Timing of Prenatal Tdap Immunization and Protection Against Pertussis

Tetanus, Diphtheria, Acellular Pertussis Vaccination During Pregnancy and Risk of Pertussis in the Newborn in Publicly and Privately Insured Mother-infant Pairs in the United States

BACKGROUND

In the United States, pertussis circulation persists and primarily infects infants and children, despite routine vaccinations. To minimize infant morbidity and mortality from the disease before the first DTaP dose, the Advisory Committee on Immunization Practices recommends maternal Tdap vaccination in weeks 27-36 of pregnancy.

METHODS

Cohorts of mother-infant pairs in the Medicaid Analytic eXtract (MAX) (2010-2014) and IBM MarketScan (2011-2015) databases were analyzed to estimate the effectiveness of prenatal Tdap vaccination compared with no vaccination to prevent infant pertussis in the first 6 months. Hazard ratios were estimated with Cox proportional hazards models and adjusted for potential confounders via inverse probability weights. The impact of preterm delivery on the risk of pertussis was analyzed. Results from the 2 databases were pooled.

RESULTS

In MarketScan, women received Tdap vaccination before delivery in 114,067 (25.6%) of 445,638 pregnancies and in MAX, 33,286 (4.8%) of 695,262 pregnancies. Among pregnancies with preterm delivery, only 21.2% and 3.8% in MarketScan and MAX had been vaccinated. The risk of pertussis in unvaccinated term infants was 3.5 (MarketScan) and 17 (MAX) per 10,000; and in preterm infants, it was 8.4 (MarketScan) and 19.8 (MAX) per 10,000. The pooled hazard ratio for Tdap vaccination any time before delivery versus no vaccination was 0.64 [95% confidence interval (CI): 0.41-1.00]. The hazard ratio was 0.11 (95% CI: 0.03-0.36) for preterm and 0.78 (95% CI: 0.48-1.29) for term infants vaccinated before 37 weeks. The incidence of pertussis was higher and the protective hazard ratio stronger during pertussis outbreaks.

CONCLUSIONS

Prenatal Tdap vaccination reduces the risk of pertussis infections in the infants' first 6 months by 36%. Vaccination soon after 27 weeks of pregnancy, before when deliveries began, ensures vaccination includes those born preterm, who are at highest risk for pertussis and benefit particularly from this vaccination.

The Effect of Timing of Tetanus-Diphtheria-Acellular Pertussis Vaccine Administration in Pregnancy on the Avidity of Pertussis Antibodies

Background: Optimal timing of gestational tetanus-diphtheria-acellular pertussis (Tdap) vaccination is not well-defined. No well-established specific anti-pertussis antibody level correlates with protection, suggesting the importance of antibody quality such as avidity. We aimed to determine the effect of timing of vaccination with Tdap in pregnancy on the avidity of cord anti-pertussis toxin (PT) immunoglobulin G (IgG).

Methods: Prospective study of newborns in a tertiary hospital (Melbourne, Australia) born to women vaccinated with Tdap in pregnancy. Ammonium thiocyanate was used as a bond-breaking agent to measure the avidity of anti-PT IgG using concentrations between 0.25 M (to measure low avidity antibodies) and 3 M (to measure very high avidity antibodies). Anti-PT IgG levels achieved at each ammonium thiocyanate concentration in cord samples of women vaccinated during 28–32 weeks gestation (WG) vs. 33–36 WG, and women vaccinated 5–12 vs. 1–4 weeks prior to delivery were compared using t-tests.

Results: Newborns of women vaccinated with Tdap during 28–32 WG (n = 43) had statistically significant higher concentrations of medium and high avidity anti-PT IgG compared with newborns of women vaccinated during 33–36 WG (n = 47), 11.6 IU/ml (95% CI, 8.8–15.2) IU/ml vs. 6.7 IU/ml (95% CI, 5.2–8.6) and 10.1 IU/ml (95% CI, 7.4–13.8) vs. 5.7 (95% CI, 3.6–8.9) IU/ml (p = 0.007 and p = 0.035), respectively. Newborns of women vaccinated 5–12 weeks before delivery (n = 64) had statistically significant higher concentrations of high and very high avidity anti-PT IgG compared with newborns of women vaccinated within 4 weeks before delivery (n = 25), 10.3 IU/mL (95% CI, 7.9–13.4) vs. 3.3 IU/mL (95% CI, 1.7–6.4), 12.6 IU/mL (95% CI, 9.4–16.9) vs. 4.3 IU/mL (95% CI, 2.2–8.5) (all p < 0.03), respectively.

Conclusions: Quantification of levels of anti-PT IgG with different avidities demonstrated that pertussis vaccination 5–12 weeks before delivery was associated with higher anti-PT IgG avidity compared with vaccination within 4 weeks before delivery. Pertussis vaccination during 28–32 WG was associated with higher anti-PT IgG avidity compared with vaccination during 33–36 WG, supporting vaccination at 28–32 over 33–36 WG for optimal protection against pertussis in infancy.