Early on, they didn't know if masks were effective against covid, because of the size of the particles. But they did still recommend them for healthcare workers:

https://www.news-medical.net/news/20200130/Do-masks-protect-...

They were saying not to buy masks in order to preserve them for healthcare workers. There was already a severe shortage of them.

https://news.yahoo.com/cdc-warns-save-respirator-masks-for-h...