The genomes of the two separate cases of coronavirus disease 2019 (COVID-19) are identical over the full length of close to 30 kb and are furthermore identical to five other sequences (four from Wuhan and one from Zhejiang); together these sequences form the largest cluster of identical cases within the early outbreak, comprising a core of at least indirectly linked cases (Figure 2). Within-outbreak sequence divergence is generally low with 0–9 nt differences over the whole genome and mutations unique to individual strains are possibly related to quality differences of the samples and noise of the methods used for sequencing.