Hooper freely acknowledges that this scenario remains unproven, and takes pains to include an appendix in The River listing 27 points of evidence for, and four against, the theory. Koprowski, now 85, categorically denies that he used African chimp kidneys as a tissue culture. He insists that CHAT was made with kidneys of rhesus macaque primates from India and the Philippines. But Hooper writes that in 1992, after an earlier version of the theory first hit the media, Koprowski was quoted as giving at least four different responses as to what type of simian species was used, before finally settling on the Asian variety with which he has stuck ever since. Normally, such a simple scientific detail could be easily checked by reading the medical literature of that period.
Researchers routinely list the species used for vaccine production.
Yet Hooper's exhaustive search for all journal articles and conference presentations about CHAT by Koprowski found an unexplained absence of this information. Also lending credence to suspicions of a cover-up is the mysterious disappearance of key relevant documents. Of course, critical evidence could come from the few CHAT samples still held in various labs' freezers. The story quickly generated major media coverage. Koprowski sued Rolling Stone for libel.