What does it mean when an epidemic's R number is above one?
One number decides whether an outbreak fizzles or explodes: how many people each sick person infects.
▶ Start the storyIt means the outbreak is growing: each infected person is, on average, passing the disease to more than one other person. The reproduction number counts exactly that. The basic version, R0 ("R nought"), is the expected number of cases directly generated by one case in a population where everyone is susceptible. If it is above 1 the chain of infections gets longer; below 1 it fades out.
R above 1
- Each case causes more than one new case
- Chains of infection get longer
- Outbreak grows
R below 1
- Each case causes fewer than one new case
- Chains of infection die out
- Cases gradually fall to zero
Once some people are immune, the number that matters is the effective reproduction number, the average number of new infections caused by one infected person at a given time in a partly susceptible population. When it is pushed and kept below 1, cases gradually decrease until the disease has been eliminated. That is what herd immunity means: the disease cannot spread because each infected person, on average, can only transmit it to less than one other contact.
The idea came from watching real children. In the 1930s, after a significant number of children had become immune to measles, the number of new infections was seen to decrease temporarily.
The number is not magic, though. R0 does not by itself say how fast an infection spreads, and it is not a biological constant of a germ: environment and behaviour change it.
Quiz me
0/3
Recap
Above one an outbreak grows, below one it shrinks, and R depends on the model and on behaviour, not only on the germ.
💡 A trick to remember it · One sick person passes it to more than one, the fire grows; to fewer than one, it runs out of wood.
Surprising fact · Measles has a herd immunity threshold above 95%.
Connects to
- 🐀 Did rats really spread the Black Death?
- 💉 How did the world wipe out smallpox, and why only smallpox?
- 🦠 Why is the 1918 flu called 'Spanish' if it didn't start in Spain?
- 🎯 How did a 100,000-tests-a-day target make COVID numbers less trustworthy?
- 💉 How does a vaccine train your body to fight a germ it has never met?
- Herd immunity
Sources (3)
No source, no claim. Every fact in this lesson (16 claims) cites at least one of these.