3.4 Epidemiological analysis of SVV in China
SVV isolates from China showed a range of different features; some strains were closely related with isolates from the USA (cluster 2), while some strains were more related to those from Brazil (cluster 3) or Canada (cluster 1). In total, 54 SVV isolates (including CH-GX-01-2019) were collected from different regions of China, including Guangdong (33/54), Hubei (2/54), Fujian (2/54), Henan (7/54), Heilongjiang (1/54), Guangxi (3/54), Shandong (1/54), Anhui (2/54), and Sichuan (1/54) Province; two isolates came from unknown regions. Further analysis showed that Chinese isolates predominantly belonged to cluster 1 and cluster 3 between 2015 and 2016, while those from 2017 belonged to cluster 2 (Fig. 3A) .
Genetic analysis, based on the complete sequences of SVV revealed that SVV isolates from Guangdong Province were distributed in three clusters and that 33 sequences shared 95.9 - 99.9% identity with each other. Isolates from Hubei, Fujian, Henan, Heilongjiang, Shandong Anhui and Sichuan Province were classified into cluster 2, and were similar to isolated from the USA. Some isolates from Henan, Guangxi, and Guangdong Province, were classified into cluster 3, and were similar to isolates from Brazil. Other isolates, from Hubei and Guangdong Province, were classified into cluster 1, and were similar to isolates from Canada. Further studies showed that SVV was detected in Liaoning, Shanghai, Hainan, Guizhou, Yunnan, Sichuan, Gansu and Xinjiang Province in China. Analysis shows that the epidemiology of SVV in China is complicated, although most isolates are found predominantly in southern and central regions of China (Fig. 3B).