Glossary
This glossary defines key terms used in the outbreak investigation and vaccine-effectiveness course. Use your browser’s page-search function to locate a particular term quickly.
A
Attack rate
The proportion of an at-risk population that develops the disease during a specified outbreak period.
B
Basic reproduction number (R₀)
The average number of secondary infections generated by one typical infectious person in a fully susceptible population without control measures.
C
Case definition
A standard set of clinical, laboratory, person, place and time criteria used to decide whether an individual should be classified as a case for an investigation.
Confirmed case
A person who meets the investigation’s definitive confirmation criteria, commonly based on an appropriate laboratory test.
Contact tracing
The process of identifying, assessing and managing people who may have been exposed to an infectious case.
E
Epidemic curve
A graph showing the number of outbreak cases by time of symptom onset, usually presented as a histogram.
I
Incubation period
The interval between infection or exposure to an infectious agent and the onset of symptoms.
Index case
The first case identified by investigators in an outbreak or cluster; it is not necessarily the first person infected.
Infectious period
The period during which an infected person can transmit the infectious agent to others.
O
Outbreak
The occurrence of disease cases above the number normally expected in a defined population, place and period.
R
Relative risk
The risk of disease in one group divided by the risk of disease in a comparison group.
S
Secondary attack rate
The proportion of susceptible contacts who develop disease following exposure to a primary case within an appropriate incubation period.
Suspected case
A person who meets specified clinical or epidemiological criteria for the disease but has not yet met the criteria for confirmation.
V
Vaccine effectiveness
The percentage reduction in disease risk among vaccinated people compared with unvaccinated people under real-world conditions. It is commonly calculated as (1 − relative risk) × 100.