Table 3. Estimated Relative Risk of Pertussis Comparing Children Who Were Undervaccinated or Vaccinated With Delay With Those Who Received Age-Appropriate and Timely DTaP Vaccine Using Log-Binomial Models.

Model	Cohort	Cohort size	Start follow-up time, mo	End follow-up time, mo	PT at risk in exposed, mo	PT at risk in unexposed, mo	Cases exposeda	Cases unexposedb	aRR (95% CI)c
Primary series
Undervaccinated with or without delay vs age-appropriate and timely vaccination	Children ≥7 mo	298 166	7	19	949 469.4	2 671 653	54	31	4.8 (3.1-7.6)
Delayed vs timely doses among those with age-appropriate vaccination	Children ≥7 mo who received ≥3 doses	257 913	7	19	492 272.2	2 671 653	5	31	0.8 (0.3-2.2)
First booster
Undervaccinated with or without delay vs age-appropriate and timely vaccination	Children ≥19 mo	258 675	19	60	2 598 849	5 360 429	99	59	3.2 (2.3-4.5)
Delayed vs timely doses among those with age-appropriate vaccination	Children ≥19 mo who received ≥4 doses	221 928	19	60	1 732 007	5 360 429	17	59	0.8 (0.5-1.4)
Second booster
Undervaccinated with or without delay vs age-appropriate and timely vaccination	Children ≥60 mo	134 950	60	Age censored /end of study	1 327 522	2 664 899	38	17	4.6 (2.6-8.2)
Delayed vs timely doses among age-appropriately vaccinated	Children ≥60 mo or older who received ≥5 doses	111 387	60	Age censored/end of study	580 387.7	2 664 899	5	17	1.3 (0.5-3.6)
Series initiation
Undervaccinated with or without delay dose 1 vs age-appropriate and timely vaccination for primary series	Children ≥3 mo	301 494	3	12	579 975.3	3 006 319	38	54	3.5 (2.3-5.5)
Delayed vs timely dose 1 among age-appropriately vaccinated for primary series	Children ≥3 mo who received 3 doses by 7 mo of age	295 325	3	12	64 926.5	2 555 811	0	41	NA
3p vs 2p + 1 scheduled	Children ≥3 mo	301 494	3	24	4 870 512	307 677.8	1	60	3.9 (0.5-28.8)

Abbreviations: aRR, adjusted relative risk; DTaP, diphtheria-tetanus–acellular pertussis; PT, person-time.

^aExposed group: undervaccination with or without vaccination delay for models assessing effect of undervaccination on pertussis risk and vaccination delay for models assessing effect of vaccination delay only on pertussis risk.

^bUnexposed group: age-appropriate and timely vaccination with DTaP vaccine.

^cAdjusted relative risk are risk ratios adjusted for age and Neighborhood Socioeconomic Status score.

^dThe term “3p” indicates accelerated 3-dose DTaP primary series to be administered at ages 2, 4, and 6 months; “2p + 1” indicates long 3-dose DTaP primary series to be administered at ages 2, 4, and 11/12 months; 2p + 1 is the exposed group.