Pertussis vaccination, combined in Tdap, is recommended in pregnant women, but the healthy-vaccinee bias affects the observational studies in pregnancy, overestimating the vaccine effectiveness and safety. A USA survey documented objectively this bias, for the influenza and pertussis vaccination. An European prospective cohort study (for influenza vaccination) showed consistent results.
The vaccine effectiveness and safety should primarily rely on RCTs, but on this subject they are small, inconclusive, and the latest showed worrying safety results.
Large observational studies showed a significant excess of chorioamnionitis, postpartum hemorrhage, and premature rupture of membranes in Tdap recipients, but these outcomes were deemed unimportant, lacking the evidence of adverse newborn outcomes. However, the healthy-vaccinee bias affects likely also pregnant women adhering to Tdap, therefore the mothers’ excess of adverse events in observational studies could be even underestimated, and the real effect on offspring could be shifted toward the null.
Before promoting Tdap vaccinations during all pregnancies, more so in countries with low burden of the correlated diseases, large independent, pragmatic trials are needed, with long follow-up. Meanwhile, vaccination in pregnant women could be offered, but communicating the uncertainties/allowing informed choices, and promoting together other protective behaviors.
Also other vaccination (and prevention) strategies could be considered, less interfering with bacterial ecology, instead of the periodic repetition of pertussis vaccination throughout life.