The US CDC's advice for the general public not to wear masks was born of several factors:

1. A severe shortage of masks which were considered to be particularly effective, specifically N-95 masks capable of filtering viral particles. Such masks as were available were being preferentially directed to healthcare providers and first-responders.

2. A concern that the general public would not wear or use masks appropriately, and might take additional compensatory risks on the premise that masks, often utilised suboptimally, provided strong protection. Note too that at the time vented masks, which had a flap that opened on exhale (and hence transmitted any viral particles to the environment) were fairly popular and widely-utilised.

3. A thought that COVID-19 was not as subject to airborne transmission as it turned out to be --- fomite-based transmission (particles on surfaces generally transferred by hand) was thought to be significant. Subsequent evidence has shown this to be far less the case than thought at the time.

4. A general lack of hard knowledge of specifics of the novel coronavirus, resulting in recommendations based on general principles of vector reduction drawn from other diseases.

5. A decidedly muddled, politicised, and anti-scientific response from the US administration at the time. For those keeping score, that was not the present Administration, though from discussion in this thread, that might not be clear.

6. In an epistemic environment in which trust in media, government, and expertise had been under prolonged attack.

All of this was discussed at the time, and in particular Zeynep Tufekci was discussing most of these factors as early as 17 March 2020 in New York Times opinion pieces and elsewhere. Example:

"Why Telling People They Don’t Need Masks Backfired"

Subtitle reveals much: "To help manage the shortage, the authorities sent a message that made them untrustworthy."

... First, many health experts, including the surgeon general of the United States, told the public simultaneously that masks weren’t necessary for protecting the general public and that health care workers needed the dwindling supply. This contradiction confuses an ordinary listener. How do these masks magically protect the wearers only and only if they work in a particular field?

Second, there were attempts to bolster the first message, that ordinary people didn’t need masks, by telling people that masks, especially medical-grade respirator masks (such as the N95 masks), needed proper fitting and that ordinary people without such fitting wouldn’t benefit. This message was also deeply counterproductive. ...

It is of course true that masks don’t work perfectly, that they don’t replace hand-washing and social distancing, and that they work better if they fit properly. And of course, surgical masks (the disposable type that surgeons wear) don’t filter out small viral particles the way medical-grade respirator masks rated N95 and above do. ...

[P]roviding top-down guidance with such obvious contradictions backfires exactly because lack of trust is what fuels hoarding and misinformation.

<https://www.nytimes.com/2020/03/17/opinion/coronavirus-face-...>

There's much more, the piece is well worth revisiting and should be a key document in future studies of the pandemic and its senselessly botched response.

Failures to accurately describe the circumstances and context of the CDC advice, or to acknowledge that the CDC rapidly reversed course once this became evident are themselves harmful.

Science is that which adapts to new knowledge and understanding. Dogma is that which clings to articles of faith despite evidence, in the interest of advancing a specific ideological agenda.

For all its faults, the CDC followed science. Incorrectly initially as it turns out, but ultimately correcting itself rather than insisting on dogma.

That point is made in another comment on this thread: <https://news.ycombinator.com/item?id=34150615>

The further complication is in crafting effective, credible, useful, and actionable messaging in a time of crisis. Here too the CDC failed, but in large part because significant other actors were strongly intent (and remain strongly intent) in seeing it fail.