Maternal Pertussis Immunization and Immunoglobulin G Levels in Early- to Late-Term and Preterm Infants

This cohort study compares maternal-derived anti–pertussis toxin immunoglobulin G (IgG) antibody levels in 2-month-old early- to late-term and preterm infants whose mothers received tetanus, diphtheria, and acellular pertussis vaccination between 20 and 24 weeks’ vs 30 and 33 weeks’ gestation.


Introduction
According to the World Health Organization (WHO), 81% of infants worldwide (105 million) received 3 doses of a diphtheria, tetanus, and pertussis vaccine in 2021, protecting them against vaccinepreventable diseases that may cause serious, even fatal, illness and disability. 1 Despite high vaccine coverage, pertussis remains endemic in many countries.Newborns and infants too young to be fully vaccinated are at the highest risk of severe complications. 2 To protect newborns and infants in the first months of life, maternal vaccination with a tetanus, diphtheria, and acellular pertussis (Tdap) vaccine from 20 weeks' gestation onward has been offered to all pregnant women in the Netherlands since December 2019.Infant diphtheria, tetanus, and pertussis (DTaP), inactivated poliovirus (IPV), Haemophilus influenzae type b, and hepatitis B vaccinations are given at 3, 5, and 11 months of age (2 + 1 dose schedule) for protection against pertussis provided that the mother received Tdap vaccination during pregnancy.An extra vaccination at 2 months of age (3 + 1 dose schedule) after maternal Tdap vaccination is advised if an infant is born before 37 weeks' gestation or if the time interval between maternal vaccination and delivery is shorter than 2 weeks, since transfer of immunity against pertussis on maternal Tdap vaccination may be insufficient.
During pregnancy, maternal immunoglobulin G (IgG) antibodies are actively transferred across the placenta, mediated by the neonatal Fc receptor expressed on syncytiotrophoblast cells.This saturable process initiates at approximately 13 to 17 weeks' gestation and increases throughout gestation.Around 33 to 36 weeks' gestation, fetal IgG antibody levels exceed maternal IgG serum levels and increase to 150% of maternal levels near the due delivery date. 35][6][7] Maternal Tdap vaccination was reported to prevent 70% to 90% of clinically confirmed pertussis cases and about 90.5% of pertussis hospitalizations in newborns and infants younger than 3 months of age in the UK from 2013 to 2018. 8,9ere is no consensus on the optimal timing of maternal Tdap vaccination to achieve the highest antibody transfer.Most studies suggest that Tdap vaccination early in the third trimester results in the highest anti-pertussis toxin (anti-PT) IgG antibody levels at birth, [4][5][6][7] while a Swiss study favored second-trimester vaccination, potentially due to a longer time interval between Tdap vaccination and delivery. 10,11Recently, it was estimated that a period of 7.5 weeks or more before delivery optimizes antibody transfer. 5Tdap vaccination before 24 weeks' gestation may therefore be particularly relevant for preterm offspring, the group most vulnerable for severe pertussis.Preterm infants have a hospitalization rate for pertussis that is 1.5-times higher than predicted based on the total proportion of infants in the national UK birth cohort. 12Offering maternal Tdap vaccination from 20 weeks' gestation also widens the opportunity for pregnant women to receive the vaccine, but few

JAMA Network Open | Infectious Diseases
Maternal Pertussis Immunization and IgG Levels in Early-to Late-Term and Preterm Infants studies have reported antibody levels after maternal Tdap vaccination at or before 24 weeks' gestation or in preterm infants. 6,11These studies had insufficient power to draw firm conclusions.
In this study, pertussis-specific IgG antibody levels after maternal Tdap vaccination between 20 0/7 and 24 0/7 weeks' gestation were evaluated in early-to late-term (hereafter, term) and preterm offspring with follow-up until 2 months of age.We primarily assessed whether maternal Tdap vaccination between 20 0/7 and 24 0/7 weeks' gestation would be associated with similar anti-PT antibody levels in term infants at 2 months of age compared with maternal Tdap vaccination between 30 0/7 and 33 0/7 weeks' gestation.Therefore, data were compared with those from a reference study (recruitment between January 2014 and February 2016) including 55 term infants following maternal Tdap vaccination between 30 0/7 and 33 0/7 weeks' gestation. 13Additionally, we compared antibody levels in term and preterm infants following maternal Tdap vaccination between 20 0/7 and 24 0/7 weeks' gestation.

