The quote comes from a letter issued by 17 prominent virologists and it appears that they wanted to sound as officious as possible. Unfortunately, they should have used the standard epidemiological term "estimated" and not "appreciated." The statement that "the total number of human infections that led to these deaths is not known", clearly indicates that anything said after these words in conjecture. But, conjecture is not a very scientific word. When you do not know the exact number of cases, you can only estimate.
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