Why the flu disappeared
The flu disappeared because of distancing, masks, and other restrictions. Covid didn’t because it has a higher R0:
| Disease | R0 | Restriction-related ΔR | R after restrictions |
| Flu | 1.3 | −1 | 0.3 — disease disappears |
| Covid-19 | 3.3 | −1 | 2.3 — disease continues to spread |