TABLE 3.

Association between preterm birth and influenza hospitalization by age group

Age group		No. of person‐years at risk	No. of influenza cases	Rate per 10,000 py (95% CI)	Hazard ratio (95% CI)
					Unadjusted	Adjusted a
<1 year	All	237,929	235	9.9 [8.7, 11.2]
	Term (≥37 weeks)	223,106	205	9.2 [8.0, 10.5]	Ref	Ref
	Preterm (<37 weeks)	14,823	30	20.2 [14.2, 28.9]	2.20 [1.47, 3.30]	2.24 [1.44, 3.50]
1 to <2 years	All	237,000	252	10.6 [9.4, 12.0]
	Term (≥37 weeks)	222,255	216	9.7 [8.5, 11.1]	Ref	Ref
	Preterm (<37 weeks)	14,745	36	24.4 [17.6, 33.8]	2.51 [1.74, 3.63]	2.26 [1.58, 3.25]
2 to <5 years	All	705,670	267	3.8 [3.4, 4.3]
	Term (≥37 weeks)	661,811	232	3.5 [3.1, 4.0]	Ref	Ref
	Preterm (<37 weeks)	43,860	35	8.0 [5.7, 11.1]	2.28 [1.60, 3.25]	2.47 [1.66, 3.66]

Abbreviation: CI, confidence interval.

^a Adjusted for season of birth, sex, maternal age, multiple birth, maternal smoking, birth order, and parental educational level.