Kinetics and sensitivity of ELISA IgG pertussis antitoxin after infection and vaccination with Bordetella pertussis in young children.

Sera from 96 young children in a vaccine trial were analysed for kinetics of ELISA IgG anti-pertussis toxin (anti-PT) after a laboratory-verified pertussis infection. The antibody decay curves after infection were biphasic and similar in shape to those after vaccination. The change from a rapid to a slower decay after the peak occurred about 4-5 months from the first day of cough. In a group of children given a two- or a five-component acellular pertussis vaccine the proportion of sera above the tentative cut-off values for anti-PT of 20, 50 or 100 EU/ml 12 months after onset of the infection were 19%, 0% and 0% respectively. Corresponding figures for a whole-cell or placebo vaccine group of infected children were significantly higher, 73%, 39% and 30%, i.e. the antibody decay after infection in young children depends on vaccination status as well as on the pertussis vaccine given. In a large group of non-infected children vaccinated with the same five-component acellular vaccine 13%, 0% and 0% had sera above 20, 50 and 100 EU/ml at 12 months after the third vaccine dose and all were below the minimum level of detection 2 years after vaccination. In conclusion, knowledge about anti-PT kinetics is essential for the interpretation of seroepidemiological data but hardly offers the possibility to establish valid cut-off values for anti-PT in single sample serology. An option would be to identify a grey zone between the positive and negative ends of the distribution for follow-up testing by a second serum.