Sixteen-month follow-up of antibodies to pertussis toxin after primary immunization with acellular or whole cell vaccine.

Antibodies to pertussis toxin (antitoxin) were measured in three blood samples drawn during a 16- to 17-month follow-up of infants immunized with adsorbed two component acellular pertussis vaccine (JNIH-6) or plain whole cell vaccine. A significant decrease of antitoxin concentration was noted between each follow-up in the acellular vaccine groups (P less than 0.005). The higher antitoxin titers induced by three doses or by two doses spaced by 2 months compared with two doses with 1-month interval disappeared with time. The antitoxin titers among high responders to three doses of whole cell vaccine paralleled those of the acellular vaccinees but at a significantly lower level. Reported exposure to pertussis did not significantly alter the decrease of antitoxin titers. The study also showed that acellular pertussis vaccine induced an antitoxin response still measurable in greater than or equal to 97 and greater than or equal to 91% of samples drawn 10 to 11 and 16 to 17 months after primary immunization, respectively.