NEW YORK (Reuters Health) - Maternal immunization with the diphtheria toxoid acellular pertussis (Tdap) vaccine seems safe and might help protect newborns from pertussis, new research hints.

In a preliminary study, researchers found that giving pregnant women Tdap vaccine in the third trimester didn't have any harmful effects on the mother-to-be and raised antibody levels in the infant.

"Higher antibody concentrations in the first two months of life are likely to provide protection against pertussis during this period of high vulnerability," Dr. Flor M. Munoz, of Baylor College of Medicine in Houston, told Reuters Health by email.

The study was published online May 3rd in JAMA to coincide with a presentation at the annual meeting of the Academic Pediatric Societies, held this year in Vancouver.

Pertussis has re-emerged in the U.S. despite high immunization rates. Infants younger than six months are at greatest risk, accounting for more than 90% of all pertussis-associated deaths, the authors note.

Infants too young to receive the primary diphtheria and tetanus toxoids and acellular pertussis (DTaP) immunization series (recommended at two, four and six months of age) rely on maternal antibodies for protection against pertussis. However, mothers of young infants "have very little antibody against pertussis to pass to their babies transplacentally and protect them," Dr. Munoz said.

"Therefore, we thought that the most direct method to protect these babies was by enhancing the already natural process of transplacental antibody transfer, by vaccinating mothers with Tdap, the adult formulation of the pertussis vaccine which was licensed in the US in 2005-2006, and boosting their levels of antibody to provide protection to their babies during this period of high vulnerability," she explained.

Between 2008 and 2012, the researchers conducted a phase 1-2 randomized double-blind trial among 48 pregnant women; 33 received Tdap and 15 received placebo at 30 to 32 weeks' gestation, with crossover immunization postpartum.

The study found that pregnant women mount an adequate response to the Tdap vaccine and the vaccine is safe and well-tolerated during pregnancy, the researchers say.

There were no Tdap-related serious adverse events in the women or their infants. Injection site reactions after Tdap immunization were reported in 26 pregnant and 12 postpartum women. Systemic symptoms were reported in 12 pregnant and 11 postpartum women. Growth and development were similar in both infant groups. No cases of pertussis occurred.

The study also shows that "antibodies to pertussis are efficiently transferred to the fetus through the placenta so that infants of mothers who were vaccinated during pregnancy had significantly higher concentrations of antibody at the time of birth and up to two months of age, when compared to infants of mothers who were vaccinated postpartum," Dr. Munoz said.

"Infants of mothers who were vaccinated during pregnancy had adequate responses to their routine pertussis vaccines at two, four and six months of age, and had expected and adequate responses to their 4th dose of vaccine at one year of age. The absolute concentration of antibodies to some of the pertussis antigens might be modestly lower after the primary series of vaccines in some infants of mothers who were vaccinated during pregnancy, but this difference did not persist after the 4th dose," Dr. Munoz added.

She said more work is needed to further confirm the findings, noting that the study was relatively small and did not assess efficacy of vaccinating pregnant women in preventing infant pertussis infection.

"Therefore, larger studies to document the safety and efficacy of Tdap vaccination during pregnancy are needed. There are other investigators doing similar studies at this time, and additional studies are being planned," she told Reuters Health.

The co-authors of an editorial agree that more study is needed.

"Although Tdap has an excellent safety record, future cohort or surveillance studies must continue to assess safety and immunogenicity of Tdap immunization during pregnancy on infant responses," write Dr. Natalia Jimnez-Truque and Dr. Kathryn M. Edwards, of Vanderbilt University Medical Center, Nashville, Tennessee.

They add, "Continued reporting of pertussis cases will also be necessary to assess the effectiveness of administering Tdap during pregnancy. In addition, the assessment of potential interference with DTaP and other vaccine antigens administered during infancy will require large prospective studies. Last, future research must address the safety and immunogenicity of repeated doses of Tdap during each pregnancy, because frequent immunization might lead to a blunted antibody response."

SOURCES: https://bit.ly/1fJQuMm and https://bit.ly/1hs2qxA

JAMA 2014;311:1736-1737,1760-1769.