Table 3. Analyses of 1^st-borns with and without other siblings (as of March 31^st 2011).

Birth order	Events During Risk Period (Days 0–2)	Events During Control Period (Days 9–18)	Relative Incidence (95% CI)	Relative Incidence Ratio (95% CI)	RIR p-value
2-month Vaccination
2nd or higher	418	1840	0.68 (0.61–0.76)	1 (Ref)
1st with siblings	248	748	0.99 (0.86–1.15)	1.46 (1.22–1.74)	<0.0001*
1st with no siblings#	326	1101	0.89 (0.79–1.01)	1.30 (1.11–1.53)	<0.0015∧
4-month vaccination
2nd or higher	310	1713	0.54 (0.48–0.61)	1 (Ref)
1st with siblings	198	652	0.91 (0.78–1.07)	1.68 (1.37–2.05)	<0.0001*
1st with no siblings#	286	919	0.93 (0.82–1.07)	1.72 (1.44–2.06)	<0.0001∧
6-month vaccination
2nd or higher	358	1808	0.59 (0.53–0.67)	1 (Ref)
1st with siblings	213	792	0.80 (0.68–0.93)	1.34 (1.11–1.62)	<0.0026*
1st with no siblings#	271	1082	0.72 (0.63–0.83)	1.22 (1.02–1.45)	<0.0283∧
12-month vaccination
2nd or higher	2338	1845	1.27 (1.19–1.35)	1 (Ref)
1st with siblings	1450	960	1.51 (1.39–1.64)	1.19 (1.08–1.32)	<0.0007*
1st with no siblings#	1807	1353	1.34 (1.24–1.43)	1.05 (0.96–1.16)	0.2694∧

^*p-value for comparison of 1^st-borns with siblings vs. 2^nd or later-borns.

∧p-value for comparison of 1^st-borns with no siblings vs. 2^nd or later-borns.

# As of March 31^st 2011.