Safety of Tdap vaccine in pregnant women: an observational study

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

A small US randomised trial including 33 pregnant women assessed the immunogenicity and safety of Tdap during pregnancy. Safety outcomes were collected with a 7-day diary. How these were defined is not described. Pain was reported by 25 (75.8%) participants while erythema and induration/swelling were each reported by 3 (9.1%) participants. These were the same rates as reported by the non-pregnant women in the study. 31 The rates of injection site pain in our study are consistent with most other studies that report less than 10% experiencing severe pain. 16 25 26 28 Rates of swelling with a diameter of more than 50 mm ranged from 1% to 18% in clinical studies. In our study population, just 0.4% recorded a swelling of more than 50 mm and fewer than 8% reported any. 16 24-28 30 Our outcomes are lower than these other reports; however, this could be because we have differentiated between swelling and induration. Overall, both events were infrequent and mild. Erythema (redness) was relatively uncommon in our study with fewer than 6% of participants reporting any and 1.4% reporting a diameter greater than 50 mm. In clinical studies, severe erythema has been reported to range from 2% to 17%. 24-28 30 Our rates are consistent with those recently reported in the US trial in pregnant women. 31 It should be noted that the aluminium content of the NZ Boostrix formulation is higher than that of the US formulation, and the higher aluminium content may be linked to greater local reactogenicity. 32 In the Northern arm of our study, we collected information about swelling and induration according to Brighton Collaboration definitions 17 18 and are able to report induration separately from swelling. Induration occurred more frequently than swelling and appeared to have a later temporal onset than swelling. More than half of the cases of induration occurred 25-48 h later compared with around a quarter of all swellings, supporting the likelihood that each has a different aetiology. It is likely that some of the swelling reported in the Southern participants was in fact induration misclassified.