Substantial undocumented infection facilitates the rapid dissemination of novel coronavirus (COVID-19)

Ruiyun Li; Sen Pei; Bin Chen; Yimeng Song; Tao Zhang; Wan Yang; Jeffrey Shaman

The results of inference for the January 24-February 8 period are presented in Table 2 , Figure S11 and Table S1 . Control measures are continually shifting, so we show estimates for both January 24 -February 3 (Period 1) and January 24 -February 8 (Period 2). The best-fitting model for both periods has a reduced reporting delay, ! , of 5 days (vs. 10 days before January 23), consistent with more rapid confirmation of infections. Estimates of both the latency and infectious periods are relatively unchanged; however, , and 4 have all shifted considerably. The contact rate, , drops to 0.51 (95% CI: 0.39 -0.69) during Period 1 and 0.34 (95% CI: 0.27 -0.48) during Period 2, less than half the estimate prior to travel restrictions. The reporting rate, , is estimated to be 0.71 (95% CI: 0.55 -0.85), i.e. 71% of infections are documented during Period 1, up from 0.14 prior to travel restrictions, and is nearly the same in Period 2. The reproductive number is 1.51 (95% CI: 1.17 -2.10) during Period 1 and 1.00 (95% CI: 0.73 -1.38) during Period 2, down from 2.23 prior to travel restrictions. While the estimate for the relative transmission rate, , is similar to before January 23, the contagiousness of undocumented infections, represented by , is substantially reduced, possibly reflecting that only very mild and asymptomatic infections remain undocumented.