Topic 10.2
How do reproduction numbers and incidence rates help predict and control disease outbreaks?
Understanding how diseases spread starts with statistics. Concepts like incidence and reproduction numbers help us measure how likely an infection is to spread and how many people need immunity to stop it. Learn how these metrics shape public health responses to outbreaks and guide vaccination strategies.
What is Incidence?
Most germs have to be transmitted from a sick person to another person for the germ to survive.
Germs usually make you unwell for days or weeks, during which time you may infect other people. Then you recover, and your symptoms, like cough, go away. You stop spreading infection, and you become immune.
If each infected person with symptoms passes their germ onto at least one other person, the germ will survive. This relates to what epidemiologists call the reproduction number for the germ.
Incidence is the measure of how likely a particular disease is to occur in a population within a specified period of time.
An example:
About 1 in every 100,000 Australians develop meningococcal disease every year, i.e. 1 meningococcal case per 100,000 people per year (in 25 million people, that’s 250 cases every year)
But, in Australia, meningococcal disease is 5 times more common in Aboriginal children, so this group needs improved access to immunisation (see the Australian National Immunisation Program website)
Reproduction Number
The reproduction number (R₀, or “R naught”) tells us how many people one sick person will pass their disease to if no one is vaccinated or immune. It helps us understand how easily a disease can spread.
- If R₀ is more than 1, the disease will spread because each sick person infects more than one other person.
- If R₀ is 1, the disease stays steady, with each sick person infecting exactly one other person.
- If R₀ is less than 1, the disease will eventually stop spreading because each sick person infects less than one person on average.
For example:
Measles has a very high R₀ (around 10–15), meaning one person could infect 10 or more others if no one is vaccinated.
Watching the video below will help you learn more about how R₀ works and why it’s so important for understanding and stopping the spread of diseases.