Pertussis-Related Infant Mortality Rate in Argentina Decreased After Maternal Tdap Vaccination

Pregnant woman being vaccinated
Tetanus toxoid, reduced diphtheria toxoid, and acellular pertussis vaccine is effective in middle-income countries like Argentina.

Tetanus toxoid, reduced diphtheria toxoid, and acellular pertussis (Tdap) vaccine is similarly effective in preventing infant disease when given to pregnant women in high-income countries,1-7 and in middle-income countries such as Argentina, where whole-cell pertussis vaccine is used for primary immunization.8 Results supporting this finding from a study in Argentina were published in Clinical Infectious Diseases8 and showed that Tdap vaccination during pregnancy prevented pertussis in infants aged <2 months, with similar efficacy whether the vaccine is administered during the second or third trimester.

In 2012, Argentina became the first country in Latin America to implement maternal Tdap vaccination, prompted mainly by an alarming increase in pertussis-related infant mortality.9,10 As young infants are especially vulnerable to severe disease and death from pertussis, researchers evaluated the efficacy of maternal Tdap vaccination during the susceptible period before primary immunization.

They conducted a matched case-control study at 6 hospitals in 4 Argentinian provinces. The study population included infants aged <2 months. For each case, researchers attempted to enroll 5 controls matched on mother’s residential health district and attendance in a participating hospital within the same province. Pertussis cases were laboratory confirmed by conventional or real-time polymerase chain reaction.

Participants’ mothers were classified as vaccinated during current pregnancy if Tdap vaccination date was confirmed by vaccination card or immunization registry. When neither vaccination card nor immunization registry were found, the mother provided verbal confirmation of Tdap vaccination. Maternal vaccination history was verified by vaccination card or immunization registry for 94% of case patients and 93% of controls; overall, 7% of vaccination histories were confirmed verbally.

Results included 71 case patients and 300 controls. Control participants were younger than case participants (25 days vs 38 days; P <.0001) and more likely to have a history of congenital disease (12% vs 3%; P =.02). Meanwhile, case patients were more likely to be part of families with ≥3 household members <18 years of age (66% vs 51%; P =.02) or to have a household member with respiratory illness (69% vs 20%; P <.0001).

Of the 71 case patients and 300 controls, 49% and 78% had mothers who were vaccinated with Tdap during pregnancy, respectively (P <.0001). Overall, the adjusted vaccine effectiveness was 80.7% (95% CI, 52.1%-92.2%). After restricting the population to those participants whose mothers were classified as vaccinated by vaccination card or immunization registry, researchers found the adjusted vaccine effectiveness to be similar whether it was administered during the second or third trimester (77.6%; 95% CI, 39.1%-91.8% and 82.7%; 95% CI, 46.4%-94.4%, respectively).

Currently, the World Health Organization and Pan American Health Organization do not endorse routine maternal Tdap vaccination because of the lack of evidence regarding the possible interference of Tdap vaccination of pregnant women on the child’s immunologic response to whole-cell pertussis vaccine.11

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“Regardless of the vaccine type used for primary immunization, if blunting of the infant antibody response does result in reduced protection against disease, there may be a shift in pertussis disease burden from younger to older infants,” noted the study authors. They stressed that “studies evaluating the impact of maternal Tdap vaccination on older infants who have received their primary immunization, in addition to monitoring surveillance data in countries where maternal vaccination is recommended, will be critical to understanding this issue.”


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