Eleven human subjects were revaccinated with a large dose (106.1 suckling mouse i.c. LD50) of the living attenuated 17D strain yellow fever virus about 14 months after primary vaccination with this agent. None had evidence of experience with any Group B arbor virus prior to the primary vaccination.
Viremia was not detected in any volunteer in blood samples drawn between the fourth and seventh post vaccination days. The same technique had yielded positive virus isolates in 6 of 11 volunteers during a comparable period following primary vaccination.
Each revaccinated volunteer responded with a rise in yellow fever neutralizing antibody titer. Three weeks after revaccination the titer had risen, on the average, 4.7-fold over the prevaccination titer.
The second exposure to the 17D virus, through the revaccination procedure, of human subjects whose only previous experience with Group B viral agents had been the primary yellow fever vaccination yielded again neutralizing antibodies highly specific for the YF virus. As after the primary vaccination, heterologous neutralizing antibodies were not detected for the West Nile virus or for types 1 and 2 dengue viruses. This observation stands in sharp contrast to previous observations on heterologous antibody production following yellow fever vaccination of human subjects possessing naturally acquired prevaccination Japanese encephalitis virus neutralizing antibodies.
The results of this study are discussed in relation to the general problem of immunity to reinfection with arthropod-borne viral agents.
Present address: Virology Dept., Merck, Sharp and Dohme Research Laboratories, West Point, Pennsylvania.