Swedish Infants Developed Pertussis at a Younger Age if Their Mother Was the Possible Source of Infection in 2009–2015

The aim of this study was to investigate the possible source of laboratory-confirmed pertussis in infants under 6 months of age. This nationwide prospective study was based on laboratory-confirmed cases reported in the enhanced pertussis surveillance in Sweden from 2009 to 2015. The parents or carers of 345 infants were interviewed about their clinical details including the age at onset and disease severity. They were also asked to state who had been in contact with the infant prior to onset, in order to identify possible sources of infection. Only one possible source of infection was identified for 191 infants and the mother was presumed to be the source in 33%. These 63 cases showed different characteristics from those where a father, sibling, someone outside the immediate family or two to three people were identified as possible sources of infection. They were significantly younger, with a median of 31 days of age at onset (p < 0.01), and their hospitalisation rate (79%) was significantly higher (p < 0.01). Our results support general maternal vaccination against pertussis. Other preventive measures also need to be strengthened and implemented to prevent severe pertussis disease in the youngest infants with higher risk of death.