Appendix.

Multivariate Logistic Model Output

Parameter	Coding	Coefficient	Standard Error	P Value**
Outcome: Symptomatic Infection	1 = Symptomatic infection in active surveillance window (n = 34)	…	…	…
	0 = Subclinical infection in active surveillance window (n = 89)
Enrollment antibody profile (“Some_imm”)	1 = HI-negative	1.593	0.467	<.001
	0 = HI-negative
Two years post-1st infection† (“Time2”)	1 = Infected in year 2	0.193	0.305	.379
	0 = Infected in year 1 or 3
Three years post-1st infection (“Time3”)	1 = Infected in year 3	0.195	0.391	.734
	0 = Infected in year 1 or 2
Some_imm*Time2	Interaction term for 2 years post-1st infection and immunity	0.651	0.317	.065
Some_imm*Time3	Interaction term for 3 years post-1st infection and immunity	0.921	0.385	.011
Rise in summed HI titers following the 1st infection	1 = Less than or equal to the mean	0.643	0.317	.040
	0 = Greater than the mean

Abbreviation: HI, hemagglutination inhibition.

* Conditioned on subdistrict of residence and study period of enrollment (KPS1 or KPS2).

** Exact P values were calculated.

^† Year 4 was not assessed in this model because no subclinical infections occurred 4 years after the first.