A positive Pfeiffer reaction obtained with the serum from persons recently recovered from an infection clinically suspected of having been yellow fever provides confirmatory evidence of the disease. On the other hand, a negative Pfeiffer reaction may occasionally be obtained in the case of persons who have had an undoubted attack of yellow fever, hence the diagnosis of yellow fever clinically established cannot be set aside because of the negative outcome of a Pfeiffer test. The duration of the humoral signs of immunity, such as are brought out by the Pfeiffer reaction, is comparatively short and gives no precise indication of the state of immunity of the entire body, such as develops as the result of a genuine infection with yellow fever and usually persists for the remainder of life. In the case of a person who had yellow fever four years ago, the Pfeiffer reaction was found to be still very marked. In this particular instance, a possible reinforcement of immunity through frequent exposure to infection, up to two years previous to the test, cannot be excluded.
A negative Pfeiffer reaction was obtained with the serum of an individual born and resident in Mérida, where several epidemics of yellow fever have occurred in the past thirty years, without his ever having contracted the disease.
With the serum of an individual who had been vaccinated but had contracted yellow fever in Brazil during the period before protection from the vaccination could have developed, a strong Pfeiffer reaction was obtained 65 days from the time of onset of the disease. The presumption is warranted that the reaction was due not so much to the vaccination as to the natural infection, hence the etiological agent in this case of yellow fever in Brazil must have been identical with the strains of Leptospira icteroides isolated elsewhere.