Neutralizing and haemagglutination-inhibiting antibodies to yellow fever 17 years after vaccination with 17D vaccine.

The duration of immunity conferred by yellow fever vaccine is as yet undetermined. In this study the neutralizing and haemagglutination-inhibiting antibodies to yellow fever were investigated in 108 persons living in Pouso Alegre, Brazil, where yellow fever has never been reported. These persons had been vaccinated with 17D yellow fever vaccine between 27 December 1940 and 5 February 1941, but not again; and their antibody pattern was compared with that of 78 controls who had never been vaccinated. In the vaccinated group the majority had neutralizing and haemagglutination-inhibiting antibodies present in the serum, whereas in the unvaccinated group antibodies were all but completely absent. Heterologous antibodies to other viruses of Casal's Group B were also found, and the significance of this finding is discussed.