Comparable groups of adult African women were vaccinated (a) with a mixed 17 D yellow fever-vaccinia vaccine administered by scarification, or (b) by inoculation subcutaneously with 17 D vaccine followed by scarification with vaccinia vaccine. There was no significant difference in the response of the two groups to the vaccinia vaccine. In the group inoculated with the mixed vaccine, of those whose sera contained no demonstrable yellow fever antibody prior to vaccination, 66·6% had developed antibody when their sera were tested 28 days later. In the other group 100% had developed antibody by the twenty-eighth day after vaccination.

It is suggested that the difference in the response of the groups in this study might be due to some local interference which prevented invasion by the 17D virus in some cases.

While there is good evidence for the efficiency of 17D vaccine as an immunizing agent when administered by scarification (Hahn, 1951; Dick, 1952), the present study indicates that the percentage of those who became immune after vaccination with the mixed vaccine used in this trial is not sufficiently high to suggest that this type of mixed vaccine should be used routinely.

In none of our patients was any reaction noted to the mixed or double vaccinations. Further information is required on the reaction of both negro and white races to combined vaccinations with yellow fever and vaccinia viruses. It is suggested that a careful follow-up of persons vaccinated by the combined Dakar vaccine might produce some valuable information in this respect.

Our thanks are due to Dr George Campbell Young for his very great co-operation, and to Messrs W. A. Whittaker, L. E. Hewitt and D. Santos for technical assistance.