Table 2.

Odds Ratios for Association of Maternal Influenza Immunization With Infant Outcomes, Among Infants Born During Period of 2009 Influenza A (H1N1) Virus Circulation

	H1N1 Vaccine (n = 1125)	No Vaccine (n = 1581)	Adjusteda	Unadjusted
Outcome	Has Outcome, No. (%)	Has Outcome, No.	OR or Difference (95% CI)	OR or Difference (95% CI)
Preterm birth (27–36 wk)	86 (7.6%)	191 (12.1%)	.63 (.47–.84)b	.60 (.46–.79)
Birth at 27–33 wk	19 (1.7%)	52 (3.3%)	.53 (.30–.95)	.49 (.29–.83)
Birth at 34–36 wk	67 (6.0%)	139 (8.8%)	.66 (.48–.92)	.65 (.48–.87)
Low birth weight, <2500 gc	68 (6.4%)	132 (8.8%)	.79 (.56–1.10)	.71 (.52–.96)
Small for gestational agec	99 (9.3%)	123 (8.2%)	1.26 (.94–1.69)	1.15 (.87–1.52)
Birth weight, gc, mean (95% CI)	3308.5 (3276.6–3340.4)	3245.3 (3216.5–3274.2)	45.1 (1.8–88.3)	63.2 (20.0–106.3)

Reference group for all models is group that was not exposed to any 2009–2010 inactivated influenza vaccine.

Abbreviations: CI, confidence interval; OR, odds ratio.

^a Adjusted models control for a priori confounders: maternal age (years), maternal asthma, gestational diabetes, maternal cardiovascular disease, hypertension during pregnancy, any pregnancy/birth complication, multiple birth, any antiviral use during pregnancy, and Kaiser Permanente site.

^b Interpretation: During the period of 2009–2010 H1N1 influenza circulation, the adjusted odds of third-trimester preterm birth were 37% lower among infants of mothers who received 2009 influenza A (H1N1) vaccine, as compared to infants of unvaccinated mothers.

^c Birth weight and small for gestational age outcomes reported for 1064 births exposed to H1N1 vaccine and 1505 births exposed to no vaccine.