A CDC evaluation found Tdap vaccination during the third trimester of pregnancy prevents 78% of pertussis cases in infants younger than 2 months of age. These findings are similar to other studies from the United Kingdom and the United States that suggest that vaccinating the mother during pregnancy is highly effective at protecting infants against pertussis.
When infants do get pertussis, their infection is less severe if their mother received Tdap during pregnancy. A CDC evaluation found maternal vaccination is 90% effective at preventing infant hospitalization from pertussis. Another U.S. study showed that infants whose mothers got Tdap during pregnancy had a significantly lower risk of hospitalization and shorter hospital stays. That same study showed that no infants born to vaccinated mothers required intubation or died of pertussis.
Links to published research on Tdap vaccination during pregnancy are available here: www.cdc.gov/pertussis/pregnant/research.html.