HAIFA, ISRAEL-- Vaccinated children and adults may be silent reservoirs for transmission of Bordetella pertussis infection, researchers found after investigating the case of a 4-month-old infant who died of pertussis in Israel.[1]

Srugo et al examined the infant's five family members as well as 46 children (ages 2 to 6 years) at day care centers that the infant's siblings had attended. Nasopharyngeal specimens were taken from all of the contacts for culture and for polymerase chain reaction (PCR) testing. In addition, serum samples were analyzed for immunoglobulin antibodies to B pertussis.

Both the infant's family members and the other children had received all four doses of the diphtheria-tetanus-pertussis (DPT) vaccine. The baby had received only the first dose of the vaccine.

The infant's mother had a three-month history of persistent cough, and the baby's three siblings had experienced paroxysmal cough for four to five weeks. However, none of the family members had had a whooping episode.

INCIDENCE OF PERTUSSIS INFECTION

Four of the five family members (80%) and five of the 46 children (11%) had positive PCR results, indicating B pertussis colonization of the nasopharynx. Two of these five children with positive PCR results met the modified World Health Organization (WHO) definition of pertussis infection (ie, three weeks of paroxysmal cough).[2] The other three children had mild, nonspecific cough. In addition, all five family members and 17 children (37%) had IgA and/or IgM antibodies to B pertussis. None of the culture results were positive.

IMPLICATIONS

The immunity conferred by the DPT vaccine diminishes within five to 10 years, Srugo et al explained. The present findings suggest that immunity does not persist into early childhood in some cases and that the DPT vaccine does not fully protect children against clinical disease, as defined by WHO criteria.

In addition, the vaccine only protects against clinical disease and not against infection. Thus, vaccinated people "may serve as reservoirs and potential transmitters of infection," the researchers suggested. Furthermore, because vaccinated persons who are infected with pertussis develop nonspecific symptoms, they may serve as silent reservoirs of infection.

--Kristin Della Volpe

References
1. Srugo I, Benilevi D, Madeb R, et al. Pertussis infection in fully vaccinated children in day-care centers, Israel. Emerg Infect Dis. 2000;6:526-529.
2. WHO meeting on case definition of pertussis, Geneva, January 10-11, 1991. Geneva, Switzerland: World Health Organization;1991:4-5 (issue no. MIN/EPI/PERT/91.1).