# Answering FAQs: Part 1

## What is the reproduction number? What about future risks?

To help the Royal Statistical Society, I wrote answers to frequently asked questions about COVID-19.

The main constraint was a limit of 400 words. These are the original version of the articles. The posts were later improved by other authors.

# What is R and how is it determined?

**What is the reproduction number (R)?**

The **reproduction number **is the average number of direct infections from one case. This is over the whole time while people are infectious.

If the reproduction number is 2, we expect 100 infected people to infect 200 more people. If the reproduction number is 0.5, the average group of 100 infected people infects 50 more.

The reproduction number can change over time. If people reduce contacts, the virus has fewer transmissions.

The **basic reproduction number (R₀)** is for when the population has no immunity. This is not a biological constant. The same virus may spread in different populations at different paces. By itself, this number does not determine how fast a virus spreads. Initial ‘seed’ cases and infectious periods are important.