Table 2.

Incidence rate (per 1000 person-years) and rate ratios of seizures following vaccination overall and by gestational age category, child’s age at vaccination, and vaccine received.

		7 through 10 days after vaccination (risk interval)		15 through 42 days after vaccination (control interval)
Gestational age category	Number of children	Number of seizures	Incidence rate^a (95% CI)	Number of seizures	Incidence rate^a (95% CI)	IRR (95% CI)^b	Preterm vs full-term, ratio of IRR (95% CI), p
Overall
Preterm (<37 weeks)	45,343	31	62 (42–89)	56	16 (12–21)	3.9 (2.5–6.0)	1.2 (0.76–1.9), 0.41
<35 weeks	16,586	10	55 (26–100)	22	17 (11–26)	3.2 (1.5–6.7)	1.0 (0.47–2.1), 0.99
35–36 weeks	28,757	21	67 (41–100)	34	15 (11–22)	4.3 (2.5–7.4)	1.4 (0.77–2.4), 0.29
Full-term (≥37 weeks)	487,032	232	43 (38–49)	510	14 (12–15)	3.2 (2.7–3.7)
Vaccine received
MMR
Preterm	37,262	22	54 (34–82)	48	17 (12–22)	3.2 (1.9–5.3)	1.2 (0.70–2.0), 0.51
Full-term	403,238	163	37 (31–43)	425	14 (12–15)	2.7 (2.2–3.2)
MMRV
Preterm	8081	9	100 (47–190)	8	13 (5.6–25)	7.9 (3.0–20)	1.4 (0.51–3.8), 0.52
Full-term	83,794	69	76 (59–95)	85	13 (11–16)	5.7 (4.1–7.8)
Child’s age at vaccination
12–15 months
Preterm	41,391	27	60 (39–87)	51	16 (12–21)	3.7 (2.3–5.9)	1.3 (0.77–2.1), 0.36
Full-term	442,919	200	41 (36–47)	477	14 (13–15)	2.9 (2.5–3.5)
16–23 months
Preterm	3952	4	92 (25–237)	5	17 (5.4–39)	5.6 (1.5–21)	0.82 (0.20–3.4), 0.79
Full-term	44,113	32	66 (45–94)	33	10 (6.7–14)	6.8 (4.2–11)

Abbreviations: MMR, measles-mumps-rubella; MMRV, measles-mumps-rubella-varicella; IRR: incident rate ratio, CI, confidence interval.

Per 1000 person-years.

IRR (7–10 days versus 15–42 days after vaccination).