Abstract: Case 1, a 4-month-old girl, was admitted to hospital because of cough for half a month, while Case 2, a 1-month-old girl, was hospitalized with an 8-day cough. Both 2 infants were diagnosed with pertussis by polymerase chain reaction (PCR) detection of Bordetella pertussis nucleic acid in nasopharyngeal swabs. The parents of Case 1 and the parents and elder brother of Case 2 were all diagnosed as pertussis by either IgG antibody levels against pertussis toxin (PT-IgG) or nucleic acid detection. Infants who have not been fully vaccinated with diphtheria, tetanus and pertussis combined vaccine are the main population of pertussis, but older children and adults often become the major infectious reservoir due to atypical symptoms. The key transmission route of pertussis in family clusters is from adults (mainly parents) to infants. Recent infection with Bordetella pertussis can be indicated by measuring a 2- to 4-fold increase of PT-IgG in the convalescent and acute phases or a single PT-IgG detection increase of more than 80 to 100 IU/ml. However, the serological test results are not reliable for children who have received pertussis vaccine within 1 year, while a comprehensive analysis of data from multiple laboratory tests is required.

Key words: Pertussis, Infant, Family clusters, Laboratory diagnosis