Table 3. Unadjusted and adjusted incidence rate ratios (IRR), 95% confidence intervals (CI) for influenza-coded health care encounters, comparing infants born to A/H1N1-vaccinated mothers with unexposed infants by influenza time period.

Influenza time period	Influenza
	Unadjusted	Adjusted
	IRR, 95% CI	IRR, 95% CIa
2009 A/H1N1 pandemic b	0.61, 0.32–1.17	Indeterminate c
Post 2009 A/H1N1 pandemic	1.06, 0.83–1.34	1.05, 0.81–1.37
Pre-2010-2011 season	1.11, 0.85–1.44	1.08, 0.80–1.44
2010–2011 season	0.99, 0.87–1.13	0.88, 0.76–1.02
Post-2010-2011 season	0.76, 0.54–1.07	0.72, 0.50–1.04
Pre-2011–2012 season	Indeterminate c	Indeterminate c

^a Adjusted using high-dimensional propensity scores

^b Second wave of the 2009 A/H1N1 pandemic only

^c A stable estimate during this time period could not be generated due to the low number of outcomes