Abstract
This national database study compares pertussis hospitalizations among infants before and after the 2012 Advisory Committee on Immunization Practices recommendation that all pregnant women receive the tetanus toxoid, reduced diphtheria toxoid, and acellular pertussis vaccine.
Pertussis infection can cause serious complications, particularly among infants younger than 2 months, who are too young to be vaccinated.1 To reduce pertussis morbidity and mortality among young infants, the Advisory Committee on Immunization Practices (ACIP) issued a series of recommendation changes regarding antenatal administration of the tetanus toxoid, reduced diphtheria toxoid, and acellular pertussis (Tdap) vaccine, the most recent of which was issued in 2012 and recommends administration in every pregnant woman, regardless of prior receipt.2 Pertussis incidence among US infants younger than 1 year decreased after 20123; however, data on trends in young infants are scarce. We examined trends in pertussis hospitalizations among infants younger than 2 months before and after the changes to the ACIP recommendations in 2012.
Methods
We used data from the Truven Health Analytics Marketscan Commercial databases, which contain deidentified patient-level data for individuals enrolled in US employer-sponsored private health insurance.4 The database includes all 50 states and is generally reflective of the privately insured population. We identified infants with a birth hospitalization occurring between January 1, 2009, and October 31, 2017, using International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) diagnosis codes (V30.xx-V39.xx) and International Classification of Diseases, Tenth Revision, Clinical Modification (ICD-10-CM) diagnosis codes (Z38.xx). Because infant age in months was not available, we assigned the birth hospitalization admission date as the infant’s date of birth. Infants who were not enrolled for at least 59 days after birth were excluded. We were unable to identify infant deaths.
The outcome of interest was pertussis hospitalization occurring between the date of birth and 2 months (60 days) of age. We defined a pertussis hospitalization as an inpatient admission with report of a pertussis-related code (ICD-9-CM code 033.0 or 033.9 or ICD-10-CM code A37.0x or A39.0x).
We calculated monthly pertussis hospitalization rates by dividing the total number of hospitalizations with a pertussis diagnosis in the first 2 months of life by the total number of infants younger than 2 months during a given month. We used an interrupted time-series design with segmented quasi-Poisson regression models to calculate risk ratios (RRs) and 95% CIs comparing infant pertussis hospitalizations occurring between the 2009-2012 period and the 2013-2017 period. We assumed a level change in pertussis hospitalizations (tested using an interaction term) and no lag. We used SAS statistical software version 9.4 (SAS Institute Inc) for data analysis. A 2-sided P < .05 was considered statistically significant. This study was deemed exempt by Emory University’s institutional review board.
Results
We identified 2 031 775 infants with a birth hospitalization; 94.6% (n = 1 921 892) were enrolled for at least 59 days. Approximately 51.6% of infants were male and 34.3% lived in the South (Table).
Table. Characteristics of Infants in the Study Population, 2009-2017.
| Year of Birth, No. (%) | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| 2009 | 2010 | 2011 | 2012 | 2013 | 2014 | 2015 | 2016 | 2017a | |
| Total No. | 250 813 | 261 196 | 277 142 | 271 507 | 208 293 | 211 987 | 157 144 | 160 588 | 123 222 |
| Sex | |||||||||
| Male | 129 098 (51.5) | 134 310 (51.4) | 142 998 (51.6) | 139 941 (51.5) | 107 898 (51.8) | 109 004 (51.4) | 81 497 (51.9) | 83 031 (51.7) | 63 685 (51.7) |
| Female | 121 715 (48.5) | 126 886 (48.6) | 134 144 (48.4) | 131 566 (48.5) | 100 395 (48.2) | 102 983 (48.6) | 75 647 (48.1) | 77 557 (48.3) | 59 537 (48.3) |
| Region of residence | |||||||||
| Northeast | 49 942 (19.9) | 52 642 (20.1) | 58 255 (21.0) | 55 844 (20.6) | 43 821 (21.0) | 51 764 (24.4) | 30 970 (19.7) | 30 009 (18.7) | 24 428 (19.8) |
| North central | 79 595 (31.7) | 75 270 (28.8) | 83 129 (30.0) | 76 691 (28.2) | 53 404 (25.6) | 53 058 (25.0) | 38 869 (24.7) | 42 778 (26.6) | 33 273 (27.0) |
| South | 77 000 (30.7) | 91 693 (35.1) | 93 129 (33.6) | 93 786 (34.5) | 70 115 (33.7) | 69 222 (32.6) | 59 322 (37.7) | 61 429 (38.2) | 46 465 (37.7) |
| West | 32 381 (12.9) | 35 574 (13.6) | 38 604 (13.9) | 42 298 (15.6) | 37 096 (17.8) | 34 303 (16.2) | 27 236 (17.3) | 25 517 (15.9) | 18 334 (14.9) |
| Unknown | 11 895 (4.7) | 6017 (2.3) | 4025 (1.4) | 2888 (1.1) | 3857 (1.8) | 3640 (1.7) | 747 (0.5) | 855 (0.5) | 722 (0.6) |
Excludes infants born after October 31, 2017, to allow for 60 days of follow-up.
Compared with the 2009-2012 rate of pertussis hospitalizations of 8.4 per 100 000 infants (95% CI, 7.2-9.7 per 100 000; 177 of 2.1 million infants), the 2013-2017 rate decreased to 3.3 per 100 000 infants (95% CI, 2.6-4.3 per 100 000; 57 of 1.7 million infants). After adjusting for time, there was a 48% decrease in the rate of pertussis hospitalization (RR, 0.52; 95% CI, 0.31-0.89; Figure). There was no difference in slope between the 2 periods (P = .95).
Figure. Monthly Infant Pertussis Hospitalizations Between Birth and 2 Months of Age, 2009-2017.
Circles indicate observed rates; dashed line is the predicted rate.
Discussion
The results of this large national study indicate a decline in pertussis hospitalization rates among young infants after the 2012 recommendation change, adding to the growing body of literature suggesting that maternal Tdap vaccination is associated with lower rates of death5 and hospitalization and reduced disease severity among young infants with pertussis.6
The data used for this study were derived from a privately insured population; findings may not be representative of all infants. Use of ICD codes to identify pertussis hospitalizations may have resulted in misclassification. Infant death data were not available, so pertussis complications were underestimated. Maternal Tdap vaccination status was not assessed; therefore, correlations between maternal vaccination and infant pertussis hospitalization could not be examined. This study examined trends in infant pertussis hospitalizations and could not account for infants with less severe illness who were not hospitalized.
These findings underscore the public health importance of maternal Tdap vaccination in reducing pertussis burden among young infants. Clinicians should continue to promote maternal Tdap vaccination and find ways to overcome barriers to vaccination during pregnancy.
Section Editor: Jody W. Zylke, MD, Deputy Editor.
References
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