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I like how you completely ignored the part where I said "that doesn't matter" and argued the wrong point anyway.

Whether you consider them reputable or not doesn't matter. Those are THE organizations (some of them, anyway) that decide these things. They are THE experts in the field. If a person were to say "a lot of people/organizations say , so it must be true", that would be argumentum ad populum. But since they are saying "a lot of <authorities/experts in the field x> claim , so it must be true", that's not a fallacy, that's a valid appeal to authority.

CDC, WHO, NIH, etc. could all be wrong, they could've interpreted the "scientific evidence" incorrectly and come to the wrong conclusions. But we know that this is an unlikely scenario for so many independent experts in the field to reach a consensus on something that is wrong. Therefore, our best bet is to trust their conclusions.

To reiterate, whether those organizations are right or wrong doesn't matter, because they are not a random majority—they are the organizations you're supposed to rely on in this situation; it's a valid appeal to authority. Hence, it's not a fallacy, let alone argumentum ad populum.