Table 1.

Characteristics of five U.S. counties.

Demographic characteristic	County, State
	Allegheny, PA	Wayne, MI	Santa Clara, CA	Sacramento, CA	Salt Lake, UT
Population, n	1,164,879	1,778,979	1,604,122	1,315,644	941,139
Household size, mean	2.22	2.51	2.74	2.61	2.85
Schools, n	532	813	615	532	409
School size, mean number of students	373	479	508	520	520
Work places, n	48,704	68,281	68,975	60,318	48,000
Workplace size, mean number of workers	11.8	10.5	11.4	10.2	10.0
Age, mean years	40.3	36.3	35.8	35.2	32.8
Proportion aged 0–18 years, %	21.7	28.1	25.7	27.6	30.8
Proportion aged 19–65 years, %	62.0	60.2	64.0	61.9	61.1
Proportion aged 65+ years, %	16.2	11.6	10.1	10.4	8.0
Influenza attack rate from Figs. 1a–1d	11.1	15.0	15.0	15.3	17.8