Study Participants
In this prospective, multicenter cohort study, antenatal care practitioners working in birthing centers or hospitals recruited pregnant women aged 18 years or older between August 2019 and November 2021.The study design and procedures were previously described. 14In brief, women were included through 2 recruitment routes; from August 2019, healthy pregnant women were invited to participate and received Tdap vaccination between 20 0/7 and 24 0/7 weeks' gestation as part of the study.In addition, after 2019, once the Dutch National Immunisation Programme (NIP) offered Tdap vaccination to all pregnant women from 20 weeks' gestation onward, women with imminent preterm labor were recruited on presentation at the hospital provided that they received Tdap vaccination between 20 0/7 and 24 0/7 weeks' gestation.These women were vaccinated through the NIP, unrelated to this study but with the same Tdap vaccine as used in the study.Women were excluded if they had received Tdap vaccination within the past 2 years or if there was a known or suspected underlying condition that could interfere with study results.Other exclusion criteria were previously described. 14Mother-infant pairs were followed up until 2 months after delivery.Data on Bordetella pertussis-specific IgG antibodies from mother-infant pairs in the study were compared with data from the reference study performed between January 2014 and February 2016 that comprised term infants at age 2 months after maternal Tdap vaccination between 30 0/7 and 33 0/7 weeks' gestation. 13Both studies used identical vaccines and study procedures for collection and timing of collection of blood samples.Laboratory procedures were performed in the same laboratory using identical procedures.This study was conducted in accordance with the Declaration of Helsinki 15 and approved by the Central Committee on Research Involving Human Subjects in the Netherlands.
Oral and written informed consent was obtained from parents or legal guardians.The study followed the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) reporting guideline.

Blood Sampling
Finger-stick blood samples (Յ300 μL) were collected from mothers within 24 hours after delivery.
Umbilical cord blood samples (Յ2 mL) were collected at delivery, and heel-stick blood samples from infants (Յ300 μL) were collected during home visits before primary vaccination at age 2 months (±5 days).For preterm infants, who often start receiving vaccinations between 6 and 9 weeks in the

Laboratory Analyses
Immunoglobulin G antibody concentrations against PT, FHA, Prn, DT, and TT were measured by bead-based fluorescent multiplex immunoassay using Luminex xMAP technology (ThermoFisher Scientific), as previously described. 17For the B pertussis antigens, the assay was calibrated against the WHO international standard for pertussis antiserum (serum reference 06/140), interpolated using a 5-parameter fit, and expressed in international units (IU/mL).

Statistical Analysis
Anti-PT IgG antibody levels following maternal Tdap vaccination are associated with prevention of clinical pertussis. 18Our primary outcome was to assess noninferiority of anti-PT antibody levels in term infants at 2 months of age following maternal Tdap vaccination between 20 0/7 and 24 0/7 weeks' gestation compared with Tdap vaccination between 30 0/7 and 33 0/7 weeks' gestation (reference cohort).The lower limit of the 95% CI of the geometric mean concentration ratio (GMR) between the main and the reference cohorts was set at 0.5 or greater for noninferiority.Secondary outcomes were the geometric mean concentration (GMC) of PT IgG levels in preterm infants at 2 months' postnatal age after maternal Tdap vaccination between 20 0/7 and 24 0/7 weeks' gestation compared with the term cohort after maternal Tdap vaccination between 20 and 24 weeks' gestation and the IgG antibody levels against all Tdap vaccine antigens (ie, PT, FHA, Prn, DT, and TT) in blood samples from infants at 2 months of age, the umbilical cord, and mothers at delivery among term and preterm mother-infant pairs (eFigure in Supplement 1).
To assess our primary research question and allow 80% power and an α of 5%, 58 term and 54 preterm mother-infant pairs were required. 14We aimed for inclusion of 60 pairs in each group to allow loss to follow-up regarding the available blood samples.
For the scope of this study, we defined preterm as birth between 24 0/7 and 34 6/7 weeks' gestation since offspring antibody levels are expected to exceed maternal antibody levels at the end of this time window, and these late-preterm offspring may therefore resemble offspring born at full term regarding transplacental antibody transfer. 3Term birth was defined as 37 0/7 or more weeks' gestation.
Comparison of baseline characteristics was done using either t, Mann-Whitney U, or Fisher exact test.The IgG-antibody concentrations against all antigens were log-transformed and computed into GMCs with corresponding 95% CIs.In all groups, including the reference cohort, 13 GMCs at different time points were assessed using generalized estimating equation models with a gaussian distribution with identity link function.An exchangeable correlation structure enabled adjustment for similarities in antibody levels among siblings who were twins or triplets.No additional adjustment was applied.
The GMRs were calculated from the GMCs within different groups and expressed with 95% CIs.R, version 2023.03.1 (R Project for Statistical Computing) was used with the geepack package for analyses. 19Findings were based on available data, and missing data were handled by complete participant analyses.Two-sided P < .05 was considered significant.

Results
In total, 221 pregnant women who received second-trimester Tdap vaccination were included.IU/mL]).No differences between term and preterm offspring were observed for PT, DT and TT (Table 2 and Figure 3).
Comparing antibody levels at delivery among mothers of term offspring in the study and reference groups, the anti-PT GMC at delivery was significantly higher in the study group (61.e Seven pairs of dichorionic-diamniotic twins, 4 pairs of monochorionic-diamniotic twins, 2 pairs of monochorionic-monoamniotic twins, and 2 sets of trichorionic-triamniotic triplets.Numbers sum to 60 infants (77 including siblings) for the total number of mother-infant pairs, but only 73 of 77 infants (94.8%) had a blood sample obtained at 2 months of age.
f Birth weight and birth weight percentiles were presented for the firstborn infant only if there were multiple pregnancies.
g Blood samples at age 2 months were obtained as close as possible to infant immunization but may have been obtained earlier than at 2 months of age because, in the Netherlands, routine preterm primary vaccinations are administered between 6 and 9 weeks after birth.
analyses using a Tdap vaccination cutoff of February 27, 2020, the first day of COVID-19 social distancing measures in the Netherlands, were conducted and found no differences in antibody levels after birth among mothers or infants before vs during COVID-19-measures.

