Table 4.

 Variables associated with influenza A/H1N1, by conditional logistic regression analysis

Characteristic	Adjusted odds ratio (95% CI)	P value
Participants (60 cases, 180 controls):
Vaccinated v unvaccinated (2008-9 winter season)	0.27 (0.11 to 0.66)	0.004
Men	0.72 (0.37 to 1.37)	0.3
Underlying conditions*	0.15 (0.08 to 0.30)	<0.001
Participants admitted to hospital (59 cases, 61 controls):
Vaccinated v unvaccinated (2008-9 winter season)	0.23 (0.07 to 0.78)	0.018
Men	0.85 (0.37 to 1.97)	0.7
Underlying conditions*	0.20 (0.09 to 0.45)	<0.001
Participants with no underlying conditions* (45 cases, 60 controls):
Vaccinated v unvaccinated (2008-9 winter season)	0.14 (0.04 to 0.50)	0.003
Men	0.73 (0.31 to 1.73)	0.479

*Medical conditions conferring a higher risk of influenza related complications.