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—It should be noted that in the current Italian field trial to test the efficacy of several acellular pertussis vaccines, which began in 1992, 90% of the participants received a pertussis vaccine and only 10% a placebo. In surveys done before the trial started, only 44% of Italian children were immunized against pertussis. Thus, the present trial is immunizing more than twice as many Italian children against the disease as were receiving vaccine before the trial began. In addition, few participants have dropped out of the trial because of dissatisfaction with the trial's design.
Marwick C. The Italian Pertussis Vaccine Trial: Ethical Issues-Reply. JAMA. 1994;272(24):1899. doi:10.1001/jama.1994.03520240026028