Safety of Tdap vaccine in pregnant women: an observational study

Petousis-Harris, Helen; Walls, Tony; Watson, Donna; Paynter, Janine; Graham, Patricia; Turner, Nikki

SAEs occurred in our study population during the study period. None were considered by clinical review as likely to be caused by the exposure to Tdap vaccine. In NZ, up to 15% of pregnancies have obstetric complications, and approximately 1 per 10 infants are born preterm or low birth weight. 33 34 Annually, there are approximately 600 perinatal deaths (∼1% of births) of which 14% are unexplained. These figures have remained consistent over time. 22 On the basis of NZ data, the rates of SAEs in our study were not higher than the expected background rate for such a cohort. Reports to the US Vaccine Adverse Event Reporting System of pregnant women inadvertently given Tdap have been summarised. 35 Between January 2005 and June 2010, there were 132 reports identified, 20 following Boostrix with no non-pregnancy SAEs reported. In the US trial, 31 SAEs occurred in 7/33 pregnant women, without known risks to pregnancy at enrolment, followed to 4 months postpartum, and none of these were nonpregnancy SAEs. The events occurred at variable time periods following immunisation and none were considered attributable to the vaccine. Recently, obstetric events and birth outcomes for 123 494 women from two Californian Vaccine Safety Datalink sites were evaluated; 26 229 women received Tdap with no increased risk for hypertensive disorders of pregnancy or preterm or small for gestational age birth found. There was a small increased risk of chorioamnionitis diagnosis. 9 Further investigations have not found an association. 7 8 A matched cohort study from the UK in 20 074 pregnant women and a matched historical unvaccinated control group found no evidence of any increase for predefined pregnancy-related AEs including stillbirth. 10 These studies all support the safety profile of Tdap in pregnancy.