A large study that looked at the impacts of tetanus, diphtheria, and pertussis (Tdap) vaccination during pregnancy found that vaccine effectiveness (VE) was high for preventing pertussis infections in infants younger than 2 months old. Though researchers found evidence of blunting in babies after they got their vaccine doses, the team didn’t find an increase in pertussis cases during the first year of life.
The study, published today in Pediatrics, included 279,418 infant-mother pairs in three Australian regions. Researchers looked at administrative health records from 2013 through 2017 from the three regions. To gauge impacts after infants received their diphtheria, tetanus, and pertussis DTaP doses, the group examined primary immunization records from the two biggest regions. The vaccine was given to about half of women in the three regions, usually between 28 to 31 weeks gestation.
VE from maternal vaccination in infants younger than 2 months was 70.4% (95% confidence interval 50.5% to 82.3%). Researchers didn’t observe any differences by gestational age for mothers’ vaccine receipt.
However, for infants ages 7 to 8 months, VE from maternal Tdap declined to 43.3%, and by 8 months it was no longer effective. Also, the team found that VE point estimates for the third dose of infant pertussis vaccine in babies whose mothers received Tdap were lower than for babies born to unvaccinated mothers, (76.5% vs 92.9%, P = .0024), suggesting a blunting effect.
Following babies over their first 18 months of age, researchers found that 331 pertussis cases, including 119 in those whose moms were vaccinated, and 212 in babies born to unvaccinated moms, suggesting a disease reduction in the early months and overall fewer cases during the first 18 months of life.
In a related commentary, Kathryn Edwards, MD, professor of pediatrics at Vanderbilt University Medical Center, wrote that the data are encouraging, and more efforts are needed to boost Tdap immunization levels in pregnant women. She also said it’s important to continue to monitor the consequences of maternally derived antibodies on infant responses to ensure that blunting has no impact on disease control.
