Table 2. Factors associated with HI titre ≥1∶40 after the pandemic wave (multivariable analysis).

Variables			Adjusted PR (95% CI)	p
Age group 20–29 years compared	H1N1 vaccination	Titre
to 30–70 years *
	Not vaccinated	Titre <1∶40 pre-pandemic	4.99 [2.65, 9.38]	<0.001
	Vaccinated	Titre <1∶40 pre-pandemic	1.33 [0.91, 1.94]	0.14
	Not vaccinated	Titre ≥1∶40 pre-pandemic	1.07 [0.87, 1.32]	0.50
	Vaccinated	Titre ≥1∶40 pre-pandemic	1.12 [0.85, 1.48]	0.41
H1N1 Vaccination compared	Age-group	Titre
to no vaccination *
	20–29	Titre <1∶40 pre-pandemic	2.57 [1.53, 4.32]	<0.001
	20–29	Titre ≥1∶40 pre-pandemic	1.30 [0.94, 1.80]	0.11
	30–70	Titre <1∶40 pre-pandemic	9.63 [6.49, 14.30]	<0.001
	30–70	Titre ≥1∶40 pre-pandemic	1.25 [1.02, 1.53]	0.03
HI Titer ≥1∶40 before pandemic	Age-group	H1N1 Vaccination
compared to HI titer <1∶40 *
	20–29	Not vaccinated	2.51 [1.41, 4.47]	0.002
	20–29	Vaccinated	1.27 [0.86, 1.87]	0.23
	30–70	Not vaccinated	11.65 [7.61, 17.82]	<0.001
	30–70	Vaccinated	1.51 [1.25, 1.82]	<0.001
Sex - Male compared to female			1.33 [0.95, 1.87]	0.10
Type of site - Mobile urban compared to fixed			1.71 [1.22, 2,38]	0.002
Type of site - Mobile rural compared to fixed			1.36 [1.09, 1.71]	0.007
ILI symptoms compared to no ILI symptoms			1.31 [0.86, 2.00]	0.21

^*Prevalence ratios calculated using variables forming the statistical triple interaction identified (age group, H1N1 vaccination and pre-pandemic titre ≥1∶40).