Maternal Tdap Blunts Preschoolers’ Pertussis Shot Response, but Booster Revives Them

Children whose mothers were vaccinated against tetanus, diphtheria, and pertussis (Tdap) during pregnancy saw significantly reduced immunity response to their own early childhood vaccination, although childhood vaccine boosters at age 4 normalized immunity levels, according to a small observational cohort study.

All children in the study received primary diphtheria, tetanus, and acellular pertussis (DTaP) vaccination at 3, 5, and 12 months of age. By age 3 years, however, children whose moms received the Tdap vaccine had geometric mean concentrations (GMC) of anti-pertussis toxin immunoglobulin G (anti-PT IgG) of 2.44 IU/mL, compared with 5.76 IU/mL in kids of unvaccinated moms (P=0.002). That significant gap persisted at age 4.

After the DTaP booster at age 4, however, immunologic parity prevailed against all the DTaP antigens. Kids of vaccinated moms had anti-PT IgG GMCs similar to those of unvaccinated mothers (247 vs 270 IU/mL), Lauri Ivaska, MD, PhD, of the University of Turku in Finland, and colleagues reported in the Journal of Infectious Diseases.

“Maternal Tdap vaccination is a safe and effective strategy for protecting young infants from pertussis, which remains the principal rationale for its use during pregnancy,” Ivaska and colleagues wrote. “These data suggest that, although maternally derived antibodies may shape antibody kinetics into childhood, the response to booster vaccination appears preserved.”

Ivaska’s study was an extension of the Finnish MIFI trial, which explored the blunting effect that maternal Tdap vaccination can have on infants’ response to their own primary DTaP vaccination.

In the U.S., the CDC and American College of Obstetricians and Gynecologists recommend maternal Tdap shots to provide newborns with protection in the particularly vulnerable period until they can begin primary DTaP vaccination at age 2 months. Infants younger than 3 months are most at risk of severe outcomes from a pertussis infection, and about 1 in 100 who get infected die.

The findings reinforce the message that all pregnant women should get Tdap, Lori Handy, MD, associate director of the Vaccine Education Center at the Children’s Hospital of Philadelphia, told MedPage Today.

“An important finding in this study is that infants born to women who were vaccinated during pregnancy had robust levels of titers at birth and at 3 months,” Handy noted.

Ivaska and colleagues found that children born to Tdap-vaccinated mothers had significantly greater cord-blood anti-PT IgG GMCs than those born to unvaccinated mothers (93.7 vs 18.4 IU/mL, P<0.001). Children of vaccinated mothers also had significantly greater anti-diphtheria toxin (DT) seroprotection rates than those born to unvaccinated mothers (100% vs 62%, P<0.001).

Between April 2023 and July 2025, Ivaska’s team assessed preschoolers’ pertussis, diphtheria, and tetanus responses at age 3 years, 4 years, and after their 4-year DTaP boosters. At 4 years, 30 children of vaccinated mothers and 15 children of unvaccinated mothers completed a pre-booster visit.

The investigators considered anti-DT IgG concentrations of at least 0.1 IU/mL to be fully protective, while anti-PT IgG concentrations of at least 5 IU/mL were considered seropositive.

Children of vaccinated and unvaccinated mothers alike saw pertussis seropositivity and diphtheria seroprotection rates slide by age 3. Anti-PT seropositivity was significantly lower in children of vaccinated mothers than those of unvaccinated moms (21% vs 62%, P=0.003). Anti-DT seroprotection rates also fell in both groups by age 3 (8% and 25%, respectively), though the difference wasn’t significant.

After the age-4 booster, anti-DT IgG GMCs rose in both groups of kids: to 1.21 IU/mL in children of vaccinated mothers and to 1.14 IU/mL in those with unvaccinated mothers.

Moving to a pertussis-only vaccine during pregnancy might deliver pertussis protection in early infancy without blunting children’s later diphtheria antibody responses, Ivaska and colleagues noted. But that wouldn’t get around the blunted response to pertussis primary vaccination. Another option could be to move the preschool DTaP booster to a younger age, they added.

But, it may not be worthwhile to change the U.S. schedule if there are no clinical differences between DTaP-vaccinated children of Tdap-vaccinated and unvaccinated mothers, Handy cautioned. Any such change would need more data on population-level infection rates and risks, she noted.

Study limitations included the small sample size and loss of some children during the 4 years of follow-up.