Table A.5.

Estimated county-level $\alpha$ and IFR using profile travel data.

County	$\alpha$	$\raisebox{1ex}{$U$}\!\left/ \!\raisebox{-1ex}{$R$}\right.$	National $\alpha =4.09\text{\%}$			County $\alpha$
			U-A	mIFR	cIFR	U-A	mIFR	cIFR
Broward, FL	31.13%	2.21	364	0.16%	0.25%	48	1.20%	1.90%
Clark, NV	12.69%	6.88	192	0.32%	0.32%	62	0.98%	1.01%
Cook, IL	6.54%	14.30	335	0.20%	0.21%	210	0.32%	0.33%
Dallas, TX	3.20%	30.27	290	0.33%	0.39%	371	0.26%	0.30%
Denver, CO	38.02%	1.63	158	0.13%	0.13%	17	1.23%	1.18%
Essex, NJ	0.45%	222.55	871	0.36%	0.27%	7973	0.04%	0.03%
Fulton, GA	3.27%	29.58	302	0.27%	0.50%	378	0.21%	0.40%
Harris, TX	5.97%	15.76	185	0.14%	0.12%	127	0.21%	0.18%
Honolulu HI	1.03%	96.40	342	0.02%	$<.01$%	1364	$<.01$%	$<.01$%
Los Angeles, CA	1.89%	51.89	178	0.35%	0.36%	386	0.16%	0.17%
Mecklenburg, NC	2.18%	44.89	1881	0.02%	$<.01$%	3533	0.01%	$<.01$%
Miami-Dade, FL	0.32%	307.31	2064	0.04%	0.05%	26053	$<.01$%	$<.01$%
New York City, NY	6.69%	13.95	1194	0.63%	0.75%	731	1.03%	1.22%
Philadelphia, PA	4.04%	23.75	692	0.16%	0.24%	701	0.16%	0.24%
Ramsey, MN	10.02%	8.98	138	0.40%	0.48%	57	0.98%	1.18%
San Francisco, CA	2.23%	43.80	89	0.13%	0.19%	164	0.07%	0.11%
Suffolk, MA	6.52%	14.34	101	0.11%	0.11%	64	0.17%	0.18%
Wayne, MI	0.86%	115.87	1246	1.74%	1.85%	5960	0.36%	0.39%
Median	3.66%	26.67	318.5	0.18%	0.24%	374.5	0.21%	0.27%

We estimate each counties’ death rate on March 13 using several measures of both the infection fatality rate and estimated infected. $\alpha$ is the estimated fraction of reported infections for each county accounting for an 8-day lag. $\frac{U}{R}=\frac{1-\alpha }{\alpha }$ gives the ratio of unreported to reported infections for that county, again accounting for the fact that observed reported infections have an 8 day lag. U-A gives the under-ascertainment rate on Mar 13 given by the total infected on Mar 13 (accounting for lag) divided by the reported infected on Mar 13. mIFR matches cohorts of infected using a log-normal fatality lag distribution to determine the death rate, and cIFR compares cumulative deaths 15 days later. All of these calculations are based estimating the number of total infected on March 13 by reported infected on March 21st divided by $\alpha$, to account for an 8-day reporting lag. National $\alpha$ is taken from Table A.4 full EU travel. County $\alpha$ uses each county’s individually computed $\alpha$ rather than the nationally computed $\alpha$. $<.01\text{\%}$ indicates positive numbers that round down to 0.00%.