
This presents a problem if you want to know when people are catching it and developing symptoms; therefore, it must be considered as part of the margin of error in these investigations.
Example
In a population of 5000 inhabitants, we want to determine the probability that someone will be infected with an STD (Sexually Transmitted Disease) in a period of 15 years.
At the beginning of the study, 350 cases of STDs were found within the population. These people should be excluded from the study, as they cannot develop the disease again and would damage the results of the attack rate.
Two years after the first evaluation, a second one is carried out and it is determined that 100 more cases of STD have arisen in the population. Then, 2 years later, a study is carried out again and it is determined that 70 more cases arose.
To measure the attack rate, it is evaluated how many people were infected and for how long they contributed to the results of the study.
In certain cases, it is difficult to determine when each person developed the disease, which causes the problem mentioned in the time of incidence.
However, there is a calculation that is applied in these cases to reduce the margin of error: it is assumed that the person was infected in the middle of the study time.
That is, if a study is conducted every two years and a healthy person was infected during one of the studies, it is assumed that they contracted the disease in the middle of the study (one year ago).
References
- Incidence: Risk, Cumulative Incidence (Incidence Proportion), and Incidence Rate, Boston University, (nd). Taken from bu.edu
- Attack rates and case fatality, Field Epidemiology Manual, 2014. Taken from Europa.eu
- Incidence rate and incidence proportion, V. Schoenbach, 2002. Taken from epidemolog.net
- Lesson 3: Measures of Risk, Centers for Disease Control and Prevention, (nd). Taken from cdc.gov
- Attack Rate, S. Pettygrove for Encyclopaedia Britannica, 2016. Taken from Britannica.com