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In the past two decades, cases of infant pertussis have increased from 34 to 103 cases per 100,000 infants. Where does it come from? In an international prospective study, researchers investigated 95 laboratory-confirmed cases of pertussis in infants younger than 6 months and 404 of their contacts (206 adult and 127 child household contacts and 71 nonhousehold contacts).
A laboratory-confirmed primary source of infection could be identified for 44 infants (48%). Most source cases (86%) were older than 13 years. The primary source was far more likely to be a household contact than a nonhousehold contact (76% vs. 25%): 55% were parents, 16% were siblings, and 29% were other contacts. In sensitivity analysis that expanded the definition of a pr…