Table 1.

Estimated average fraction of reported infections.

	${\alpha }_{US}$	$\frac{1-{\alpha }_{US}}{{\alpha }_{US}}$
	8 day lag
China and Italy Travel Data
	.0416	23.1
Only Chinese Travel Data
	.0458	20.8
China, Italy, Spain, Germany, UK
	.0427	22.4
	5 day lag
China and Italy Travel Data
	.0161	61.2
Only Chinese Travel Data
	.0169	58.3
China, Italy, Spain, Germany, UK
	.0164	60.1

We report estimated $\alpha$ by OLS without a constant for several specifications of the model. We use $T_0$ as Feb 23 and $T_1$ as March 10,13 for each lag respectively. For the versions including European data, European travel to both Iceland and the United States is considered. King County, WA is omitted from the calculation.