The reasons for the recurrence of the epidemic may include the following points:
1. The contagiousness of the virus itself.
2. Decline in crowd prevention and control awareness.
3. Increase in external defense input pressure.
4. Seasonal climate changes.
5. Seasonality of viruses. Outside the tropics, many common respiratory viruses reappear during specific seasons.
6. Immune system factors. Insufficient adaptive immune response, weakened immunity, and immune escape of the human immune system can destroy or avoid the bactericidal properties of the immune system, leading to reinfection.
It should be noted that there are many reasons for the re-emergence of the epidemic, and it requires the joint efforts of the whole society to prevent and control it.