Infants whose mothers received the tetanus, diphtheria, acellular pertussis (TDAP) vaccine during pregnancy had lower rates of pertussis, commonly known as whooping cough, but timing was critical, according to a new study. The report, published in the American Journal of Preventive Medicine, was the first to track clinical outcomes of the vaccine for the first 18 months of life in offspring.

University of North Carolina Chapel Hill–led researchers reviewed insurance data on more than 675,000 pregnancies in the United States from 2010 to 2014 to determine who had received Tdap during pregnancy. At the same time, they gathered information on hospitalizations and outpatient visits for pertussis in those infants through age 18 months.

The study team posited that vaccination of the mothers would confer immunity to the fetus during the first 6 months of life, at which point the infants could receive the pertussis vaccine themselves. Results indicate a 75% reduction of pertussis hospitalizations and a 46% reduction of any pertussis cases in infants whose mothers were immunized during pregnancy.

An unexpected finding was that response to the pertussis vaccine series by those infants was unaffected by their mothers’ immunization, in contrast to what had been suggested in other immunological studies.

“This just adds more fuel to the fire for encouraging women to get Tdap during pregnancy,” explained Sylvia Becker-Dreps, MD, MPH, associate professor in the departments of family medicine in the UNC School of Medicine and epidemiology in the UNC Gillings School of Global Public Health in a press release. “A lot of women are concerned about vaccines in general, but you really might be harming your baby by not getting this vaccine.”

Becker-Dreps also pointed out that timing was important, adding, “Our results showed that getting it during the third trimester, but at least two weeks before delivery, is best to optimize the benefits of the vaccine.”

A reduction in pertussis was found in infants whose mothers received the vaccine in the third trimester, while no benefits of the vaccine were observed when mothers received it earlier in the pregnancy. The CDC urges receipt of the Tdap vaccine between 27 and 36 weeks of pregnancy.

“Infants of mothers who received prenatal Tdap experienced half the rate of pertussis as compared with infants of unimmunized mothers,” study authors conclude. “These results do not provide evidence to support changing the currently recommended timing of Tdap administration in pregnancy.”