SALT LAKE CITY — Americans who are fully vaccinated against COVID-19 should go back to wearing masks in public indoor settings, along with everyone at K-12 schools, to slow the spread of the highly contagious delta variant in virus hot spots, according to new recommendations by the Centers for Disease Control and Prevention released Tuesday. The change comes because of new science showing the delta variant — now responsible for 80% of coronavirus cases in the United States — not only is causing breakthrough cases of COVID-19 in fully vaccinated people but also allows them to spread the deadly disease, said Dr. Rochelle Walensky, CDC director.