Wild-type yellow fever virus isolates from monkeys may not elicit protective immune responses against pathogenic strains in humans.

“if we take a virus out of a dead monkey that's died of yellow fever simply preserved not the same so one or two individuals didn't make any responses against the wild type primary ISIL because these viruses that come right out of a monkey for example are incredibly well adapted to replicating in the monkey and have not been adapted to replicate in tissue culture”