Prenatal Tdap vaccination prevents the occurrence of and reduces the severity of pertussis in infants, two retrospective cohort studies showed.

In 2012, the Advisory Committee on Immunization Practices recommended that pregnant women receive a Tdap vaccination during their third trimester of pregnancy to optimize the transfer of pertussis antibodies to the fetus. Since the committee’s recommendation, no studies to evaluate the effectiveness of this strategy have been conducted in the United States, Kathleen Winter of the California Department of Public Health and her associates reported ( Clin. Infect. Dis. 2016. doi: 10.1093/cid/ciw634 ) .

Therefore, Ms. Winter and her associates conducted two separate retrospective cohort studies: one to compare the effectiveness of prenatal versus postpartum Tdap vaccination in preventing pertussis and the second to investigate the effectiveness of prenatal Tdap vaccination on pertussis severity in infants.

For the comparison study, researchers identified 42,941 mothers who were vaccinated during pregnancy and 31,563 mothers who were vaccinated following delivery. The stage of pregnancy at the time of vaccination was documented for 42,218 of the mothers vaccinated prenatally, and 77% were vaccinated during the recommended window of 27-36 weeks’ gestation. For the remaining mothers, 14% received vaccinations before 27 weeks’ gestation, and 9% were vaccinated after 36 weeks’ gestation. Infants whose mothers received the Tdap vaccine at any point during pregnancy were less likely to develop pertussis before 8 weeks of age (odds ratio, 0.36; 95% confidence interval, 0.15-0.89) or 12 weeks of age (OR, 0.47; 95% CI, 0.24-0.92).

Moreover, infants whose mothers received the vaccine during 27-36 weeks’ gestation were less likely to develop pertussis than were infants whose mothers were vaccinated during pregnancy but outside the 27- to 36-week time frame (OR, 0.22; 95% CI, 0.08-0.63).

Overall, Tdap vaccination during 27-36 weeks’ gestation was 85% more effective in reducing pertussis in infants younger than 8 weeks old and 72% more effective in preventing pertussis in infants younger than 12 weeks old, compared with postpartum vaccination.

In a companion paper, researchers described the results of a separate retrospective cohort study, the “first known study demonstrating that prenatal Tdap vaccination reduces severity of disease in infants who are infected with pertussis,” according to Ms. Winter and her associates ( Clin. Infect. Dis. 2016. doi: 10.1093/cid/ciw633 ).

For this study, the researchers identified 420 infants born between January 2011 and December 2015 who reported with pertussis at less than 63 days of age and had known maternal vaccination status. Of those 420 infants, only 49 mothers (12%) received Tdap vaccination during pregnancy, and only 14 received Tdap during the recommended window of 27-36 weeks’ gestation.

Infants born to mothers who received the Tdap vaccine during pregnancy were significantly less likely to be hospitalized when they developed pertussis (OR, 0.4; 95% CI, 0.2-0.9), were less likely to be admitted to the intensive care unit (OR, 0.5; 95% CI, 0.2-1.2), and had shorter hospital stays (median, 3 days vs. 6 days; P = .019). Infants born to vaccinated mothers also were older when they developed pertussis and were less likely to display common pertussis symptoms: paroxysmal cough, apnea, cyanosis, and whoop.

“Prenatal Tdap vaccination was 58% effective in preventing hospitalizations in infants infected with pertussis,” the researchers wrote, adding that “prenatal Tdap vaccination of mothers is a critical strategy for reducing the morbidity and mortality from pertussis.”

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