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The Rockefeller Institute continued their work on yellow fever back in New York; it was highly dangerous work. The Rockefeller vaccine was used in the United States and England. At the same time, the Pasteur Institute in Tunis developed a one-dose vaccine (usually combined with smallpox), and it was used in France and their African colonies. The French vaccine was given by scarification (scratching into the skin). It risked febrile and central nervous system reactions but could be done in en mass.

In 1932, new technique of cultivating the virus in embryonated eggs and freeze drying them came into use. The 17D virus strain from a chance mutation at the Rockefeller Institute was attenuated enough that it didn’t need protective immune serum. Trials continued in South America but there were problems with encephalitis and jaundice.

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