The attack rate is the proportion of an at‑risk population that develops a disease over the course of an outbreak or defined period. It is calculated as the number of new cases divided by the number of susceptible individuals exposed, expressed as a percentage.
Explanation
Attack rate is used in epidemiology to describe the intensity of an outbreak and to compare the risk associated with different exposures. Unlike incidence rate, which considers person‑time, the attack rate is a cumulative incidence restricted to a discrete time period and a specific population. Investigators compute attack rates for those who ate a particular food at a picnic or attended an event to identify the source of a food‑borne outbreak. Secondary attack rate measures the spread of disease among contacts of primary cases, such as household members, and provides insight into transmissibility. Because it is contingent on accurate case counts and denominator data, the attack rate can be biased by under‑reporting or misclassification of exposed individuals. It is especially useful for acute infectious diseases with short incubation periods.
Outbreak examples
During a norovirus outbreak on a cruise ship, 120 of 600 passengers developed gastrointestinal illness; the attack rate was 20%. At a company picnic, 75 of 100 people who ate egg salad became ill with salmonellosis, whereas only 5 of 50 who did not eat the salad were sick, suggesting the egg salad as the vehicle of infection. In a measles outbreak, the secondary attack rate among unvaccinated household contacts may exceed 90%, reflecting high contagiousness. In contrast, the attack rate of influenza among vaccinated nursing home residents may be low, indicating vaccine protection. Attack rates have been calculated for COVID‑19 clusters associated with weddings, choirs and sporting events to assess the effectiveness of mitigation measures.
By quantifying the proportion of people who become ill in a defined group, attack rate helps public health officials characterize outbreaks and implement targeted control strategies.
Related Terms: Incidence, Outbreak, Secondary attack rate, Case fatality rate, Epidemiology