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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]

From: The use of relative incidence ratios in self-controlled case series studies: an overview

  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) - -
  1. ap-value for interaction of time period (whole cell or acellular periods) with risk