Subclinical pertussis in incompletely vaccinated and unvaccinated infants

Incidental to a phase II study of acellular and whole-cell pertussis vaccines involving 342 infants who were clinically observed from birth until the age of 9 months, subclinical pertussis was retrospectively diagnosed in 10 infants on the basis of serological evidence. IgG and IgA to filamentous haemagglutinin (FHA), pertussis toxin (PT) and agglutinogens 2 and 3 (AGG2, 3) were assayed by enzyme-linked immunosorbent assay (ELISA) in serum obtained at birth and at 2, 4, 6 and 9 months of age. All 10 infants had > or = 4-fold rises in at least two different pertussis IgG antibodies. Nine of the 10 infants had > or = 4-fold increases in all three IgG antibodies measured. One infant had > or = 4-fold increases in IgG-FHA and IgG-AGG2,3 but not IgG-PT. Seven infants had raised IgA antibodies to PT and FHA and 4 infants had raised IgA antibodies to AGG2,3. Subclinical infection provoked differing degrees of antibody production in response to multiple antigens. Subclinical infection was detected in both unvaccinated infants (4) and in infants who had been vaccinated from 2 months of age with either acellular (4) or whole-cell vaccines (2). Subjects were 8 months of age or younger and only 1 had completed primary vaccination. Other infections of infancy were commonly detected; 4 infants had upper respiratory disease about the time of subclinical pertussis. None had a household member with symptomatic pertussis. Likelihood of subclinical infection was related to significantly lower levels of maternally acquired pertussis IgG-AGG2,3 antibodies but not associated with infants' nutritional status.