Pertussis sources of infection and routes of transmission in the vaccination era.

Vaccination against pertussis has resulted in reduction of the infection pressure of Bordetella pertussis (partial herd immunity), but the circulation of B. pertussis has persisted as a consequence of waning of vaccine-induced and naturally acquired immunity. An increase in the reported incidence of B. pertussis infection in older children, adolescents and young adults has been noted, resulting in a perceived resurgence of the disease in these age groups. Regardless of whether this resurgence is real or not, older groups are increasingly recognized as playing an important role in transmitting B. pertussis infection to incompletely immunized infants, in whom pertussis disease continues to cause severe and fatal illness, albeit at much lower levels than in the prevaccine era. Several studies have suggested that mothers, in particular, are a significant source of infection for infants. Adolescents, grandparents and health care workers can also play a role. By contrast, most adolescents acquire the infection from schoolmates and friends, whereas for adults the main sources are children and work colleagues. Furthermore teachers, child care workers and health care workers could be at increased risk of being exposed to, and transmitting, B. pertussis infection. Current immunization strategies inadequately control the circulation of B. pertussis, in part because of suboptimal adherence to current pediatric immunization guidelines. In addition to efforts to improve pertussis immunization rates in children, the expansion of pertussis immunization to target specific groups should be considered. Besides reducing morbidity in the targeted groups, these strategies could decrease the residual burden of pertussis morbidity and mortality in infants.