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Despite effective vaccines, pertussis remains endemic in the U.S.; indeed, rates have risen since the late 1980s, in part owing to waning immunity from acellular pertussis vaccination. Because the greatest pertussis-related morbidity and mortality occurs during the first months of life, pregnant women are advised to receive tetanus toxoid, reduced diphtheria toxoid, and acellular pertussis (Tdap) vaccine during each pregnancy to protect the infant by passive transfer of antibodies. However, the effectiveness and optimal timing of this approach remain uncertain. In a case-control study (with neonatal pertussis cases in the offspring as the outcome for determining maternal vaccine effectiveness [VE]), investigators analyzed 240 cases of infan…