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How effective is the Tdap vaccine during pregnancy?

In this study, researchers assess the effectiveness of the Tdap vaccine in pregnant mothers to prevent pertussis in infants.

Bordetella pertussis is a bacterium that causes the respiratory disease pertussis, which can be quite severe in infants. Pertussis incidence has increased over the years, reaching 20,762 confirmed cases in 2015. At least half the deaths due to pertussis occurs in infants, which could explain why the Centers for Disease Control and Prevention (CDC) recommend diphtheria, tetanus, acellular pertussis (DTaP) vaccine for infants at ages two, four, and six months, and booster shots at 15-18 months and at 4-6 years old.

CDC recommends Tdap booster vaccine in pregnant women

Since these shots and boosters are received in scheduled doses, usually only children at six months of age who have received the third dose of the vaccine are considered to be fully protected. The booster immunization of tetanus, diphtheria, acellular pertussis (Tdap) has not shown as much promise as expected in lowering incidence of pertussis in infants. As a result, a recommendation was made by the CDC to include a Tdap booster for pregnant women that can provide immunity to their unborn child in the placenta.

Debates regarding the optimal time to administer booster

Through additional research, it seemed like this too, did not fully provide the anticipated protection to the unborn infant, leading to the debate on whether there is an optimal time to give the pregnant women the booster in order to maximize protection of the infant against pertussis. Researchers still debate on whether administration in the second or third trimester provides the best protection to the unborn child. Currently, the third trimester is when the booster is administrated, by CDC recommendation.

Becker-Dreps and colleagues attempted to determine how effective Tdap is at different gestational time points. This American study was published in the American Journal of Preventive Medicine. The researchers examined 1,079,034 pregnant women from 2010 to 2014. Groups were made based on when the Tdap vaccine was received: prenatal Tdap (greater than two weeks before delivery), or the postpartum Tdap group (less than two weeks, the day of delivery or seven days following delivery). The researchers investigated if pertussis occurred in infants of mothers who received Tdap vaccination during pregnancy and women who did not.

Effectiveness of Tdap vaccine in infants when delivered before and after pregnancy

For the mothers who received prenatal Tdap vaccination, they found that there was a 43% decrease in the rate of pertussis in infants. Inpatient-only pertussis was also 68% lower compared to infants whose mothers did not receive the vaccine before or after pregnancy. They also found that if mothers received the Tdap vaccine at the 27-week mark or later, the children would be less likely to experience pertussis compared to the group of mothers without the vaccine.

Rates did not change if the Tdap vaccine was received at less than 27 weeks. In the postpartum Tdap group, infants also experienced fewer pertussis infections compared to the group that was not immunized. Overall, the effectiveness of the vaccine against pertussis in the children could vary by the age (in months) of the children.

The benefits of this immunization seemed to not be as prominent as in other studies, which may be attributed to the fact that this study examined infants for a longer duration (18 months) compared to other studies.

Fewer rates of pertussis in infants with third-trimester administration of Tdap

Overall, if Tdap was received in the third trimester, there were fewer rates of pertussis in their infants compared to pregnant mothers who did not receive the vaccine at all. In the time periods before the third trimester or the postpartum time period, statistical evidence could not suggest any benefit to receiving the vaccines at these specific times. So, there seems to be no evidence, at least in this study, that the current third-trimester immunization program should be changed.

There were limitations as well. Misclassifications may have occurred. These could occur if pertussis cases were confirmed by symptoms instead of the more accurate lab testing. There may also be rare technical errors which may have occurred, such as incorrect matching of mothers to their infants. Early immunization estimates may have been not as precise as the estimates for the post-27 week group, which had more subjects than the former group. Finally, this study was done on groups under insurance, so the results of this study may not necessarily reflect the uninsured population.

Written by Olajumoke Marissa Ologundudu B.Sc. (Hons)

Reference: Becker-Dreps, S et al. Effectiveness of Prenatal Tetanus, Diphtheria, Acellular Pertussis Vaccination in the Prevention of Infant Pertussis in the U.S. Am J Prev Med.(2018). doi: 10.1016/j.amepre.2018.04.013.



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