Table 3 Estimated domestic transmission rates in the US.

${\chi }_1^{US}$

9.18 year^-1

${\chi }_2^{US}$

5.54 year^-1

${\chi }_3^{US}$

1.48 year^-1

${\chi }_4^{US}$

2.31 year^-1

${\chi }_5^{US}$

1.50 year^-1

${\chi }_6^{US}$

0.57 year^-1

${\chi }_7^{US}$

0.58 year^-1