A PRELIMINAEY EVALUATION OF THE IMMUNIZING POWER OF CHICK-EMBRYO 17D YELLOW FEVER VACCINE INOCULATED BY SCARIFICATION1

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Abstract
Two experiments are recorded in which the immunizing power of 17D vaccine administered by scarification was tested. In the first experiment, the sera of 51 volunteers who had no demonstrable yellow fever antibody prior to vaccination by scarification with 17D vaccine were tested 32 days after vaccination. Forty-three (84.3 per cent) of these postvac-cination sera gave positive results and 3 (5.9 per cent) and 5 (9.8 per cent) gave inconclusive and negative results, respectively. In a comparable group of 68 volunteers vaccinated with 17D vaccine of approximately the same titer, 64 (94.1 per cent) developed positive postvaccination sera and 4 (5.9 per cent) showed inconclusive results. In this first experiment an attempt was also made to compare the immunizing effect of 17D vaccine inoculated by scarification with Dakar mouse-brain vaccine of the same mouse intracerebral titer. The preparation of Dakar vaccine used was estimated to contain 16,000 m.Ld. per inoculum as compared with 8,000 m.l.d. in the case of the 17D vaccine. In view of the methods employed in estimating the titers, it is not certain if the quantity of 17D vaccine used for each scarification was indeed 8,000 m.l.d. In any event, it was shown that 98.2 per cent of a group of 56 volunteers with negative prevaccination sera developed positive sera after vaccination with the Dakar vaccine. It appeared from this experiment that Dakar vaccine is a better antigen than 17D vaccine when administered by scarification. In a second experiment, groups of patients were inoculated by scarification with aliquots of the pooled contents of two ampules of 17D vaccine suspended in gum arabic (group 1A) or in saline solution (group 1B). Of 22 and 23 patients in groups 1A and 1B, 20 (95.2 per cent) and 22 (95.7 per cent) respectively, developed positive sera by the twenty-eighth day after vaccination. A control group of 19 patients with negative prevaccination sera were inoculated subcutaneously with the same quantity of virus as was used for scarification. All in this group developed positive post-vaccination sera. In combining the results of both experiments 85 of 91 (93.4 per cent) persons (with negative prevaecination sera) were immunized by scarification, with 17D vaccine. The relative merits of Dakar vaccine and 17D vaccine are discussed. It is suggested that by using either a crude extract of embryos or 17D mouse-brain virus a preparation of 17D vaccine could be administered by scarification which might prove to be a satisfactory method of mass vaccination of persons living in endemic yellow fever areas or for troops drafted on overseas service. The use of such a preparation would reduce the cost of the vaccine, and the dispensing with syringes would facilitate mass vaccinations and reduce the chance of the syringe transmission of disease.

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