Fig. 3.

Transmission mechanism of COVID-19 on social networks. The group of infected but non-isolated (blue and yellow nodes) is the main propagating source of virus through physical contacts (droplet), especially over the incubation period [32], [33]. The infected patients would die or produce antibodies without treatment but undoubtedly the herd immunity would take a long time to achieve and uncountable individuals would lose their lives in this process. Therefore, the diagnosed patients should be isolated from social networks to cut off the transmission links (yellow edges). Due to the red edges are more dangerous for the exposed persons, the comprehensive identification policy for all people and the nucleic acid testing strategy for symptomatic patients should be taken to remove their connections with others. Meanwhile, some carrier would migrate to other place through the global transportation networks. The experiences from China demonstrated that the external input is the largest threat in the post-transmission period [34]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)