Discussion
In this prospective cohort study, anti-PT IgG levels in term infants at 2 months of age following maternal Tdap vaccination between 20 0/7 and 24 0/7 weeks' gestation were inferior to those in the group with Tdap vaccination between 30 0/7 and 33 0/7 weeks' gestation, with an approximate 2-fold reduction in GMCs of anti-PT IgG levels.As long as the mechanisms of protection following maternal Tdap vaccination are not fully understood and no correlate of protection is available, anti-PT IgG levels are often used in studies like ours as surrogate markers for protection. 18Anti-PT levels in umbilical cord blood are correlated with protection against pertussis, 20 and lower anti-PT IgG levels may point to less protection against pertussis in newborns.We also observed a reduction in anti-Prn antibody levels after maternal Tdap vaccination between 20 and 24 weeks' gestation compared with the reference group.
The GA at which to administer maternal Tdap vaccination for the highest antibody transfer may vary per vaccine antigen.Many studies have suggested that Tdap vaccination between 27 0/7 and 30 0/7 weeks' gestation results in maximal pertussis-specific antibody levels and avidity in term b Women vaccinated between 20 0/7 and 24 0/7 weeks' gestation who delivered at term or preterm were included in the general cohort; women vaccinated between 30 0/7 and 33 0/7 weeks' gestation who delivered at term were included as the reference cohort.
2][23][24][25][26] In contrast, an observational Swiss study suggested that Tdap vaccination earlier in pregnancy led to higher maternal antibody transfer, potentially because of the longer transfer time before delivery. 10The recent Optimising the Timing of Whooping Cough Immunisation in Mums (OpTIMUM) randomized clinical trial observed the highest pertussis-specific IgG antibody levels in umbilical cord serum when mothers received the Tdap vaccine early in the third trimester (28-32 weeks' gestation) compared with earlier than 24 weeks' and 24 to 27 weeks' gestation. 6Notably, the number of preterm offspring included in that study was too small to draw conclusions for this most vulnerable group (15 [4%]; 5 per study group). 6e reduction in anti-B pertussis antibodies in offspring following maternal Tdap vaccination before 24 weeks' gestation may possibly be explained by the fact that peak levels of anti-B pertussis antibodies following vaccination are achieved when maternofetal antibody transfer is still

Figure 2 .
Figure 2. Individual Immunoglobulin G (IgG) Antibody Concentrations and Geometric Mean Concentrations (GMCs) After Second-vs Third-Trimester Tetanus, Diphtheria, and Pertussis Vaccination in Early-and Full-Term Mother-Infant Pairs at Different Time Points

Figure 3 . 7
Figure 3. Individual Immunoglobulin G (IgG) Antibody Concentrations and Geometric Mean Concentrations (GMCs) After Second-Trimester Tetanus, Diphtheria, and Pertussis Vaccination in Early-and Full-Term vs Preterm Mother-Infant Pairs at Different Time Points

Table 1 .
Baseline Characteristics at the Mother and Infant Level for Preterm and Early-and Full-Term Mother-Infant Pairs a 8 IU/mL[95% CI, 46.8-81.7 IU/mL] vs 32.9 IU/mL [95% CI, 26.0-41.6IU/mL]).No differences for the other Tdap antigens were found (Table2and Figure2).Comparing preterm and term mother-infant pairs following Tdap vaccination between 20 0/7 and 24 0/7 weeks' gestation, mothers had significantly higher GMCs after preterm than term delivery for all antigens (eg, PT 60.4 IU/mL [95% CI, 44.1-82.7 IU/mL] vs 32.9 IU/mL [95% CI, 26.0-41.6IU/mL])exceptPrn(Table2andFigure3).Sensitivity b Data are presented as number (percentage) of participants unless otherwise indicated.c In the study cohort, 73 preterm infants born to 60 mothers (due to multiple pregnancies) had a blood sample obtained at 2 months of age, as did 66 term infants.d In the reference cohort, infants had a blood sample obtained at 2 months of age.

Table 2 .
GMCs and GMRs in Preterm and Early-and Full-Term Infants With Mothers Vaccinated at 20 0/7 to 24 0/7 Weeks' Gestation and Term Infants With Mothers Vaccinated at 30 0/7 to 33 0/7 Weeks' Gestation a Abbreviations: GMC, geometric mean concentration; GMR, geometric mean concentration ratio; NA, not applicable.aTerm birth was defined as a gestational age of 37 0/7 weeks or more and preterm birth as less than 35 0/7 weeks' gestation.