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Attack Rate

Discover the shocking truth about attack rates and how they affect your safety, revealing eye-opening statistics and empowering insights to help you protect yourself.

USMLE Guide: Attack Rate


In the context of epidemiology, the attack rate is a crucial measure used to quantify the occurrence of new cases of a disease within a defined population during a specific period of time. It is particularly useful in outbreak investigations and provides valuable insights into the transmission dynamics and risk factors associated with the disease. This guide aims to provide a comprehensive understanding of attack rate, its calculation, interpretation, and significance.

Key Concepts

Attack Rate

  • Attack rate, also known as the incidence proportion, represents the proportion of individuals who develop a specific disease within a population at risk during a given time period.
  • It is expressed as a percentage and is calculated by dividing the number of new cases by the total number of individuals at risk, then multiplying by 100.

Population at Risk

  • The population at risk refers to individuals who are susceptible to the disease under investigation and are exposed to the source of infection or risk factors during the defined time period.
  • It is important to exclude individuals who are already affected by the disease or are immune, as they are not at risk of developing the disease during the specified time frame.

Outbreak Investigation

  • Attack rates play a fundamental role in outbreak investigations as they help identify the magnitude of the outbreak and evaluate the effectiveness of control measures.
  • Comparing attack rates among different populations or subgroups can help identify risk factors, modes of transmission, and potential sources of infection.


To calculate the attack rate, follow these steps:

  1. Determine the total number of individuals in the population at risk (excluding those already affected or immune).
  2. Identify the number of new cases of the disease during the specified time period.
  3. Divide the number of new cases by the total number of individuals at risk.
  4. Multiply the result by 100 to express it as a percentage.

Interpretation and Significance

  • Attack rates provide valuable information about the likelihood of developing a specific disease within a defined population during a given time frame.
  • Higher attack rates indicate a greater risk of acquiring the disease, suggesting a more significant outbreak or increased vulnerability of the population.
  • Comparing attack rates among different populations or subgroups can reveal variations in susceptibility, exposure, or effectiveness of preventive measures.
  • Attack rates can guide public health interventions, such as implementing control measures, allocating resources, and designing targeted interventions to reduce the spread of the disease.


Understanding the attack rate is vital for healthcare professionals, particularly in the context of outbreak investigations and public health interventions. By calculating and interpreting attack rates, healthcare providers can assess the magnitude and impact of a disease outbreak, identify risk factors, and implement appropriate control measures.

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