Prior to the Covid-19 pandemic, mainstream epidemiology and public health entities doubted – or even rejected – the efficacy of lockdowns and mass quarantines because they were considered ineffective.
This all changed in March 2020, when sentiment flipped in support of lockdown measures.
Still, there is a vast body of evidence explaining their original stance and why these mandates do not work.
Fauci said that shutting down the country does not work. (January 24, 2020)
Early into 2020, Fauci spoke to reporters saying, "That's something that I don't think we could possibly do in the United States, I can't imagine shutting down New York or Los Angeles, but the judgement on the part of the Chinese health authorities is that given the fact that it's spreading throughout the provinces… it's their judgement that this is something that in fact is going to help in containing it. Whether or not it does or does not is really open to question because historically when you shut things down it doesn't have a major effect."
World Health Organization Report discusses NPIs and why quarantine is ineffective. (2019)
In a table, WHO lists their recommendations of NPIs depending on severity level. Quarantine of exposed individuals is categorized as "not recommended in any circumstances." The report explains that "home quarantine of exposed individuals to reduce transmission is not recommended because there is no obvious rationale for this measure, and there would be considerable difficulties in implementing it."