Table 2.

ER Visits and Admissions in the first 72 h following vaccination versus days 9 to 18 for the time period when whole cell pertussis vaccine was used compared to the time period when acellular pertussis vaccine was used [23]

	Risk events (first 72 h)	Control events (Day 9–18)	Relative incidence	95 % CI	Relative incidence ratio	95 % CI	P-valuea
Apr 94–Mar96 (whole-cell)	1323	3663	1.08	1.02–1.15	1.82	1.64–2.01	P < 0.0001
Apr98–Mar00 (acellular)	697	3508	0.60	0.55–0.65	1 (ref)	-	-

^a p-value for interaction of time period (whole cell or acellular periods) with risk