Table 1. Results of the analysis of covariance and linear regressions (see methods).

	Covariance analysis		Regression (“Mekong River”)			Regression (“national road”)
year	n1	p-value of interaction	n1	p-value2	1/slope estimate3	n1	p-value2
2002	40	0.005	22	0.02	11	18	0.70
2003	44	<0.001	22	0.001	9	22	0.22
2004	31	<0.001	22	<0.001	8	9	0.07
2005	38	<0.001	19	<0.001	8	19	0.09
2006	40	<0.001	22	0.004	13	18	0.22
2007	52	0.001	26	0.006	17	26	0.378

¹Number of districts where an annual epidemic occurred according to the national threshold, along the Mekong River, the national road (Figure 4A), or, for the covariance analysis, both.

²P-value of the regression slope estimate.

³Inverse of the estimate of the regression slope β1, in km per week (see methods).