As long as the mechanisms of protection following maternal Tdap vaccination are not fully understood and no correlate of protection is available, anti-PT IgG levels are often used in studies like ours as surrogate markers for protection.18 Anti-PT levels in umbilical cord blood are correlated with protection against pertussis,20 and lower anti-PT IgG levels may point to less protection against pertussis in newborns. Here, F2 is linked to pertussis.