This study was planned to collect evidences of familial pertussis transmission to infants younger than 6 months of age. Understanding the dynamics of transmission of pertussis in families is essential to plan effective prevention strategies that could be integrated in pertussis control.

The seroprevalence of IgG antibodies to pertussis toxin (PT-IgG) and prolonged cough symptoms were evaluated in parents of 55 infants aged <6 months hospitalized for confirmed pertussis. Parents of 33 infants with lower respiratory tract infection (LRTI) and parents of 57 healthy infants admitted as outpatients for hip ultrasound examination (HE) were enrolled as controls.

Parents of pertussis cases had PT-IgG levels significantly higher as compared to LRTI and HE parents. More than 40 % were compatible as transmitters of pertussis to their babies, since they had a level of PT-IgG ≥ 100 IU/ml, which is considered diagnostic for a recent pertussis episode. Based on serology, the percentage of pertussis cases that had at least one parent as source of infection was 49.1 %. When cough symptoms were taken into account, the percentage of parents putative transmitters of the infection to their infants increased to 56.4 %.

Parents are scarcely aware of the household transmission of pertussis to their newborns. Our study highlights the need to advise parents about the likelihood of transmission to the newborn and to be particularly aware of coughing symptoms in the household. Since infection can be asymptomatic, a serological survey of family members should also be considered.